**Neutron Diffraction Studies of the Magnetic Oxide Materials**

J.B. Yang1, Q. Cai2, H.L. Du1, X.D. Zhou2, W.B. Yelon2 and W.J. James2 *1School of Physics, Peking University 2Materials Research Center, Missouri University of Science and Technology 1P.R. China 2USA* 

### **1. Introduction**

24 Will-be-set-by-IN-TECH

100 Neutron Diffraction

Cuello, G. & Granada, J. (1997). Thermal neutron scattering by debye solids: A synthetic

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Howe, M. A., McGreevy, R. L. & Zetterström, P. (1996). Computer code correct: Correction

Kropff, F., Latorre, J. R., Granada, J. R. & Castro Madero, C. (1984). Total neutron cross section of D2O at 20 °C between 0.0005 and 10 ev, *Technical Report EXFOR 30283001*, IAEA. Lovesey, S. (1986). *The Theory of Neutron Scattering from Condensed Matter*, International series

Mughabghab, S., Divadeenam, M. & Holden, N. (1984). *Neutron Cross Sections: Neutron*

Academic Press. URL: *http://books.google.com/books?id=cgk6AQAAIAAJ* Paalman, H. H. & Pings, C. J. (1962). Numerical evaluation of X-Ray absorption factors for cylindrical samples and annular sample cells, *J. Appl. Phys.* 33: 2635. Rodríguez Palomino, L. A., Blostein, J. J. & Dawidowski, J. (2011). Calibration and

URL: *http://www.sciencedirect.com/science/article/pii/S0168900211008989* Rodríguez Palomino, L. A., Dawidowski, J., Blostein, J. J. & Cuello, G. J. (2007). Data

URL: *http://www.sciencedirect.com/science/article/pii/S0168583X07003862* Schmunk, R. E., Randolph, P. D. & Brugger, R. M. (1960). Total cross sections of Ti,V,Y, Ta and

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absolute normalization procedure of a new deep inelastic neutron scattering spectrometer, *Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment* 646(1): 142 – 152.

processing method for neutron diffraction experiments, *Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms*

*problems*, Addison-Wesley series in computer science and information processing, Addison-Wesley Pub. Co. URL: *http://books.google.com/books?id=TP9QAAAAMAAJ* Squires, G. L. (1978). *Introduction to the theory of thermal neutron scattering*, Cambridge

URL: *http://www.sciencedirect.com/science/article/pii/0168900287903706* Granada, J. R. (1985). Slow-neutron scattering by molecular gases: A synthetic scattering

ambient water using h/d isotopic substitution, *Journal of Physics, Conference Series* .

(2002). D4c: A very high precision diffractometer for disordered materials, *Appl.*

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URL: *http://www.sciencedirect.com/science/article/pii/S0306454996000503* Cuello, G. J. (2008). Structure factor determination of amorphous materials by neutron

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Neutron diffraction (ND) is an extremely valuable tool for the investigation of magnetic materials, because of its ability to directly observe periodic magnetic structures, determine magnetic moment directions and magnitudes, to observe light elements (H, C, N, O, F etc.) that are otherwise difficult to locate from x-ray diffraction due to the strong scattering of heavy elements, or to distinguish nearby elements in the periodic chart. Neutron diffraction has proven to be a very useful technique in the study of the magnetic oxides such as perovskites and transition metal oxides.

The perovskite-type ABO3(A=La, Sr, B=Fe, Co, Ni, Cu, Mn, Ti) perovskites has attracted much attention due to their mixed electronic and oxygen ion conductivity, which shows potential to serve as oxygen separation membranes, oxygen sensors, and solid oxide fuel cell (SOFC), which results from the large number of oxygen vacancies (Kamata et al, 1978, Mizusaki et al., 1991, Kuo et al, 1989.) It is recognized that the physical properties of ABO3 are largely dependent on the oxygen deficiency (Goodenough, 1955, Srilomsak et al., 1989) . The mixed conductivity can be enhanced through the substitution of La3+ by Sr2+ at A sites, and the substitution of Fe3+ by other transition metal ions at B sites. The charge imbalance and overall charge neutrality can be maintained by the presence of charged oxygen vacancies and mixed valence state ions at the B sites. These point defects are the origin of the mixed electronic and oxygen ion conductivity.

A three-dimensional framework made up of BO6 octahedra characterizes the ABO3 perovskite crystal structure. The interstices of this framework are sites for the large A cations, and the small B cations are at the centers of the octahedra. Every two neighboring BO6 octahedra share a common O atom. The perovskite structure is often stable with both divalent (Sr, Ba, Ca, etc.) and trivalent (e.g. lanthanide) A site occupation, and the charge imbalance and overall charge neutrality will be maintained by the formation of electrons, holes and/or charged oxygen vacancies. These point defects can take part in the electronic and/or oxygen ion conductivity, and this gives the material scientist an opportunity to alter the transport properties of a given oxide.

When perovskite materials that contain transition metal ions at the B sites are heated to sufficiently high temperature, they can equilibrate with the ambient oxygen by oxygen

Neutron Diffraction Studies of the Magnetic Oxide Materials 103

oxygen vacancies were prepared by heating the samples in N2, O2, and CO/CO2 mixtures. The effect of oxygen vacancies on the structural, magnetic and electronic properties was determined using neutron diffraction, magnetic measurements and Mössbauer spectra. It was found that magnetic ordering, charge disproportionation and charge ordering in these compounds shows strong dependence on the oxygen vacancies. The oxygen vacancies will changes the Fe valence states, the unit cell volume and the Fe-O-Fe bond angle. These dramatically affect the Fe-O-Fe superexchange coupling. Therefore by creating oxygen vacancies or having excess oxygen, the exchange interaction of Fe-O and the valence state of Fe ions are affected, and lead to large changes in the magnetic and transport properties, such as the magnetic ordering temperature, the magnetic moments, the hyperfine interactions

The liquid-mixing method (Eror et al., 1986) was used to synthesize fine LSFO powder. An aqueous solution of Fe nitrate was first prepared and thermogravimetrically standardized. The lanthanum carbonate, and strontium carbonate were added to form a clear solution. Citric acid and ethylene glycol were added to the nitrate solution and then heated slowly to form a polymeric precursor, which was heated to 250ºC to form an amorphous resin. This resin was calcined at 800ºC for 8 hours. The powders were pressed at 207 MPa to form a dense bar. The bar was sintered at 1000–1200°C for 24 h under different environments( air, N2, O2 or CO/CO2), followed by quenching to room temperature. We assume the oxygen nonstoichiometry of the quenched samples is the same as before quenching. The magnetization curves of the samples were measured using a superconducting quantum interference device SQUID magnetometer in a eld of up to 6 T from 1.5 K to 800 K. A magnetic eld of 50 Oe was used for the eld cooling(FC) and zero eld cooling(ZFC) process. The crystal phase was identied by x-ray diffraction analysis using Cu *Kα* radiation. The powder neutron diffraction experiments were performed at the University of Missouri-Columbia research reactor using neutrons of wavelength 1.4875 Å. The data for each sample were collected over 24h In order to study the oxygen stoichiometry change in air at high temperature, in-situ neutron diffraction studies were carried out at high temperature (up to 800C) for the samples unquenched and quenched at 1500C in air. The Rietveld method was used to refine the data by using the FULLPROF code (Rodriguez-Carvajal 2000), in

and electric conductivity in the pervoskite structure.

which the magnetic ordering was modeled by a separate phase.

**2.1.1 The effect of heat treatment under different gas environments** 

Figure 1 shows the typical ND patterns of La0.6Sr0.4FeO3 powders at different heat treatment conditions. Similar patterns are observed for all samples, showing them to be single phase. The symmetry of the samples remains rhombohedral (space group *R*¯3*c*) throughout the series. A cubic structure (space group *Pm*¯3*m*) was also proposed for these compounds, but it was found that the data could be better tted in a hexagonal structure (space group *R*¯3*c*) as conrmed by the following neutron diffraction data renement. The model is a rhombohedral structure with space group R 3 c. The lattice parameters (hexagonal setting) are 2 *<sup>p</sup> ab a* , 2 3 *<sup>p</sup> c a* , ==90, and =120, where ap is the lattice parameter of

**2. Experimental methods** 

**2.1 Results and discussion** 

exchange. If the oxygen activities at the two sides of the perovskite are different, the oxygen ions can be transported from the high oxygen activity side to the low one. Therefore perovskites can be used as oxygen separation membranes or electrodes of SOFCs.

Strontium-doped lanthanum manganate is most commonly used as the cathode of SOFCs operated at high temperature. LaMnO3 is a p-type perovskite. At high temperature, it can have oxygen excess, stoichiometry, or oxygen deficiency, depending on the oxygen partial pressure.( Kamata et al, 1978, Mizusaki et al., 1991, Kuo et al, 1989.) For example, at 1200C, the oxygen content of LaMnO3 ranges from 3.079 to 2.947 under oxygen partial pressures of 1 to 10-11 atmosphere pressure.(Kamata et al, 1978) In addition to oxygen nonstoichiometry, LaMnO3 can also have La deficiency or excess. LaMnO3 with La excess may contain La2O3 as a second phase, and therefore LaMnO3 with La deficiency is recommended for use in SOFCs.

In recent years, work has focused on SOFCs that can operate at an intermediate temperature (IT) range (600-800C). In this temperature range, the metal alloy, such as stainless steel, which is cheap and easily fabricated, can be used as the interconnect, and the reliability of SOFCs is thus improved. In order for a SOFC to be operated in the IT range, the electrode kinetics have to be at least as fast as those occurring at high temperature. LSM, the current cathode material, is not suitable for use below 800C due to its very low oxygen vacancy concentration. Therefore, research has to be done to find new material for use as the cathode of SOFCs operating in the IT range. In general, most cathodes have relied on the B-site cation, and doping on both A-sites and B-sites to improve electrical conductivity and catalytic performance. One approach to finding a new cathode material is to replace the Mn by other transition metals, such as Fe, Co, and Ni etc.

The strontium-doped lanthanum ferrite La1-xSrxFeO3- (LSFO) has been intensively studied since it has good mixed conductivity at high temperature, and, thus, can be used as an oxygen membrane and is a candidate for the cathode of the SOFC. Undoped LaFeO3 has a low electrical conductivity and oxygen vacancy concentration, but the electrical conductivity and oxygen vacancy concentration are increased by the substitution of La by Sr. M. V. Patrakeev et al., 2003 studied the electron / hole and ion transport in LSFO in the oxygen partial pressure range of 10-19-0.5 atm and temperatures between 750-950C, and found that the electronic and ionic conductivity increase with Sr content and attain maximal values at x=0.5. The conductivity can be explained by electron hopping between Fe3+ and Fe4+ in the high PO2 range and between Fe3+ and the Fe2+ in the low PO2 range. The LSFO material is stable when the PO2>10-16 atm at 950C; oxygen vacancies will appear at 600-800C in air. LSFO has shown excellent cathode performance at 750C (Sun et al., 2005).

It is important to measure the oxygen vacancy concentration in LSFO. It is well known that neutron powder diffraction coupled with Rietveld refinement(Rodriguez-Carvajal 1998) can be used to determine the oxygen vacancy concentration in many oxides, since the sensitivity of neutron scattering to oxygen is comparable to other atoms. However, the precision of such a determination is unknown, especially at low vacancy content. In addition, neutron diffraction is a very sensitive direct probe of the magnetic moment, and LSFO exhibits magnetic ordering. If the magnetic moment of Fe is sensitive to the oxygen vacancy concentration, it can be used as an indirect probe of the oxygen vacancy concentration. To understand the physical properties of La0.6Sr0.4FeO3- and its behavior in SOFC and to be able to optimize its behavior, it is important to have a good knowledge of the defect chemistry of La0.6Sr0.4FeO3. In the present paper, samples of La0.6Sr0.4FeO3 with different amounts of oxygen vacancies were prepared by heating the samples in N2, O2, and CO/CO2 mixtures. The effect of oxygen vacancies on the structural, magnetic and electronic properties was determined using neutron diffraction, magnetic measurements and Mössbauer spectra. It was found that magnetic ordering, charge disproportionation and charge ordering in these compounds shows strong dependence on the oxygen vacancies. The oxygen vacancies will changes the Fe valence states, the unit cell volume and the Fe-O-Fe bond angle. These dramatically affect the Fe-O-Fe superexchange coupling. Therefore by creating oxygen vacancies or having excess oxygen, the exchange interaction of Fe-O and the valence state of Fe ions are affected, and lead to large changes in the magnetic and transport properties, such as the magnetic ordering temperature, the magnetic moments, the hyperfine interactions and electric conductivity in the pervoskite structure.

### **2. Experimental methods**

102 Neutron Diffraction

exchange. If the oxygen activities at the two sides of the perovskite are different, the oxygen ions can be transported from the high oxygen activity side to the low one. Therefore

Strontium-doped lanthanum manganate is most commonly used as the cathode of SOFCs operated at high temperature. LaMnO3 is a p-type perovskite. At high temperature, it can have oxygen excess, stoichiometry, or oxygen deficiency, depending on the oxygen partial pressure.( Kamata et al, 1978, Mizusaki et al., 1991, Kuo et al, 1989.) For example, at 1200C, the oxygen content of LaMnO3 ranges from 3.079 to 2.947 under oxygen partial pressures of 1 to 10-11 atmosphere pressure.(Kamata et al, 1978) In addition to oxygen nonstoichiometry, LaMnO3 can also have La deficiency or excess. LaMnO3 with La excess may contain La2O3 as a second phase, and therefore LaMnO3 with La deficiency is recommended for use in SOFCs. In recent years, work has focused on SOFCs that can operate at an intermediate temperature (IT) range (600-800C). In this temperature range, the metal alloy, such as stainless steel, which is cheap and easily fabricated, can be used as the interconnect, and the reliability of SOFCs is thus improved. In order for a SOFC to be operated in the IT range, the electrode kinetics have to be at least as fast as those occurring at high temperature. LSM, the current cathode material, is not suitable for use below 800C due to its very low oxygen vacancy concentration. Therefore, research has to be done to find new material for use as the cathode of SOFCs operating in the IT range. In general, most cathodes have relied on the B-site cation, and doping on both A-sites and B-sites to improve electrical conductivity and catalytic performance. One approach to finding a new cathode material is to replace the Mn

The strontium-doped lanthanum ferrite La1-xSrxFeO3- (LSFO) has been intensively studied since it has good mixed conductivity at high temperature, and, thus, can be used as an oxygen membrane and is a candidate for the cathode of the SOFC. Undoped LaFeO3 has a low electrical conductivity and oxygen vacancy concentration, but the electrical conductivity and oxygen vacancy concentration are increased by the substitution of La by Sr. M. V. Patrakeev et al., 2003 studied the electron / hole and ion transport in LSFO in the oxygen partial pressure range of 10-19-0.5 atm and temperatures between 750-950C, and found that the electronic and ionic conductivity increase with Sr content and attain maximal values at x=0.5. The conductivity can be explained by electron hopping between Fe3+ and Fe4+ in the high PO2 range and between Fe3+ and the Fe2+ in the low PO2 range. The LSFO material is stable when the PO2>10-16 atm at 950C; oxygen vacancies will appear at 600-800C in air.

It is important to measure the oxygen vacancy concentration in LSFO. It is well known that neutron powder diffraction coupled with Rietveld refinement(Rodriguez-Carvajal 1998) can be used to determine the oxygen vacancy concentration in many oxides, since the sensitivity of neutron scattering to oxygen is comparable to other atoms. However, the precision of such a determination is unknown, especially at low vacancy content. In addition, neutron diffraction is a very sensitive direct probe of the magnetic moment, and LSFO exhibits magnetic ordering. If the magnetic moment of Fe is sensitive to the oxygen vacancy concentration, it can be used as an indirect probe of the oxygen vacancy concentration. To understand the physical properties of La0.6Sr0.4FeO3- and its behavior in SOFC and to be able to optimize its behavior, it is important to have a good knowledge of the defect chemistry of La0.6Sr0.4FeO3. In the present paper, samples of La0.6Sr0.4FeO3 with different amounts of

LSFO has shown excellent cathode performance at 750C (Sun et al., 2005).

perovskites can be used as oxygen separation membranes or electrodes of SOFCs.

by other transition metals, such as Fe, Co, and Ni etc.

The liquid-mixing method (Eror et al., 1986) was used to synthesize fine LSFO powder. An aqueous solution of Fe nitrate was first prepared and thermogravimetrically standardized. The lanthanum carbonate, and strontium carbonate were added to form a clear solution. Citric acid and ethylene glycol were added to the nitrate solution and then heated slowly to form a polymeric precursor, which was heated to 250ºC to form an amorphous resin. This resin was calcined at 800ºC for 8 hours. The powders were pressed at 207 MPa to form a dense bar. The bar was sintered at 1000–1200°C for 24 h under different environments( air, N2, O2 or CO/CO2), followed by quenching to room temperature. We assume the oxygen nonstoichiometry of the quenched samples is the same as before quenching. The magnetization curves of the samples were measured using a superconducting quantum interference device SQUID magnetometer in a eld of up to 6 T from 1.5 K to 800 K. A magnetic eld of 50 Oe was used for the eld cooling(FC) and zero eld cooling(ZFC) process. The crystal phase was identied by x-ray diffraction analysis using Cu *Kα* radiation. The powder neutron diffraction experiments were performed at the University of Missouri-Columbia research reactor using neutrons of wavelength 1.4875 Å. The data for each sample were collected over 24h In order to study the oxygen stoichiometry change in air at high temperature, in-situ neutron diffraction studies were carried out at high temperature (up to 800C) for the samples unquenched and quenched at 1500C in air. The Rietveld method was used to refine the data by using the FULLPROF code (Rodriguez-Carvajal 2000), in which the magnetic ordering was modeled by a separate phase.

#### **2.1 Results and discussion**

#### **2.1.1 The effect of heat treatment under different gas environments**

Figure 1 shows the typical ND patterns of La0.6Sr0.4FeO3 powders at different heat treatment conditions. Similar patterns are observed for all samples, showing them to be single phase. The symmetry of the samples remains rhombohedral (space group *R*¯3*c*) throughout the series. A cubic structure (space group *Pm*¯3*m*) was also proposed for these compounds, but it was found that the data could be better tted in a hexagonal structure (space group *R*¯3*c*) as conrmed by the following neutron diffraction data renement. The model is a rhombohedral structure with space group R 3 c. The lattice parameters (hexagonal setting) are 2 *<sup>p</sup> ab a* , 2 3 *<sup>p</sup> c a* , ==90, and =120, where ap is the lattice parameter of

Neutron Diffraction Studies of the Magnetic Oxide Materials 105

This peak proves to be purely magnetic and the change reflects a large increase in magnetic moments for the CO/CO2-reduced samples. An antiferromagnetic structure has been confirmed for all samples. Fe atoms at (0,0,0) couple antiferromagnetically with those at (0,0,1/2) along the c-axis. The Fe atoms show a magnetic moment 3.8B in the CO/CO2 treated La0.6Sr0.4FeO3 as compared to 1.2B in the O2-treated samples at 290 K. The extent of oxygen vacancy in these compounds has been obtained by refinements of neutron diffraction data. The O2-, and N2-quenched samples show similar patterns, and there are less than 1% vacancies on the oxygen sites. The oxygen vacancy concentration is around 7-10% for the samples quenched in the CO/CO2 mixtures. It is noticed that these values are about 2% less than those measured from the iodometric method. This difference might cause from

160

12

0

4

8

165

170

Fe-O-Fe Angle (deg.)

Oxygen Vacancy (%)

Fig. 2. The lattice parameters and unit cell volumes of La0.6Sr0.4FeO3 treated at different gases(a), the bond angle, oxygen vacancy and magnetic moments of La0.6Sr0.4FeO3 treated at

The typical neutron patterns of the samples (N2, O2, and 50%CO/50%CO2) at 290 K were collected( data not shown here). The N2-treated sample shows a rhombohedral structure and are rened in the space group *R* 3 c. The diffraction peak half width for the CO/CO2 treated samples is dramatically reduced compared to the N2-treated samples which is similar to the XRD patterns. The rhombohedral splitting of the peaks is not too obvious for the CO/CO2 treated samples. Thus, we have used both a cubic cell space group *Pm* 3 *m* and a rhombohedral cell, space group *R* 3 *c* to rene the structure. The rhombohedral structure gives much better renement results. For example, χ2 is 7.56 for the renement using space group *R* 3 *c* and 15.8 for space group *Pm* 3 *m* for the N2-treated sample. Therefore, all samples have been rened with a rhombohedral cell space group *R* 3 *c*). The first diffraction peak [101] at about 19°) is much stronger in the reduced samples than in the air-, N2-, and O2-quenched samples. This peak proves to be purely magnetic and the change reflects a large increase in magnetic moments for the CO/CO2-reduced samples. The air-, O2-, and N2 quenched samples show similar patterns, and there is less than 1%. The 57Fe Mössbauer

175

(b)

0

N2 Air O2 CO/CO2 =10:90, 50:50, 90:10

2

Magnetic moment (B)

4

the systematic errors of the two techniques.

352

5.4 5.5 5.6 13.2 13.4 13.6

a axis

N2 O2

different gases(b)

CO:CO2

=10:90 50:50 90:10

c axis

356

360

Unit cell volume (A3

Lattice parameters ( A )

)

364

(a)

Fig. 1. Neutron diffraction patterns of La0.6Sr0.4FeO3 quenched at O2 and CO/CO2. atmospheres

the basic cubic perovskite cell. In this model, the large cations La3+/Sr2+ occupy the 6a sites, the small cations Fe3+/Fe4+ occupy the 6b sites, and the oxygen ions occupy the 18e sites. The magnetic structure is modeled as having antiferromagnetic ordering with the moments in the hexagonal plane, which reverse between the positions (0, 0, 0) and (0, 0, ½). The magnetic model has the symmetry R 3 , and the magnetic moments on the two sites are constrained to be equal. It is difcult to use XRD patterns to determine the structural distortion, and the oxygen vacancy concentration. Accordingly, neutron diffraction was employed to distinguish the differences between the structures of the samples, and to determine the oxygen vacancy concentration.(Yang et al., 2002)

As evidenced from this figure, all samples crystallize in the rhombohedral structure with space group R-3c. The lattice parameters and bond angle are shown in Fig. 2(a) and (b). It can be seen that the unit cell volume increases when the La0.6Sr0.4FeO3 is quenching in the CO/CO2 atmosphere. The unit cell volume of three CO/CO2-reduced samples are larger by about 5Å3 than those of the O2- and N2-quenched samples. The ratio of the lattice parameters a/c changes from 0.4113 for the O2-quenched sample to 0.40840 for the reduced samples, which decreases the distortion from cubicity. This change affects the peak positions and the sharpness of the diffraction peaks. The diffraction peak splitting for the CO/CO2 treated samples is dramatically reduced compared to the O2-treated samples. The first diffraction peak is much stronger in the reduced samples than in the O2-quenched samples.

20 40 60 80

the basic cubic perovskite cell. In this model, the large cations La3+/Sr2+ occupy the 6a sites, the small cations Fe3+/Fe4+ occupy the 6b sites, and the oxygen ions occupy the 18e sites. The magnetic structure is modeled as having antiferromagnetic ordering with the moments in the hexagonal plane, which reverse between the positions (0, 0, 0) and (0, 0, ½). The magnetic model has the symmetry R 3 , and the magnetic moments on the two sites are constrained to be equal. It is difcult to use XRD patterns to determine the structural distortion, and the oxygen vacancy concentration. Accordingly, neutron diffraction was employed to distinguish the differences between the structures of the samples, and to

As evidenced from this figure, all samples crystallize in the rhombohedral structure with space group R-3c. The lattice parameters and bond angle are shown in Fig. 2(a) and (b). It can be seen that the unit cell volume increases when the La0.6Sr0.4FeO3 is quenching in the CO/CO2 atmosphere. The unit cell volume of three CO/CO2-reduced samples are larger by about 5Å3 than those of the O2- and N2-quenched samples. The ratio of the lattice parameters a/c changes from 0.4113 for the O2-quenched sample to 0.40840 for the reduced samples, which decreases the distortion from cubicity. This change affects the peak positions and the sharpness of the diffraction peaks. The diffraction peak splitting for the CO/CO2 treated samples is dramatically reduced compared to the O2-treated samples. The first diffraction peak is much stronger in the reduced samples than in the O2-quenched samples.

2 ( degree )

CO:CO2 Yobs =90:10

quenched Yobs

CO:CO2 =10:90

O2

determine the oxygen vacancy concentration.(Yang et al., 2002)

atmospheres

Count number ( 103 )

> Yobs Ycal Yobs-Ycal

 Ycal Yobs-Ycal

Fig. 1. Neutron diffraction patterns of La0.6Sr0.4FeO3 quenched at O2 and CO/CO2.

 Ycal Yobs-Ycal This peak proves to be purely magnetic and the change reflects a large increase in magnetic moments for the CO/CO2-reduced samples. An antiferromagnetic structure has been confirmed for all samples. Fe atoms at (0,0,0) couple antiferromagnetically with those at (0,0,1/2) along the c-axis. The Fe atoms show a magnetic moment 3.8B in the CO/CO2 treated La0.6Sr0.4FeO3 as compared to 1.2B in the O2-treated samples at 290 K. The extent of oxygen vacancy in these compounds has been obtained by refinements of neutron diffraction data. The O2-, and N2-quenched samples show similar patterns, and there are less than 1% vacancies on the oxygen sites. The oxygen vacancy concentration is around 7-10% for the samples quenched in the CO/CO2 mixtures. It is noticed that these values are about 2% less than those measured from the iodometric method. This difference might cause from the systematic errors of the two techniques.

Fig. 2. The lattice parameters and unit cell volumes of La0.6Sr0.4FeO3 treated at different gases(a), the bond angle, oxygen vacancy and magnetic moments of La0.6Sr0.4FeO3 treated at different gases(b)

The typical neutron patterns of the samples (N2, O2, and 50%CO/50%CO2) at 290 K were collected( data not shown here). The N2-treated sample shows a rhombohedral structure and are rened in the space group *R* 3 c. The diffraction peak half width for the CO/CO2 treated samples is dramatically reduced compared to the N2-treated samples which is similar to the XRD patterns. The rhombohedral splitting of the peaks is not too obvious for the CO/CO2 treated samples. Thus, we have used both a cubic cell space group *Pm* 3 *m* and a rhombohedral cell, space group *R* 3 *c* to rene the structure. The rhombohedral structure gives much better renement results. For example, χ2 is 7.56 for the renement using space group *R* 3 *c* and 15.8 for space group *Pm* 3 *m* for the N2-treated sample. Therefore, all samples have been rened with a rhombohedral cell space group *R* 3 *c*). The first diffraction peak [101] at about 19°) is much stronger in the reduced samples than in the air-, N2-, and O2-quenched samples. This peak proves to be purely magnetic and the change reflects a large increase in magnetic moments for the CO/CO2-reduced samples. The air-, O2-, and N2 quenched samples show similar patterns, and there is less than 1%. The 57Fe Mössbauer

Neutron Diffraction Studies of the Magnetic Oxide Materials 107

CO:CO2 =10:90


T=20K

T=130 K, Hext=5T

T=130 K

T=300 K

Velocity (mm/s)

=90:10


T=20K

T=130 K, Hext=5T

T=130 K

T=300 K

Velocity (mm/s)

Fig. 5. Mössbauer spectra of the CO/CO2-quenched La0.6Sr0.4FeO3 samples measured at

Fig. 4. The Mössbauer spectra of the CO/CO2-quenched samples at different temperatures

1.00 CO:CO2

0.97 0.98 0.99 1.00

> 0.97 0.98 0.99 1.00

> 0.94 0.96 0.98 1.00

Relative Transimission

different temperatures

0.92 0.94 0.96 0.98 1.00

0.94 0.96 0.98

0.94 0.96 0.98 1.00

0.92 0.94 0.96 0.98 1.00

0.94 0.96 0.98 1.00

Relative Transimission

spectra of La0.6Sr0.4FeO3- treated in N2 measured at different temperatures are shown in Fig. 3 (a). The hyperfine parameters are listed in Table. I. The Mössbauer spectroscopy of N2 at room temperature shows a paramagnetic behavior. Two singlets were used in the fitting for the spectrum. The isomer shifts are 0.26mm/s and 0.18mm/s, which are exactly what would be expected for the Fe3+ and Fe4+ ions. The ratio of Fe3+/Fe4+ is 63.9/36.1, corresponding to an oxygen vacancy, =0.023. At 130 K and 20 K, the spectra are comprised of three superimposed magnetic hyperfine patterns.

Fig. 3. 57Fe Mössbauer spectra of La0.6Sr0.4FeO3- treated in N2 (a) and O2 (b) measured at different temperatures

The parameters of these spectra are mostly applicable to the Fe3+ and Fe5+ valence states. These observations indicate a change from a paramagnetic Fe4+ state to a mixed valence state Fe3+/Fe5+ resulting from the charge disportortionation reaction, 2Fe4+ Fe3++Fe5+ (Takano et al., 1997). Here it was assumed that the two subspectra with the larger hyperfine fields correspond to the Fe3+ ions, while the one with the lowest hyperfine field (~26T) corresponds to Fe5+ ions, which was used by Dann *et al, 1994*.

Similar spectra were observed for O2-quenched samples as shown in Fig. 3(b). At room temperature the rhombohedral relaxation of the hyperfine fields appears and therefore a Voigt peak-shaped sextet and a singlet were used in the fitting to account for the Fe3+ and Fe4+ ions. At low temperature, the charge disportortionation also takes place. The oxygen deficiency obtained from the relative areas of the Mössbauer spectra of the Fe3+ and Fe4+ ions in O2-treated sample is nearly zero, which agrees well with the neutron diffraction measurements. The Mössbauer spectra of the CO/CO2-quenched samples are shown in Fig. 4. They show a typical sextet due to the antiferromagnetic coupling of the Fe atoms, which is

spectra of La0.6Sr0.4FeO3- treated in N2 measured at different temperatures are shown in Fig. 3 (a). The hyperfine parameters are listed in Table. I. The Mössbauer spectroscopy of N2 at room temperature shows a paramagnetic behavior. Two singlets were used in the fitting for the spectrum. The isomer shifts are 0.26mm/s and 0.18mm/s, which are exactly what would be expected for the Fe3+ and Fe4+ ions. The ratio of Fe3+/Fe4+ is 63.9/36.1, corresponding to an oxygen vacancy, =0.023. At 130 K and 20 K, the spectra are comprised of three

superimposed magnetic hyperfine patterns.

1.00 O2

T=300 K

0.97 0.98 0.99 1.00 0.96

0.98

Relative Transimission

different temperatures

1.00

0.88 0.92 0.96


T=20 K

T=130 K

(a)


Velocity (mm/s)

T=20 K

T=130 K, Hext=5T

T=130 K

(b)

0.97 0.98 0.99 1.00

0.94 0.96 0.98 1.00

Relative Transimission

Fig. 3. 57Fe Mössbauer spectra of La0.6Sr0.4FeO3- treated in N2 (a) and O2 (b) measured at

The parameters of these spectra are mostly applicable to the Fe3+ and Fe5+ valence states. These observations indicate a change from a paramagnetic Fe4+ state to a mixed valence state Fe3+/Fe5+ resulting from the charge disportortionation reaction, 2Fe4+ Fe3++Fe5+ (Takano et al., 1997). Here it was assumed that the two subspectra with the larger hyperfine fields correspond to the Fe3+ ions, while the one with the lowest hyperfine field (~26T)

Similar spectra were observed for O2-quenched samples as shown in Fig. 3(b). At room temperature the rhombohedral relaxation of the hyperfine fields appears and therefore a Voigt peak-shaped sextet and a singlet were used in the fitting to account for the Fe3+ and Fe4+ ions. At low temperature, the charge disportortionation also takes place. The oxygen deficiency obtained from the relative areas of the Mössbauer spectra of the Fe3+ and Fe4+ ions in O2-treated sample is nearly zero, which agrees well with the neutron diffraction measurements. The Mössbauer spectra of the CO/CO2-quenched samples are shown in Fig. 4. They show a typical sextet due to the antiferromagnetic coupling of the Fe atoms, which is

0.92 0.94 0.96 0.98 1.00

0.80 0.84 0.88 0.92 0.96

1.00 N2

T=300 K

Velocity (mm/s)

corresponds to Fe5+ ions, which was used by Dann *et al, 1994*.

Fig. 4. The Mössbauer spectra of the CO/CO2-quenched samples at different temperatures

Fig. 5. Mössbauer spectra of the CO/CO2-quenched La0.6Sr0.4FeO3 samples measured at different temperatures

Neutron Diffraction Studies of the Magnetic Oxide Materials 109

confirmed by neutron diffraction and magnetic measurements. The best fitting can be reached by using two sextets for the fitting of the entire spectra by assuming two different Fe valence states. The average hyperfine field is 50-53 T at room temperature, which is of the same order as that of the Fe-oxide (such as Fe2O3). This large hyperfine field corresponds to a valence state between Fe2+ and Fe3+. There is no evidence in the spectra for the presence of any Fe5+ at low temperatures. An attempt to find evidence for the presence of distinct Fe4+ or Fe2+ lines in the spectra also failed. The two sextets have values of an isomer shift somewhere between Fe2+ and Fe3+ regions. The true oxidation state of Fe seems to be neither Fe2+ nor Fe3+ but an intermediate state (such as Fe(3-x)+,0<x<1.0). Because the CO/CO2 treated samples have much higher oxygen vacancy concentrations, it suggests that some Fe4+ or even Fe3+ in the non-reduced samples are reduced to Fe(3-x)+ (0<x<1.0), in order to maintain charge balance in these compounds. It is evident that the change of the Fe valence state from Fe4+ to Fe(3-x)+ results in a large hyperfine field and a large magnetic moment. The average quadrupole splitting of the CO/CO2-quenched samples is smaller than that of the non-reduced sample, which indicates a decrease in the distortion from the cubic structure. This is consistent with the neutron diffraction data, which show a decrease of the distortion from cubic. It is found that the CO/CO2-treated samples have the same a/c ratio (0.40855) as those (a/c=0.40850) of La1-xSrxFeO3- (x=0.6 and 0.7), which is, near the boundary between the rhombohedral and cubic structures (Dann et al., 1994). The spectra measured under a magnetic field of 5 T at 130K were also included in Figs. 3-5. As compared with the spectra at 130 K without the magnetic field, it is found that the intensities of the first and sixth line decrease. This indicates that there might be a canted magnetic moment in the magnetic

sublattice, which implies that the antiferromagnetic structure is not perfect.

Figure 6 shows the typical ND patterns of La0.6Sr0.4FeO3 powders quenched at different temperatures in the air. The first peak at low angle is a pure magnetic peak; its intensity increases with the quenching temperature, and some of the split peaks merge into one peak

Figure 7 shows the change of lattice parameters a\* and c\* with the quenching temperature, in which a\* represents *a* / 2 and c\* represents *c* /2 3 . The symmetry of L6SF quenched to room temperature from 700 to 1100C remains rhombohedral, but the rhombohedral distortion becomes small when the quenching temperature is high. When the quenching temperature T1200C, the unit cell appears to be cubic. The presence of oxygen vacancies

According to the direct refinement, the oxygen vacancy concentration increases from almost zero for the unquenched sample to about 0.2 for the 1500C quenched sample. The statistical uncertainty shows that the direct refinement is reliable only when the oxygen vacancy

The unit cell volume increases as the quenching temperature increases by a total of 3.8Å3 from the unquenched sample to the 1500C quenched sample. The statistical uncertainty in the unit cell volume is about 0.04Å3. It shows that, the unit cell volume can be a good metric for the determination of vacancy concentration. However, the unit cell volumes are relative

**2.1.2 The effect of quenching temperatures** 

can relax the strain in the structure and reduce the distortion.

when the quench temperature is high.

concentration is high.


Parameter constrained to the given value, and 0<x<1.0.

Table 1. Hyperfine parameters of the La0.6Sr0.4FeO3 treated at different gases measured at different temperatures.

N2-quenched

O2-quenched

CO/CO2=10:90

CO/CO2=90:10

Parameter constrained to the given value, and 0<x<1.0.

20K

130K

130 K (H= 5T)

 20K

130K

130K (H=5T)

130K (H=5T)

different temperatures.

IS (mm/s) QS (mm/s) Bhf (kOe) Int.(%)

Fe3+ 0.442 -.058 540.9 26.3 Fe3+ 0.381 .001 515.0 58.4 Fe5+ -0.005 0.047 262.3 15.3

Fe3+ 0.420 -.106 517.0 43.9 Fe3+ 0.386 -.077 488.9 39.3 Fe5+ -0.085 -0.118 264.2 16.8

Fe3+ 0.415 -.051 513.3 44.9 Fe3+ 0.387 -.039 478.0 33.2 Fe5+ -0.033 0.251 253.4 21.9

Fe4+ 0.180 - - 36.1

Fe3+ 0.422 -0.023 534.3 51.2 Fe3+ 0.399 -0.001 507.1 33.5 Fe5+ -0.053 -0.034 268.9 15.3

Fe3+ 0.427 -0.046 524.6 33.2 Fe3+ 0.399 -0.252 497.1 48.4 Fe5+ -0.058 -0.015 259.6 18.4

Fe4+ 0.202 40.1

Fe(3-x)+ 0.376 0.012 533.1 27\*

Fe(3-x)+ 0.379 -0.052 522.7 27\*

Fe(3-x)+ 0.435 -0.032 551.4 73\* Fe(3-x)+ 0.375 -0.068 521.6 27\*

Fe(3-x)+ 0.236 0.020 501.1 27\*

Fe(3-x)+ 0.417 -0.007 535.4 40\*

Fe(3-x)+ 0.386 -0.074 526.4 40\*

Fe(3-x)+ 0.434 -0.007 553.1 60\* Fe(3-x)+ 0.388 -0.055 524.1 40\*

Fe(3-x)+ 0.272 -0.063 504.1 40\*

RT Fe3+ 0.261 - - 63.9

RT Fe3+ 0.331 0.047 191.0 59.9

20K Fe(3-x)+ 0.479 -0.026 561.6 73\*

130K Fe(3-x)+ 0.442 0.014 555.8 73\*

RT Fe(3-x)+ 0.324 0.050 535.5 73\*

20K Fe(3-x)+ 0.469 0.024 565.5 60\*

130K Fe(3-x)+ 0.439 0.009 557.4 60\*

RT Fe(3-x)+ 0.324 0.035 535.3 60\*

Table 1. Hyperfine parameters of the La0.6Sr0.4FeO3 treated at different gases measured at

confirmed by neutron diffraction and magnetic measurements. The best fitting can be reached by using two sextets for the fitting of the entire spectra by assuming two different Fe valence states. The average hyperfine field is 50-53 T at room temperature, which is of the same order as that of the Fe-oxide (such as Fe2O3). This large hyperfine field corresponds to a valence state between Fe2+ and Fe3+. There is no evidence in the spectra for the presence of any Fe5+ at low temperatures. An attempt to find evidence for the presence of distinct Fe4+ or Fe2+ lines in the spectra also failed. The two sextets have values of an isomer shift somewhere between Fe2+ and Fe3+ regions. The true oxidation state of Fe seems to be neither Fe2+ nor Fe3+ but an intermediate state (such as Fe(3-x)+,0<x<1.0). Because the CO/CO2 treated samples have much higher oxygen vacancy concentrations, it suggests that some Fe4+ or even Fe3+ in the non-reduced samples are reduced to Fe(3-x)+ (0<x<1.0), in order to maintain charge balance in these compounds. It is evident that the change of the Fe valence state from Fe4+ to Fe(3-x)+ results in a large hyperfine field and a large magnetic moment. The average quadrupole splitting of the CO/CO2-quenched samples is smaller than that of the non-reduced sample, which indicates a decrease in the distortion from the cubic structure. This is consistent with the neutron diffraction data, which show a decrease of the distortion from cubic. It is found that the CO/CO2-treated samples have the same a/c ratio (0.40855) as those (a/c=0.40850) of La1-xSrxFeO3- (x=0.6 and 0.7), which is, near the boundary between the rhombohedral and cubic structures (Dann et al., 1994). The spectra measured under a magnetic field of 5 T at 130K were also included in Figs. 3-5. As compared with the spectra at 130 K without the magnetic field, it is found that the intensities of the first and sixth line decrease. This indicates that there might be a canted magnetic moment in the magnetic sublattice, which implies that the antiferromagnetic structure is not perfect.

#### **2.1.2 The effect of quenching temperatures**

Figure 6 shows the typical ND patterns of La0.6Sr0.4FeO3 powders quenched at different temperatures in the air. The first peak at low angle is a pure magnetic peak; its intensity increases with the quenching temperature, and some of the split peaks merge into one peak when the quench temperature is high.

Figure 7 shows the change of lattice parameters a\* and c\* with the quenching temperature, in which a\* represents *a* / 2 and c\* represents *c* /2 3 . The symmetry of L6SF quenched to room temperature from 700 to 1100C remains rhombohedral, but the rhombohedral distortion becomes small when the quenching temperature is high. When the quenching temperature T1200C, the unit cell appears to be cubic. The presence of oxygen vacancies can relax the strain in the structure and reduce the distortion.

According to the direct refinement, the oxygen vacancy concentration increases from almost zero for the unquenched sample to about 0.2 for the 1500C quenched sample. The statistical uncertainty shows that the direct refinement is reliable only when the oxygen vacancy concentration is high.

The unit cell volume increases as the quenching temperature increases by a total of 3.8Å3 from the unquenched sample to the 1500C quenched sample. The statistical uncertainty in the unit cell volume is about 0.04Å3. It shows that, the unit cell volume can be a good metric for the determination of vacancy concentration. However, the unit cell volumes are relative

Neutron Diffraction Studies of the Magnetic Oxide Materials 111

unquenched

3.85

Quenching Temperature

temperatures.

3.86

3.87

3.88

a\*(Å)

3.89

3.90

3.91

3.92

700C 800C 900C

Fig. 7. Lattice parameters of La0.6Sr0.4FeO3 vs. quenching condition.

1000C 1100C

a (Å) c (Å) Vol. (Å3) Total O Mag.(RT)

No Quench 5.52818(17) 13.44049(56) 355.72(2) 2.992(12) 1.31(3) 0.008(12)

700C 5.52942(15) 13.44457(52) 355.99(2) 2.980(12) 1.42(2) 0.020(12)

800C 5.52946(15) 13.44860(51) 356.10(2) 2.964(12) 1.67(2) 0.036(12)

900C 5.53072(16) 13.46527(58) 356.71(2) 2.962(12) 2.07(2) 0.038(12)

1000C 5.53352(17) 13.48996(65) 357.72(2) 2.936(16) 2.44(2) 0.064(16)

1100C 5.53456(16) 13.50656(62) 358.30(2) 2.920(14) 2.72(2) 0.080(14)

1200C 5.53091(41) 13.55839(190) 359.20(6) 2.872(18) 2.98(2) 0.128(18)

1200C/24h 5.53026(40) 13.55285(191) 358.97(6) 2.856(14) 3.07(2) 0.144(14)

1300C 5.53098(25) 13.55746(110) 359.18(4) 2.830(16) 3.26(2) 0.170(16)

1400C 5.53510(29) 13.55662(126) 359.69(4) 2.818(16) 3.39(2) 0.182(16)

1500C 5.53455(25) 13.55276(108) 359.52(4) 2.804(14) 3.44(2) 0.196(14)

Table 2. The crystal structure parameters of La0.6Sr0.4FeO3 at various quenching

1200C 1200C/24h 1300C

a\*

c\*

1400C

(B)

1500C

3.85

3.86

3.87

3.88

C\*

(Å)

3.89

3.90

3.91

3.92

due to the uncertainty in wavelength and sample position. Thus, the volumes of a series of samples can easily be compared, but the unit cell volume of a single specimen cannot be used to estimate its oxygen vacancy concentration. The small downturn in volume at the 1500C quenched samples appears to be an artifact, since the data on the samples treated in different reducing atmospheres with vacancy concentration up to 9.6% show a continued increase in volume.

The unit cell expansion associated with the formation of oxygen vacancies can be explained by: a): The repulsive force arising between those mutually exposed cations when oxygen ions are absent in the lattice. b): The increase in cation size due to the reduction of Fe ions from a high valence state to a lower valence state, which must occur concurrently with the formation of oxygen vacancies in order to maintain electrical neutrality.

Fig. 6. The neutron diffraction patterns of unquenched, 900C, 1100C, 1200C, and 1500C air quenched specimens. The black dots are the observed intensity, the solid red line is the calculated intensity, and their difference (blue) is under them. The upper tic marks show Bragg positions for the nuclear phase, and the lower ones are for the magnetic phase.

due to the uncertainty in wavelength and sample position. Thus, the volumes of a series of samples can easily be compared, but the unit cell volume of a single specimen cannot be used to estimate its oxygen vacancy concentration. The small downturn in volume at the 1500C quenched samples appears to be an artifact, since the data on the samples treated in different reducing atmospheres with vacancy concentration up to 9.6% show a continued

The unit cell expansion associated with the formation of oxygen vacancies can be explained by: a): The repulsive force arising between those mutually exposed cations when oxygen ions are absent in the lattice. b): The increase in cation size due to the reduction of Fe ions from a high valence state to a lower valence state, which must occur concurrently with the

16 Iobs

 Ical Diff Bragg\_peak

20 40 60 80 100

2 (degree)

Fig. 6. The neutron diffraction patterns of unquenched, 900C, 1100C, 1200C, and 1500C air quenched specimens. The black dots are the observed intensity, the solid red line is the calculated intensity, and their difference (blue) is under them. The upper tic marks show Bragg positions for the nuclear phase, and the lower ones are for the magnetic phase.

formation of oxygen vacancies in order to maintain electrical neutrality.

1500o C

0 4 8

3)

12 unquenched

900 o C

1100o C

1200o C

increase in volume.

Fig. 7. Lattice parameters of La0.6Sr0.4FeO3 vs. quenching condition.


Table 2. The crystal structure parameters of La0.6Sr0.4FeO3 at various quenching temperatures.

Neutron Diffraction Studies of the Magnetic Oxide Materials 113

RT

0.00 0.05 0.10 0.15 0.20

Moment

Volume

354

356

o

)

Unit Cell Volume (A3

358

360

oxygen vacancy

Fig. 9. Magnetic moment and unit cell volume at room temperature vs. oxygen vacancy

The magnetic moment on the Fe sites at room temperature was found to increase from1.31<sup>B</sup> for the untreated sample to 3.44B for the sample quenched from 1500C. The statistical uncertainty in the magnetic moment is less than 2.3% of the total moment, significantly less than the uncertainty in the vacancy concentration directly determined by the crystallographic refinement. The small increase in moment between the untreated and 700C quenched samples probably reflects the production of a small oxygen vacancy

0.00 0.05 0.10 0.15 0.20

Fig. 10. Oxygen vacancy concentration vs. saturation moment for La0.6Sr0.4FeO3

linear fit

1.0

concentration for La0.6Sr0.4FeO3.

1.5

2.0

2.5

Magnetic Moment (B)

3.0

3.5 La0.6Sr0.4FeO3-

concentration, outside the limit of the direct determination.

3.8 La0.6Sr0.4FeO3-

2.2

2.4

2.6

2.8

3.0

Saturation Moment (

)

3.2

3.4

3.6

Fig. 8. The changes of oxygen vacancy, magnetic moment, and the unit cell volume with the quench temperature for the La0.6Sr0.4FeO3 samples.

Figure 9 shows the magnetic moment and the unit cell volume of L6SF samples as a function of oxygen vacancy concentration. It appears that, in this range, the magnetic moment shows smoother behavior than does the unit cell volume, and may be the more reliable indirect measurement of the oxygen vacancy concentration.

magnetic moment

Oxygen vacancy

1200C/24h

Fig. 8. The changes of oxygen vacancy, magnetic moment, and the unit cell volume with the

Figure 9 shows the magnetic moment and the unit cell volume of L6SF samples as a function of oxygen vacancy concentration. It appears that, in this range, the magnetic moment shows smoother behavior than does the unit cell volume, and may be the more reliable indirect

1300C 1400C 1500C

Unquenched

quench temperature for the La0.6Sr0.4FeO3 samples.

measurement of the oxygen vacancy concentration.

 700C 800C 900C 1000C 1100C 1200C

c)

b)

a)

Vol

355

356

357

Vol (Å3

)

(

)

358

359

360

1.8

2.4

3.0

3.6

0.00

0.05

0.10

0.15

0.20

0.25

1.2

Fig. 9. Magnetic moment and unit cell volume at room temperature vs. oxygen vacancy concentration for La0.6Sr0.4FeO3.

The magnetic moment on the Fe sites at room temperature was found to increase from1.31<sup>B</sup> for the untreated sample to 3.44B for the sample quenched from 1500C. The statistical uncertainty in the magnetic moment is less than 2.3% of the total moment, significantly less than the uncertainty in the vacancy concentration directly determined by the crystallographic refinement. The small increase in moment between the untreated and 700C quenched samples probably reflects the production of a small oxygen vacancy concentration, outside the limit of the direct determination.

Fig. 10. Oxygen vacancy concentration vs. saturation moment for La0.6Sr0.4FeO3

Neutron Diffraction Studies of the Magnetic Oxide Materials 115

magnetic moment is not affected by the sample position and the model used for the refinement. Third, it is necessary to collect full data sets in order to accurately refine the vacancy concentration, while it is possible to extract the magnetic moment with the low angle range data only, and such data may be collected in as little as 1 hour. Of course, at very high vacancy concentration (>0.2), Fe2+ is expected to appear, with a smaller moment than Fe3+, the average moment at Fe sites will become small as increases, and the linear relationship between the saturation moment and the oxygen vacancy will break down. At those concentrations, however, the direct determination of oxygen vacancy concentration by

On the other hand, collection of a full data set allows us to extract all the three determinations: volume, oxygen occupancy, and magnetic moment. These three determinations are essentially independent. The unit cell volume is based only on peak positions and not peak intensities, and uses the full data set. The oxygen occupancies affect the intensities at all angles and especially uses the high angle data, while the magnetic moment is based primarily on the intensities of magnetic peaks at low angle. Thus use of neutron diffraction provides

La0.6Sr0.4FeO3 shows a very small oxygen vacancy contents prepared by heating in the N2 and O2 gases. When treated in a CO/CO2 mixture, the amount of oxygen vacancies exceeds 7%. Mössbauer spectra of N2 and O2-quenched samples show a transition from high temperature paramagnetic Fe3+ and Fe4+ valence states to a low temperature antiferromagnetic mixed-valence state resulting from nominal charge disproportionation reaction 2Fe4+=Fe3++Fe5+. The Fe valence states change from Fe4+ to an intermediate valence state, Fe(3-x)+, in the CO/CO2-reduced samples. There is no charge disproportionation throughout the entire temperature range. The change of the valence state in the CO/CO2 heat treatment increases magnetic moments and the hyperfine fields of the Fe atoms in these compounds. It is found that the antiferromagnetic structure exhibits a small ferromagnetic component, which causes from canted magnetic sublattices La0.6Sr0.4FeO3 spacimens quenched from 700-1500C exhibit antiferromagnetic ordering at room temperature, the unit cell volume and the oxygen vacancy concentrations increase with increasing quenching temperature. The magnetic ordering is dominated by the Fe3+-O2--Fe3+ interactions, and the Fe3+ concentration increases with increasing oxygen vacancy . Thus, the magnetic moment and the Neel temperature increase with increasing oxygen vacancy. The saturation moment, determined by neutron diffraction at 10K, is linear with the oxygen vacancy (). At high temperature with air flowing, L6SF will absorb oxygen at 303C-655C and then lose oxygen when temperature is above 655C. The presence of oxygen vacancies increases the thermal

Neutron diffraction measurement coupled with refinement by the Rietveld method appears to be a reliable, redundant method for determining the oxygen vacancy in L6SF. The unit cell volume, the oxygen occupation, and the magnetic moment can be used to determine the oxygen vacancy. The unit cell volume can be affected by the sample position, but the magnetic moment at room temperature can provide the data more accurately than the other two parameters. The saturation moment (10K) is an even more powerful tool which can be

crystallographic refinement will become more precise.

**3. Conclusion** 

expansion coefficient.

used to determine the vacancy concentration.

redundancy and cross checks on the oxygen vacancy determination.

Fig. 10 shows the saturation moments, determined from the neutron powder diffraction measurement at 10K, as a function of oxygen deficiency. The saturation moment is linear with vacancy concentration, the highest deficiency, =0.2, corresponds to a nearly pure Fe3+ state, and its magnetic moment is about 3.8B, which is equal to the magnetic moment of Fe3+ in LaFeO3. The moment when =0 is about 2.33B, and this value is about 60% of the magnetic moment of Fe3+. This shows the magnetic moments at Fe sites are contributed by Fe3+, and that the moment of Fe4+ is almost zero. For the ferrites with high La (50% or more) in an oxidizing environment, it is reasonable to assume that the Fe atoms are in the 3+ and 4+ charge states. The great difference between the saturation moments of the two Fe ions provide a direct determination of the ratio of the two ions and, thus, of the oxygen stoichiometry.

The magnetization as a function of temperature follows the Brillouin curve: saturated at low temperature and decreasing slowly up to about 70% of TN and more rapidly as TN is approached. Thus, according the ratio of the magnetic moment at room temperature to the moment at low temperature (Table 3), TN is a little above room temperature when the quenching temperature is below 900C.


Table 3. The magnetic moment of La0.6Sr0.4FeO3 at different quenching temperature. The mol% of Fe3+ is calculated from the oxygen vacancy concentration.

La0.6Sr0.4FeO3 exhibits antiferromagnetic ordering below the Neel temperature. Comparing the magnetic moments at room temperature and 10K, the Neel temperature (TN) increases with increasing oxygen vacancies alone with the concentration of Fe3+. The magnetic interactions between Fe ions, leading to magnetic ordering in this type of oxide, are predominantly superexchange; exchange that is mediated by polarization of oxygen ions lying between the Fe near neighbors. Since the Fe4+ ions have small or zero moments, the exchange interactions in L6SF are expected to be dominated by the Fe3+-O2 - Fe3+ interactions. This is the reason why TN increases with increasing Fe3+ concentration, despite the loss of some bonding oxygen atoms.

There are a lot of advantages in using the magnetic moment as a measure of vacancy concentration in La0.6Sr0.4FeO3. First, the saturation moment gives an absolute determination without establishing the room temperature curves, while the room temperature moment may be quickly and reliably determined if the correlations have been established. The uncertainty in magnetic moment is 2% at low vacancy concentration, decreasing to less than 1% when the moment is large, thus, the vacancy concentration should be known, by this indirect determination, to a precision of 1% or 2% over the range of interest. Second, the magnetic moment is not affected by the sample position and the model used for the refinement. Third, it is necessary to collect full data sets in order to accurately refine the vacancy concentration, while it is possible to extract the magnetic moment with the low angle range data only, and such data may be collected in as little as 1 hour. Of course, at very high vacancy concentration (>0.2), Fe2+ is expected to appear, with a smaller moment than Fe3+, the average moment at Fe sites will become small as increases, and the linear relationship between the saturation moment and the oxygen vacancy will break down. At those concentrations, however, the direct determination of oxygen vacancy concentration by crystallographic refinement will become more precise.

On the other hand, collection of a full data set allows us to extract all the three determinations: volume, oxygen occupancy, and magnetic moment. These three determinations are essentially independent. The unit cell volume is based only on peak positions and not peak intensities, and uses the full data set. The oxygen occupancies affect the intensities at all angles and especially uses the high angle data, while the magnetic moment is based primarily on the intensities of magnetic peaks at low angle. Thus use of neutron diffraction provides redundancy and cross checks on the oxygen vacancy determination.

### **3. Conclusion**

114 Neutron Diffraction

Fig. 10 shows the saturation moments, determined from the neutron powder diffraction measurement at 10K, as a function of oxygen deficiency. The saturation moment is linear with vacancy concentration, the highest deficiency, =0.2, corresponds to a nearly pure Fe3+ state, and its magnetic moment is about 3.8B, which is equal to the magnetic moment of Fe3+ in LaFeO3. The moment when =0 is about 2.33B, and this value is about 60% of the magnetic moment of Fe3+. This shows the magnetic moments at Fe sites are contributed by Fe3+, and that the moment of Fe4+ is almost zero. For the ferrites with high La (50% or more) in an oxidizing environment, it is reasonable to assume that the Fe atoms are in the 3+ and 4+ charge states. The great difference between the saturation moments of the two Fe ions provide a direct determination of the ratio of the two ions and, thus, of the oxygen

The magnetization as a function of temperature follows the Brillouin curve: saturated at low temperature and decreasing slowly up to about 70% of TN and more rapidly as TN is approached. Thus, according the ratio of the magnetic moment at room temperature to the moment at low temperature (Table 3), TN is a little above room temperature when the

Quenching 700 800 900 1000 1100 1200 1300 1400 1500

(B) 1.31(3) 1.42(2) 1.67(2) 2.07(2) 2.44(2) 2.72(2) 2.98(2) 3.26(2) 3.39(2) 3.44(2)

(B) 2.40(3) 2.46(2) 2.50(2) 2.64(3) 2.82(3) 3.06(3) 3.26(3) 3.55(3) 3.68(2) 3.73(3)

 0.546 0.577 0.668 0.784 0.865 0.889 0.914 0.918 0.921 0.922 Fe3+% 61.4 64 67.2 67.4 72.8 76 85.6 94 96.4 98.4 Table 3. The magnetic moment of La0.6Sr0.4FeO3 at different quenching temperature. The

La0.6Sr0.4FeO3 exhibits antiferromagnetic ordering below the Neel temperature. Comparing the magnetic moments at room temperature and 10K, the Neel temperature (TN) increases with increasing oxygen vacancies alone with the concentration of Fe3+. The magnetic interactions between Fe ions, leading to magnetic ordering in this type of oxide, are predominantly superexchange; exchange that is mediated by polarization of oxygen ions lying between the Fe near neighbors. Since the Fe4+ ions have small or zero moments, the exchange interactions in L6SF are expected to be dominated by the Fe3+-O2

interactions. This is the reason why TN increases with increasing Fe3+ concentration, despite

There are a lot of advantages in using the magnetic moment as a measure of vacancy concentration in La0.6Sr0.4FeO3. First, the saturation moment gives an absolute determination without establishing the room temperature curves, while the room temperature moment may be quickly and reliably determined if the correlations have been established. The uncertainty in magnetic moment is 2% at low vacancy concentration, decreasing to less than 1% when the moment is large, thus, the vacancy concentration should be known, by this indirect determination, to a precision of 1% or 2% over the range of interest. Second, the


mol% of Fe3+ is calculated from the oxygen vacancy concentration.

stoichiometry.

T(C) No

RT

10K

10 *RT K* 

quenching temperature is below 900C.

the loss of some bonding oxygen atoms.

La0.6Sr0.4FeO3 shows a very small oxygen vacancy contents prepared by heating in the N2 and O2 gases. When treated in a CO/CO2 mixture, the amount of oxygen vacancies exceeds 7%. Mössbauer spectra of N2 and O2-quenched samples show a transition from high temperature paramagnetic Fe3+ and Fe4+ valence states to a low temperature antiferromagnetic mixed-valence state resulting from nominal charge disproportionation reaction 2Fe4+=Fe3++Fe5+. The Fe valence states change from Fe4+ to an intermediate valence state, Fe(3-x)+, in the CO/CO2-reduced samples. There is no charge disproportionation throughout the entire temperature range. The change of the valence state in the CO/CO2 heat treatment increases magnetic moments and the hyperfine fields of the Fe atoms in these compounds. It is found that the antiferromagnetic structure exhibits a small ferromagnetic component, which causes from canted magnetic sublattices La0.6Sr0.4FeO3 spacimens quenched from 700-1500C exhibit antiferromagnetic ordering at room temperature, the unit cell volume and the oxygen vacancy concentrations increase with increasing quenching temperature. The magnetic ordering is dominated by the Fe3+-O2--Fe3+ interactions, and the Fe3+ concentration increases with increasing oxygen vacancy . Thus, the magnetic moment and the Neel temperature increase with increasing oxygen vacancy. The saturation moment, determined by neutron diffraction at 10K, is linear with the oxygen vacancy (). At high temperature with air flowing, L6SF will absorb oxygen at 303C-655C and then lose oxygen when temperature is above 655C. The presence of oxygen vacancies increases the thermal expansion coefficient.

Neutron diffraction measurement coupled with refinement by the Rietveld method appears to be a reliable, redundant method for determining the oxygen vacancy in L6SF. The unit cell volume, the oxygen occupation, and the magnetic moment can be used to determine the oxygen vacancy. The unit cell volume can be affected by the sample position, but the magnetic moment at room temperature can provide the data more accurately than the other two parameters. The saturation moment (10K) is an even more powerful tool which can be used to determine the vacancy concentration.

**6** 

*1,3Taiwan 2South Korea* 

**Introduction of Neutron Diffractometers** 

*1Department of Chemical & Materials Engineering and Center for Neutron Beam* 

*3Department of Engineering and System Science, National Tsing Hua University* 

The design of advanced metallic materials for their structural applications requires the understanding of the strengthening mechanisms and property evolution subjected to different types of deformation modes1. These metallic systems can interact with their microstructure upon the changes of the environmental conditions, such as strain rate and temperature2. While the microstructure has been facilitated for many purposes, this chapter puts forward how to characterize the structural properties with neutron diffractometers. Moreover, nowadays, many neutron diffractometers are equipped with load frames, which advance the diffraction measurements to real-time observations. The chapter considers that deformation mechanisms and their effects on the microstructure are central to the mechanical behavior of structural materials. The main objective of the chapter is to introduce readers how to facilitate the neutron diffractometers to study the mechanical behavior of the structural materials. The reported diffractometers are summarized from the literatures, public information, and on-site visits. Some useful software for diffraction-profile and scattering-intensity analyses is briefly mentioned. The microscopic features are connected with the macroscopic states, such as the applied stresses and temperature evolution to bridge the understanding of the bulk property. What reciprocal information obtained from the diffraction profiles can be inferred to the

**1. Introduction** 

materials structural parameters will be explained.

**2. Neutron diffraction and diffraction-profile-refinement software** 

Neutrons are elementary particles with a finite mass (*m*, about 1.67 x 10-27 *kg*) and spin, without an electron charge. It carries a magnetic moment, and according to *de Broglie* law, the neutrons behave like waves with a wavelength (λ) at the levels of Å and gives rise to diffraction3. Neutron diffraction is based on the nuclear interaction between neutrons and the matters and on magnetic interaction with magnetic moments of the atoms due to its magnetic moments. Specifically, in this chapter, we focus only on the application of the

**for Mechanical Behavior Studies of** 

*2Neutron Science Division, Korea Atomic Energy Research Institute* 

E-Wen Huang1, Wanchuck Woo2 and Ji-Jung Kai3

**Structural Materials** 

*Applications, National Central University* 

### **4. Acknowledgments**

This work is supported by the National Natural Science Foundation of China (Grant No. 509701003 and 51171001), the National 973 Project (No. 2010CB833104, MOST of China), and the program for New Century Excellent Talents (NCET-10-1097) and the Scientific Research Foundation for Returned Overseas Chinese Scholars, State Education Ministry.

#### **5. References**

