**3.2 Ln0.5A0.5FeO3- (Ln= La, Nd, Sm; A= Ba, Sr) 1.25≤ <rA> ≤1.34 Å perovskites**

As in the case of the previous family of iron perovskites, this new series of compounds are of interest for their use as mixed ionic electronic conducting materials, mainly from the point of view of cathodes for Solid Oxide Fuel Cells (SOFC), although they could also be used as ceramic membranes for oxygen separation. In the present case, the degree of lanthanide substitution was fixed to x=0.5 given that previous studies have shown that it is precisely at this degree of substitution when electronic and ionic conductivity are maximised (Hansen, 2010; Vidal et al., 2007).

In the present series of compounds the effect of the <rA> variation on the properties of four different phases, Ln0.5A0.5FeO3–δ (Ln=La, Nd, Sm; A=Ba, Sr) was evaluated. For this series (Table 1), <rA> has been varied between 1.34 and 1.25 Å keeping x and σ2(rA) constant, with values of 0.5 and 0.0161 Å2, respectively.

Prior to choosing a synthesis method for all samples, two different methods were tried for one of the samples, La0.05Sm0.45Sr0.18Ba0.32FeO3**-**, for which the structural parameters and morphology were evaluated. The ceramic and glycine-nitrate routes were used in the present case.

## **3.2.1 Influence of the synthetic method on the structure and morphology of the La0.05Sm0.45Sr0.18Ba0.32FeO3-**

Laboratory X-ray diffraction data at room temperature for La0.05Sm0.45Ba0.5FeO3- obtained by ceramic and glycine-nitrate routes were extremely similar, both samples being pure to the detection limits of the technique. XRD patterns were fitted by the Rietveld method (Figure 5) considering a rhombohedral symmetry (space group *R*-3*c*) in both cases.

Synthetic Methods for Perovskite Materials – Structure and Morphology 503

Room temperature X-ray diffraction patterns of the LB134, LNSB131, LNSB128, LSSB125-gn samples show that all the samples are single phase compounds (Figure 7a). A shift of the diffraction maxima to lower diffraction angles (2 with decreasing <rA> anticipates an increase of the lattice parameters. X-ray powder diffraction patterns were indexed using a cubic symmetry (*Pm-3m* space group) for the LB134 and LNSB131 samples and a rhombohedral symmetry (*R-3c* space group) in the case of LNSB128 and LSSB125-gn

compounds. Figure 7b shows the Rietveld fits to the XRD patterns for two samples.

Fig. 7. (a) X-ray powder diffraction patterns at room temperature for all samples. (b) and (c) show the Rietveld fits to the X-ray powder diffraction patterns at room temperature for

The <rA> dependence of the lattice parameters, unit cell volume, main bond distances and Fe-O-Fe bond angles are shown in Figure 8. As it can be observed, lattice parameters and unit cell volume decrease with decreasing the average A-site ionic radius. Given that the doping level has been fixed (x=0.5), changes in the ratio Fe3+/Fe4+ and, therefore, in the Fesite ionic radius (<rFe>), are not expected. Consequently, the decrease in lattice parameters is ascribed to the variation of <rA>. A more detailed analysis is given elsewhere (Ecija et. al, 2011). This also explains the decrease of the <A-O> and <A-Fe> mean distances with decreasing <rA> (Figure 8b). Although <rFe> has been kept constant, there is a slight reduction of the <Fe-O> distances as <rA> decreases, which is a consequence of the tilting in

**3.2.2 Structural study** 

LB134-ss and LSS125-ss samples.

Fig. 5. Rietveld fits to room XRD patterns for LSSB-ss and LSSB-gn samples. In each case, lattice parameters (a, b, c), unit cell volume (V) and theoretical density (ρ) are included.

From the fits to the XRD data it was observed that both phases are nearly identical: they crystallise in the same space group and does not show significant difference among lattice parameters, cell volume or density values.

The morphological study, however, shows a different picture. As shown in Figure 6, where SEM micrographs taken at the same amplification for both samples are presented, their microstructure is quite different. The sample prepared following the ceramic route presents a microstructure with heterogeneous grain sizes and shapes, in which particles range from ~ 2 to approximately 8 m in diameter. On the other hand, the average grain size of the sample obtained by glycine-nitrate route is about 200 nm, almost an order of magnitude smaller. The higher calcination temperature and longer reaction time required to obtain the samples in the ceramic process can explain the bigger grain size showed for these samples (Melo Jorge et al., 2001).

Fig. 6. SEM micrographs taken on the surface of the La0.05Sm0.45Sr0.18Ba0.32FeO3 phases obtained by the glycine-nitrate (gn) and solid state reaction (ss) methods.

For the application as SOFC cathodes, samples with small and homogeneous particle sizes are usually preferred. As a consequence, the glycine-nitrate process was considered a more appropriate technique for preparing the perovskite samples of this series: Ln0.5A0.5FeO3- (Ln= La, Nd, Sm; A= Ba, Sr) with 1.25≤ <rA> ≤1.34 Å.

## **3.2.2 Structural study**

502 Advances in Crystallization Processes

Fig. 5. Rietveld fits to room XRD patterns for LSSB-ss and LSSB-gn samples. In each case, lattice parameters (a, b, c), unit cell volume (V) and theoretical density (ρ) are included.

parameters, cell volume or density values.

(Melo Jorge et al., 2001).

From the fits to the XRD data it was observed that both phases are nearly identical: they crystallise in the same space group and does not show significant difference among lattice

The morphological study, however, shows a different picture. As shown in Figure 6, where SEM micrographs taken at the same amplification for both samples are presented, their microstructure is quite different. The sample prepared following the ceramic route presents a microstructure with heterogeneous grain sizes and shapes, in which particles range from ~ 2 to approximately 8 m in diameter. On the other hand, the average grain size of the sample obtained by glycine-nitrate route is about 200 nm, almost an order of magnitude smaller. The higher calcination temperature and longer reaction time required to obtain the samples in the ceramic process can explain the bigger grain size showed for these samples

Fig. 6. SEM micrographs taken on the surface of the La0.05Sm0.45Sr0.18Ba0.32FeO3 phases

For the application as SOFC cathodes, samples with small and homogeneous particle sizes are usually preferred. As a consequence, the glycine-nitrate process was considered a more appropriate technique for preparing the perovskite samples of this series: Ln0.5A0.5FeO3-

obtained by the glycine-nitrate (gn) and solid state reaction (ss) methods.

(Ln= La, Nd, Sm; A= Ba, Sr) with 1.25≤ <rA> ≤1.34 Å.

Room temperature X-ray diffraction patterns of the LB134, LNSB131, LNSB128, LSSB125-gn samples show that all the samples are single phase compounds (Figure 7a). A shift of the diffraction maxima to lower diffraction angles (2 with decreasing <rA> anticipates an increase of the lattice parameters. X-ray powder diffraction patterns were indexed using a cubic symmetry (*Pm-3m* space group) for the LB134 and LNSB131 samples and a rhombohedral symmetry (*R-3c* space group) in the case of LNSB128 and LSSB125-gn compounds. Figure 7b shows the Rietveld fits to the XRD patterns for two samples.

Fig. 7. (a) X-ray powder diffraction patterns at room temperature for all samples. (b) and (c) show the Rietveld fits to the X-ray powder diffraction patterns at room temperature for LB134-ss and LSS125-ss samples.

The <rA> dependence of the lattice parameters, unit cell volume, main bond distances and Fe-O-Fe bond angles are shown in Figure 8. As it can be observed, lattice parameters and unit cell volume decrease with decreasing the average A-site ionic radius. Given that the doping level has been fixed (x=0.5), changes in the ratio Fe3+/Fe4+ and, therefore, in the Fesite ionic radius (<rFe>), are not expected. Consequently, the decrease in lattice parameters is ascribed to the variation of <rA>. A more detailed analysis is given elsewhere (Ecija et. al, 2011). This also explains the decrease of the <A-O> and <A-Fe> mean distances with decreasing <rA> (Figure 8b). Although <rFe> has been kept constant, there is a slight reduction of the <Fe-O> distances as <rA> decreases, which is a consequence of the tilting in

Synthetic Methods for Perovskite Materials – Structure and Morphology 505

also helped to understand the new phenomenon (Tai, 2000; Takeuchi, 2002; Ulyanov, 2007). Phase morphology, highly dependent on preparative conditions, was also observed to play an important role in the effect: low temperature CMR was improved as the grain size was reduced (Das, 2002). In order to help in this area, it was decided to study the effects of the synthesis method in a series of perovskite compounds with the general formula Nd0.8Sr0.2(Mn1-xCox)O3 (0.1≤x≤0.3). The sol-gel and freeze-drying techniques were used in order to compare their structural, morphological and magnetic properties. Details of the

Fig. 9. SEM micrographs of the surface of the LB134, LNSB131, LNSB128, LSSB125-gn bulk

The X-ray powder diffraction patterns (XRPD) of all the compounds studied in this section were indexed in the orthorhombic space group *Pmna* irrespective of the synthesis method used. Fig. 10 shows the XRD and Rietveld refinement profiles for all phases with the

When the lattice parameters and cell volume are compared a slight decrease with increasing cobalt content is observed in both cases. This effect is related to the changes in sizes of the B site atoms upon doping (Meera et al., 2001; Pollert et al., 2003). The oxidation states of Mn and Co in the AMn1-xCoxO3 systems has been for long debated (Goodenough et al., 1961; 1997; Toulemonde et al., 1998; Troyanchuk et al., 2000), and different mixtures of Mn4+-Mn3+ and Co3+-Co2+ have been proposed. The most likely combination, based in spectroscopic and magnetic results, seems to indicate that the cobalt is introduced as Co2+ (and not as Co3+) thus causing a mixed state (4+ and 3+) in manganese. This mixture of oxidation states has also been proved useful for Pollert et al. to explain magnetic and electrical properties of the series Nd0.8Na0.2Mn1-xCoxO3 (x≤0.1), Pr0.8Na0.2Mn1-xCoxO3 (x≤0.2) (Pollert et al., 2003a, 2003b)

samples.

**3.3.1 Structural study** 

formula Nd0.8Sr0.2(Mn1-xCox)O3.

later (magnetic properties) can be found elsewhere (Vidal et al., 2005).

the FeO6 octahedra due to the rhombohedral distortion. The decrease of the <Fe-O-Fe> bond angles is the result of the same effect (Woodward, 1998).

Fig. 8. <rA> dependence of (a) the unit cell parameters and volume per formula unit; and (b) the main average bond lengths (<A–Fe>, <A–O>, <Fe–O>) and Fe–O–Fe bond angle. Shaded area indicates the x range where the structural transition takes place.

#### **3.2.3 Morphological study**

Figure 9 shows the SEM micrographs of the LB134, LNSB131, LNSB128, LNSB125-gn bulk samples after the last heating at 1050ºC.

In this series of samples no significant differences can be found in the morphology and average particle size. All samples present some agglomeration and fine grain size. Image analysis of the micrographs has allowed us to determine that the average grain size of the samples is in the range of 150-250 nm.

#### **3.3 Synthesis and characterization of Nd0.8Sr0.2(Mn1-xCox)O3 perovskites with x = 0.1, 0.2, 0.3**

The hole-doped perovskite manganese oxides with general formula Ln1-xAxMnO3 (Ln= La, Pr, Nd; A= Ca, Sr, Ba, Pb; x<0.5) draw considerable attention in the late 1990´s due to their colossal magneto-resistance (CMR) effect at low temperatures (Rao, 1998). In the search for new CMR materials it was observed that doping on Mn site by other transition metal elements, such as Cr, Fe, Co and Ni, was an effective way to obtain new materials, which also helped to understand the new phenomenon (Tai, 2000; Takeuchi, 2002; Ulyanov, 2007). Phase morphology, highly dependent on preparative conditions, was also observed to play an important role in the effect: low temperature CMR was improved as the grain size was reduced (Das, 2002). In order to help in this area, it was decided to study the effects of the synthesis method in a series of perovskite compounds with the general formula Nd0.8Sr0.2(Mn1-xCox)O3 (0.1≤x≤0.3). The sol-gel and freeze-drying techniques were used in order to compare their structural, morphological and magnetic properties. Details of the later (magnetic properties) can be found elsewhere (Vidal et al., 2005).

Fig. 9. SEM micrographs of the surface of the LB134, LNSB131, LNSB128, LSSB125-gn bulk samples.
