**3.3.1 Structural study**

504 Advances in Crystallization Processes

the FeO6 octahedra due to the rhombohedral distortion. The decrease of the <Fe-O-Fe> bond

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.

Figure 9 shows the SEM micrographs of the LB134, LNSB131, LNSB128, LNSB125-gn bulk

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

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

**3.3 Synthesis and characterization of Nd0.8Sr0.2(Mn1-xCox)O3 perovskites with** 

Shaded area indicates the x range where the structural transition takes place.

**3.2.3 Morphological study** 

**x = 0.1, 0.2, 0.3** 

samples after the last heating at 1050ºC.

samples is in the range of 150-250 nm.

angles is the result of the same effect (Woodward, 1998).

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

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)

Synthetic Methods for Perovskite Materials – Structure and Morphology 507

Table 2 shows details of the averge interatomic <B-O>, <A-O> and <A-B> distances together with <B-O-B> bond angles. As it can be observed <B-O> and <A-B> distances decrease as the cobalt concentration increases. These tendencies can be explained following the same reasoning indicated before, which basically concludes that cobalt enters in the structure in an oxidation state with smaller radius than manganese. No significant changes are observed in the <A-O> distances or in the <B-O-B> angles. This indicates that, despite the observed changes in the B size, the cobalt doping does not cause a noticeable distortion in the

**Variables NSMC10 NSMC20 NSMC30** 

Table 2. Mean atomic distances and bond angles for the series of ABO3 (A = Nd0.8 Sr0.2; B = Mn1-xCox; x = 0.1, 0.2 and 0.3) samples prepared by the two methods described in the

In the same way, no significant changes are observed between the compounds synthesised

The SEM micrographs of the present series of compounds are shown in Fig. 11. All pictures were taken on sintered bars after they were prepared by the sol-gel and freeze-drying

As observed, the Nd0.8Sr0.2(Mn1-xCox)O3 samples prepared by the sol-gel route show a homogeneous particle size morphology distributed in small agglomerates. The grain size of the nearly spherical particles decreases with Co content from ~ 200 nm for the sample with x = 0.1 to ~ 100 nm for x = 0.3. In the case of the compounds obtained by the freeze-drying technique the morphology of the particles is also spherical and homogeneous but the grain size is quite stable in all phases and slightly higher (~250 nm) than in the previous case.

According to some studies (Kuharuangrong et al., 2004), Co doping of 20% does not usually influence the grain size of LSM, but 40 % mol Co significantly reduces the grain size (about 5-10 m) of the La0.84Sr0.16Mn1-xCoxO3 (x= 0, 0.2, 0.4) samples prepared by conventional oxide mixing process. However, some reduction in the particle size was observed in the case of La0.67Pb0.33Mn1-xCoxO3 (x= 0 and 0.3) compounds, which were also prepared by the ceramic

<A–B> 3.346 3.350 3.345 3.345 3.341 3.339(1) <A–O1> 2.749(1) 2.756(1) 2.748(1) 2.751(2) 2.748(1) 2.747(2) <A–O2> 2.588(1) 2.592(1) 2.588(2) 2.593(2) 2.587(1) 2.591(2) <A–O> 2.668 2.674 2.668 2.672 2.667 2.671 <B–O1> 1.954(4) 1.959(4) 1.950(4) 1.958(6) 1.958(4) 1.957(4) <B–O2> 1.968(1) 1.970(2) 1.967(2) 1.965(2) 1.963(2) 1.960(2) <B–O> 1.961 1.964 1.958 1.961 1.957 1.958

B-O1-B 160.8(4) 159.3(2) 161.5(4) 158.6(2) 158.5(3) 158.3(2) B-O2-B 157.9(4) 158.3(4) 158.2(4) 159.0(1) 158.5(3) 159.4(1) <B–O–B> 159.3 158.8 159.8 158.8 158.5 158.8

**sg fd sg fd sg fd** 

perovskite structure.

**distance (Å)** 

**bond** 

text.

methods.

**angle** 

**(º)** 

by the sol-gel or the freeze-drying techniques.

**3.3.2 Morphological study** 

and La0.8Na0.2Mn1-xCoxO3 (x≤0.2) (Pollert et al., 2004). On the other hand, as the Co content increases, the LSMC materials would shift from the oxygen excessive region to the oxygen deficient region, which would also contribute to the decrease in the unit volume. Wandekar et al. (Wandekar et al., 2009) carried out a detailed study on the crystal structure and conductivity of Co substituted LSM system and showed that the change in the ion radius of B-site element plays a predominant role at low Co content and the increase in oxygen vacancy becomes dominant at high Co content.

Fig. 10. Rietveld fits to the X-ray diffraction data in the orthorhombic *Pnma* space group for the Nd0.8 Sr0.2 Mn1-xCoxO3 compounds. The labels "sg" and "fd" indicates that samples were prepared using the sol gel and freeze-drying techniques, respectively. In each case, lattice parameters (a, b, c), unit cell volume (V) and theoretical density (ρ) data are inset.

In consequence, considering only the high spin states of these elements and assuming nearly oxygen stoichiometric phases at room temperature, the observed reduction of the lattice volume in the present samples is consistent with the gradual appearance of Mn4+ (of smaller size than Mn3+ and Co2+ (Shannon, 1976) and so with the reduction of the mean B-site ionic radii. This explanation is also satisfactory for the same effect in the compounds La0.7Na0.3Mn1 xCoxO3 where the authors had assumed the presence of only Mn3+–Co3+ (Meera et al., 2001).

and La0.8Na0.2Mn1-xCoxO3 (x≤0.2) (Pollert et al., 2004). On the other hand, as the Co content increases, the LSMC materials would shift from the oxygen excessive region to the oxygen deficient region, which would also contribute to the decrease in the unit volume. Wandekar et al. (Wandekar et al., 2009) carried out a detailed study on the crystal structure and conductivity of Co substituted LSM system and showed that the change in the ion radius of B-site element plays a predominant role at low Co content and the increase in oxygen

Fig. 10. Rietveld fits to the X-ray diffraction data in the orthorhombic *Pnma* space group for the Nd0.8 Sr0.2 Mn1-xCoxO3 compounds. The labels "sg" and "fd" indicates that samples were prepared using the sol gel and freeze-drying techniques, respectively. In each case, lattice

In consequence, considering only the high spin states of these elements and assuming nearly oxygen stoichiometric phases at room temperature, the observed reduction of the lattice volume in the present samples is consistent with the gradual appearance of Mn4+ (of smaller size than Mn3+ and Co2+ (Shannon, 1976) and so with the reduction of the mean B-site ionic radii. This explanation is also satisfactory for the same effect in the compounds La0.7Na0.3Mn1 xCoxO3 where the authors had assumed the presence of only Mn3+–Co3+ (Meera et al., 2001).

parameters (a, b, c), unit cell volume (V) and theoretical density (ρ) data are inset.

vacancy becomes dominant at high Co content.

Table 2 shows details of the averge interatomic <B-O>, <A-O> and <A-B> distances together with <B-O-B> bond angles. As it can be observed <B-O> and <A-B> distances decrease as the cobalt concentration increases. These tendencies can be explained following the same reasoning indicated before, which basically concludes that cobalt enters in the structure in an oxidation state with smaller radius than manganese. No significant changes are observed in the <A-O> distances or in the <B-O-B> angles. This indicates that, despite the observed changes in the B size, the cobalt doping does not cause a noticeable distortion in the perovskite structure.


Table 2. Mean atomic distances and bond angles for the series of ABO3 (A = Nd0.8 Sr0.2; B = Mn1-xCox; x = 0.1, 0.2 and 0.3) samples prepared by the two methods described in the text.

In the same way, no significant changes are observed between the compounds synthesised by the sol-gel or the freeze-drying techniques.
