**3.1.1 Structural study**

The room temperature X-ray powder diffraction patterns of these compounds are shown in Fig. 2a. A structural transition from orthorhombic symmetry (space group *Pnma*) for samples with x≤0.4 to rhombohedral symmetry (space group *R-3c)* for 0.5≤x≤ 0.7 compounds, and finally, to a mixture of rhombohedral *R-3c* and cubic *Pm-3m* perovskite phases for the x = 0.8 composition was observed. Representative Rietveld fits to the X-ray diffraction data for the samples LPS20, LNSC50 and LNSC80 are shown in Fig. 2b, 2c and 2d, respectively.

The dependence with the doping level x of the cell parameters and cell volume per formula unit and atomic distances and bond angles for all samples are represented in Figure 3a and b, respectively. There is a systematic decrease in volume, cell parameters and <Fe-O> distances with increasing doping level across the series. Given that in the system studied the A-site mean ionic radius <rA> has been kept constant as the doping level increases, the observed effect can be solely associated to a reduction of the Fe-site mean ionic radius as it

dispersive detector were used for the samples obtained by glycine-nitrate route and LSSB-ss. A Philips X'Pert-MPD X-ray diffractometer with secondary beam graphite monochromated Cu–Kα radiation was used for the samples obtained by ceramic solid-state, sol-gel and

All samples were single phase without detectable impurities. The crystal structure was refined by the Rietveld method (Rietveld, 1959) from X-ray powder diffraction data using

Microstructural analysis was carried out in a JEOL JSM 6400 scanning electron microscope (SEM) using a secondary electron detector at 30 kV and 1.10−10 A for the LPS20, LNSC30, LPSC40, LPSC50, LPSC60, LPSC70 samples and a JEOL JSM-7000F at 3 kV and 11.10-12 A for

**3.1 Characterization of Ln1−xAxFeO3−δ (Ln=La, Nd, Pr; A=Sr, Ca) perovskites with** 

The (La**1−x**Sr**x**)FeO3- (LSF) perovskite system exhibits high electronic and oxide ion conductivities at high temperatures, which make it attractive for solid oxide fuel cell (SOFC) cathodes. Several works (Ecija et al., 2011; Rodríguez-Martínez & Attfield, 1996; Vidal et al., 2007 and references therein) have shown that the physical properties of these perovskite materials are very sensitive to changes in the doping level (x) the average size of the A

The synthesis of these compounds allows us to study the effect of the variation of the doping level x on the properties of the perovskites with general formula Ln1−xAxFeO3−δ (Ln=La, Nd, Pr; A=Sr, Ca) applied as SOFC cathodes. This has been achieved by keeping constant both the

For the preparation of this series the solid state reaction route has been chosen due to its

The room temperature X-ray powder diffraction patterns of these compounds are shown in Fig. 2a. A structural transition from orthorhombic symmetry (space group *Pnma*) for samples with x≤0.4 to rhombohedral symmetry (space group *R-3c)* for 0.5≤x≤ 0.7 compounds, and finally, to a mixture of rhombohedral *R-3c* and cubic *Pm-3m* perovskite phases for the x = 0.8 composition was observed. Representative Rietveld fits to the X-ray diffraction data for the samples LPS20, LNSC50 and LNSC80 are shown in Fig. 2b, 2c and

The dependence with the doping level x of the cell parameters and cell volume per formula unit and atomic distances and bond angles for all samples are represented in Figure 3a and b, respectively. There is a systematic decrease in volume, cell parameters and <Fe-O> distances with increasing doping level across the series. Given that in the system studied the A-site mean ionic radius <rA> has been kept constant as the doping level increases, the observed effect can be solely associated to a reduction of the Fe-site mean ionic radius as it

average size (<rA>) and the size mismatch (2(rA)) to 1.22 Å and 0.003 Å2, respectively.

simplicity to obtain perovskite phases in the same synthetic conditions.

freeze-drying techniques.

the rest of samples.

**3.1.1 Structural study** 

2d, respectively.

**3. Results** 

**0.2≤x≤0.8** 

GSAS software package (Larson & Von Dreele, 1994).

cations (<rA>) and the effects of A cation size disorder (2(rA)).

oxidises from Fe3+ to Fe4+, with smaller radius (<rFe>, rFe3+=0.645 Å and rFe4+ =0.585 Å) (Shannon, 1976). Details of these effects are given elsewhere (Vidal et al., 2007).

The increase of the <A-O> distances and <Fe-O-Fe> bond angles with increasing doping level (x) can be explained due to the structural transition produced with x: when passing from orthorhombic (*Pnma,* LPS20) to a mixture of rhombohedral and cubic (*R-3c* + *Pm-3m,* LSC80) the octahedra that compose the perovskite structure reduce its cooperative tilting and the structure "expands".

These results are in nice agreement with other structural studies of related perovskites in which similar structural transitions with doping level were observed (Blasco et al., 2008; Dann et al., 1994; Tai et al., 1995).

Fig. 2. (a) X-ray diffraction patterns for the series Ln1−xAxFeO3−δ with x=0.2 to x=0.8, all obtained by the ceramic route. Rietveld fits to the X-ray diffraction data for samples LPS20 (b), LPSC50 (c) and LSC80 (d).

Synthetic Methods for Perovskite Materials – Structure and Morphology 501

Fig. 4. (a) Scanning electron microscopy (SEM) images obtained at the same magnification

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

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

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

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

**3.2.1 Influence of the synthetic method on the structure and morphology of the** 

(Figure 5) considering a rhombohedral symmetry (space group *R*-3*c*) in both cases.

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

for all Ln1−xAxFeO3−δ compositions as a function of doping level x.

maximised (Hansen, 2010; Vidal et al., 2007).

values of 0.5 and 0.0161 Å2, respectively.

present case.

**La0.05Sm0.45Sr0.18Ba0.32FeO3-**

Fig. 3. (a) Variation of the unit cell parameters and volume per formula unit with doping, x. (b) Dependence of the mean atomic distances and bond angles with doping, x. Shaded area indicates the x range where the structural transition takes place.

#### **3.1.2 Morphological study**

Microstructure of bulk samples was study by scanning electron microscopy (SEM). Images of the sintered bars at 1300ºC are shown in Fig. 4.

These micrographs show different particle size distributions with grain sizes ranging between 0.33 and 2.83 μm for sample LPS20, to 5 and 37 μm for sample LSC70.

The dispersion in particle sizes is larger for values of x≥0.4. As observed previously (Liou, 2004a, 2004b) this increase in particle size with the doping level seems to be a result of a change in the melting point of the samples that decreases increasing alkaline-earth cation content. According to Kharton et al. (Kharton et al., 2002) this effect results in a liquid-phase assisted by sintering and an enhanced grain growth.

Fig. 3. (a) Variation of the unit cell parameters and volume per formula unit with doping, x. (b) Dependence of the mean atomic distances and bond angles with doping, x. Shaded area

Microstructure of bulk samples was study by scanning electron microscopy (SEM). Images

These micrographs show different particle size distributions with grain sizes ranging

The dispersion in particle sizes is larger for values of x≥0.4. As observed previously (Liou, 2004a, 2004b) this increase in particle size with the doping level seems to be a result of a change in the melting point of the samples that decreases increasing alkaline-earth cation content. According to Kharton et al. (Kharton et al., 2002) this effect results in a liquid-phase

between 0.33 and 2.83 μm for sample LPS20, to 5 and 37 μm for sample LSC70.

indicates the x range where the structural transition takes place.

of the sintered bars at 1300ºC are shown in Fig. 4.

assisted by sintering and an enhanced grain growth.

**3.1.2 Morphological study** 

Fig. 4. (a) Scanning electron microscopy (SEM) images obtained at the same magnification for all Ln1−xAxFeO3−δ compositions as a function of doping level x.
