Fundamental material property trends in the La0.8-xNdxCa0.2FeO3-δ series: crystal structure and thermal expansion

Compounds from the series La0.8-xNdxCa0.2FeO3-δ (0.1 ≤ x ≤ 0.7) were synthesised by a sol–gel route. X-ray diffraction and Rietveld analysis showed that materials with 0 ≤ x ≤ 0.6 crystallize as single-phase orthorhombic perovskites. The smaller ionic radius of Nd3+ compared to La3+ leads to a decrease in unit cell volume with increasing x. Elemental mapping by high-resolution scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy confirms the homogeneous distribution of the A-site elements (La, Nd and Ca) in the perovskite (ABO3) lattice. The thermal expansion behaviour of La0.8-xNdxCa0.2FeO3-δ (0 ≤ x ≤ 0.6) was characterized by dilatometry at 30 ≤ T/°C ≤ 1000 and 1 × 10–3 ≤ pO2/bar ≤ 1. The thermal expansion coefficients of La0.8-xNdxCa0.2FeO3-δ, which were determined in regions I (40–530 °C) and II (530–990 °C), respectively, are almost independent of the Nd concentration in the range of (0 ≤ x ≤ 0.6) and increase slightly with decreasing pO2. The transition from orthorhombic to trigonal modification, which is observed for La0.8Ca0.2FeO3-δ at approx. 740 °C, is suppressed for all Nd-substituted compounds with x ≥ 0.1.

Introduction oxygen exchange kinetics [3][4][5][6][7]. SOC air electrode materials, which are optimized for operation temperatures of 650-850°C, usually contain La 3? and Sr 2? on the A-site and mixed-valence transition metals on the B-site (Co, Fe) [4,8]. However, the insufficient long-term stability of state-of-the-art SOC air electrodes under application-relevant conditions remains a critical factor and a challenge towards materials development. The degradation mechanism of materials from the La 1-x Sr x Co 1-y Fe y O 3-d series frequently involves the surface segregation of Sr [9][10][11] driven (among other factors) by the size-mismatch between the host (La 3? ) and substituent ion (Sr 2? ) [12], and the reaction with acidic impurities such as volatile Cr-or Si-species and SO 2 [13][14][15][16][17][18][19]. A possible approach towards the improvement of long-term stability is, therefore, the replacement of Sr with different substituents (preferably with smaller ionic radius) and the decrease in the compound basicity [20,21].
Recently, promising results with regard to fast oxygen exchange kinetics and high stability vs. SO 2 -poisoning were reported for Sr-and Co-free perovskites from the La 1-x Ca x FeO 3-d series [7,[22][23][24]. Price et al. reported that the solubility limit of Ca in La 1-x Ca x FeO 3-d is 38% [25]. In the series La 1-x Ca x FeO 3-d (0.1 B x B 0.25), La 0.8 Ca 0.2 FeO 3-d showed the highest oxygen exchange activity [7,23] and excellent long-term stability [26]. The results of long-term oxygen exchange measurements and post-test analyses indicated that the improved impurity tolerance of La 0.8 Ca 0.2 FeO 3-d is due to lower compound basicity and the island-like formation pattern of secondary phases [20,26].
In the present study, the solubility of 10-70% of Nd on the A-site of La 0.8 Ca 0.2 FeO 3-d is investigated with the aims to further decrease compound basicity and to further improve long-term stability against impurity poisoning, while at the same time (to the greatest possible extent) maintaining the excellent properties of the original compound. With an ionic radius of 1.27 Å Nd 3? (XII) is smaller than (but still relatively close to) the ionic radius of La 3? (XII), which is 1.36 Å [12]. A-site substitution with Ce 3? or Pr 3? was avoided since both cations are capable of valence changes (3 ?/4 ?), which may introduce more complex changes in the material properties or lead to problems with phase stability. Sm 3? (XII) with an ionic radius of 1.24 Å and rare earth elements with even higher atomic numbers (i.e. even smaller ionic radii) were also considered unfavourable, since these cations may lead to more significant changes in crystal structure and material properties compared to the original compound La 0.8 Ca 0.2 FeO 3-d .
The present publication describes the synthesis of the compounds La 0.8-x Nd x Ca 0.2 FeO 3-d (0.1 B x B 0.7) as well as characterization of the crystal structure and thermal expansion behaviour of the single-phase materials La 0.8-x Nd x Ca 0.2 FeO 3-d (0.1 B x B 0.6) in comparison with La 0.8 Ca 0.2 FeO 3-d . By analysis of XRD data with Rietveld refinement, information on phase purity and structural parameters (space group, bond length and angles, atom positions) was obtained. The thermal expansion behaviour was characterized by dilatometry in a wide range of temperatures (30-1000°C) and oxygen partial pressures (1 9 10 -3 B pO 2 /bar B 1).  until pH = 8 was reached and a clear dark red solution was obtained. The temperature was slowly raised to evaporate water and form a gel. Self-ignition and combustion of the dried gel occurred at approximately 500°C. The raw ash was crushed in an agate mortar and calcined at 1000°C for 4 h (5 K min -1 ramps for heating and cooling) in air. According to XRD, singlephase compounds were obtained for La 0.  Table S-1). The particle size distribution of the calcined powders was determined by use of a CILAS 1064L particle size analyser. After calcination, the d 50 values of the powders were in the range of 18.6 d 50 /lm 22.4. A bench top roll-mill was used to grind the calcined powders until 0.5 d 50 /lm 0.7 was reached.

Experimental Sample preparation
For thermal expansion measurements, cylindrical pellets were fabricated by uniaxial pressing. After sintering at 1150°C for 10 h with heating and cooling rates of 1 K min -1 , the samples were approximately 5 mm in diameter and 8 mm long. The relative densities of the sintered samples of La 0.8-x Nd x Ca 0.2 FeO 3-d with x = 0 [7] and x = 0.3-0.6 (this study) were in the range of 97-99% of the theoretical densities. (Relative densities of La 0.8-x Nd x Ca 0.2 FeO 3-d with x = 0.1-0.2 were determined with 92% of the theoretical densities; however, since these two samples were not ideally cylindrical, the relative densities may be underestimated.) X-ray diffraction and electron microscopy XRD of the calcined powders was performed with a diffractometer (BRUKER-AXS D8 Advance ECO) using a Cu Ka radiation source operated at 40 kV and 25 mA. The diffractograms were recorded at scanning rates of 0.01°s -1 . Rietveld refinement was performed by using the commercially available software TOPAS (Bruker) [27]. Lattice parameters were obtained from fits of the reflections' positions to an b Figure 1  orthorhombic unit cell (space group Pnma). Reflection broadening was described by convolution of a Voigt function with a modified pseudo-Voigt function according to Thompson-Cox-Hastings [28]. Sample preparation for the TEM investigation was carried out on a focused ion beam (FIB) microscope FEI NOVA 200 Nanolab. TEM analyses were performed with a TITAN 3 G2 60-300 from FEI at 300 kV accelerating voltage in scanning transmission electron microscopy mode (STEM). The microscope is equipped with a C S -probe corrector and a Super-X detector system (FEI). STEM images were acquired with a high-angle annular dark-field (HAADF) and a bright-field (BF) detector. The software packages GMS 3 (Gatan) and Velox 2 (Thermo Fisher Scientific) were used to acquire HAADF images and EDX spectrum images.
The cation stoichiometry (Table 1) was obtained by energy-dispersive X-ray spectroscopy (EDXS) measurements in a scanning electron microscope (Cambridge, STEREOSCAN 250 MK3) equipped with an Oxford Instruments Mod. 6272 detector. To confirm the homogeneity of cation distribution, area scans on polished samples with 10 mm diameters were measured.

Dilatometry and differential scanning calorimetry
The thermal expansion behaviour of the materials was investigated between 30 and 1000°C with heating rates of 1 K min -1 using a single-rod dilatometer DIL 402/PC4 (NETZSCH). Atmospheres with different oxygen partial pressures in range of 1 9 10 -3 B pO 2 /bar B 1 were obtained by O 2 -Ar gas mixtures using mass flow controllers (Teledyne HFC-302) at flow rates of 10 l h -1 .
Differential scanning calorimetry (DSC) was performed on selected sintered (1100°C, 2 h, air) and finely ground powders using a High Temperature DSC 404 C Pegasus (NETZSCH). The sample (approximately 20-30 mg) was placed in an 85 ll Pt-Rh crucible. To minimize the influence of adsorbed humidity or CO 2 , the powders were heated in the calorimeter to 1000°C, kept in 20% O 2 (rest Ar) and cooled to 20°C before the actual experiments were started. The DSC measurements were performed between 20 and 1000°C in 20% O 2 (rest Ar) with heating and cooling rates of 20 K min -1 and gas flow rates of 50 ml min -1 .  Fig. 1a. All reflections were assigned to the orthorhombic GdFeO 3 perovskite structure with the space group Pnma (SG#62). With increasing Nd content, the reflections shift towards higher diffraction angles. This effect is shown in detail for the reflections ascribed to the lattice planes (220) and (022) in Fig. 1(b), and for the lattice planes (321), (240), (123) and (042) in Fig. 1(c). The separation of these reflections increases with increasing Nd concentration. Rietveld refinement was performed for all compounds La 0.8-x Nd x Ca 0.2 FeO 3-d (0 x 0.6). The lattice parameters of the orthorhombic unit cells are plotted as a function of Nd concentration in Fig. 2a. Linear dependences of the unit cell parameters of the solid solutions on the Nd concentration are observed, following Vegard's law [29]. No change in space group is observed within the investigated range of compositions (see also the plot of the orthorhombicity, defined as 2(ba)/(a ? b) [30], in Fig. 2b). The lattice parameters of the Nd-substituted compounds are given in Table 2, which also includes data of the unsubstituted materials LaFeO 3 [31], CaFeO 3 [32] and NdFeO 3 [33], and of La 0.8 Ca 0.2 FeO 3-d [7]. Increasing substitution of La 3? with Nd 3? causes a linear decrease in the unit cell volume of La 0.8-x Nd x Ca 0.2 FeO 3-d ( Table 2 and Fig. 2(c)).
Goldschmidt's tolerance factor t ( Table 2), which describes the deviation from the ideal cubic perovskite structure (t = 1) and may be used as an indicator of the relative stability of the structure, was calculated according Eq. 1 [34,35].
The variables x A , x A 0 and x A 00 represent the occupancy of the A-sites of the perovskite lattice with A = La, A 0 = Ca and A 00 = Nd (for example: x A = 0.2, x A 0 = 0.2 and x A 00 = 0.6 in La 0.2 Nd 0.6 Ca 0.2 FeO 3-d ). The symbols r i denote the ionic radii of the species i (Aand B-site cations, oxygen). The following ionic radii from [12]  As an example for the compounds from the LNCF series, La 0.2 Nd 0.6 Ca 0.2 FeO 3-d was analysed with atomically resolved STEM-EDX regarding the     Fig. 4a and Fig. 4b. The atomic positions of La 0.8-x Nd x Ca 0.2 FeO 3-d (0.1 B x B 0.6) are given in Table 3. Table 4 shows bond lengths and bond angles of materials from the LNCF series in comparison with the unsubstituted compounds LaFeO 3 [31], CaFeO 3 [32], NdFeO 3 [33] and the Nd-free analogue La 0.8-Ca 0.2 FeO 3-d [7]. Bond lengths and bond angles of La 0.8-x Nd x Ca 0.2 FeO 3-d (0 B x B 0.6) are also shown as a function of Nd concentration in Fig. 5. Substitution of 10% La with Nd decreases the bond length between Fe and the apical oxygen (Fe1-O1, Fig. 5f), followed by an increase in the bond length from x = 0.1 to 0.3 in La 0.8-x Nd x Ca 0.2 FeO 3-d ( Fig. 5(a)). An increase with different slopes is observed at higher Nd concentrations (x = 0.4 to 0.6). While the bond lengths Fe1-O2 ( Fig. 5(b)) decrease with increasing x, the bond lengths Fe1-O2 0 show similar trends to those of Fe1-O1 (Fig. 5c, decrease from x = 0 to x = 0.1 and increase with further increase in x). The presence of two different equatorial oxygen positions (O2 and O2 0 ) is due to a distortion of the FeO 6 octahedra [31,33,36]. Although Fe1-O2 and Fe1-O2 0 have different bond lengths, the bond angles of Fe1-O2-Fe1 and Fe1-O2 0 -Fe1 are equal. The bond angles Fe1-O2-Fe1 and Fe1-O2 0 -Fe1 (both shown in Fig. 5d), as well as Fe1-O1-Fe1 (Fig. 5e) increase from x = 0 to x = 0.1 and decrease from x = 0.1 to 0.6. The high-resolution HAADF image shown in Fig. 6b was used to perform a Fast Fourier Transformation (FFT) (Fig. 6d) (Table 5 and Fig. 7). A threedimensional representation of the T-and pO 2 -dependence of the average a values (50 T= C 1000) is given in Figure S  of La 0.8-x Nd x Ca 0.2 FeO 3-d (0 x 0.6) is in the range of (12.8 ± 0.3) 9 10 -6 K -1 (region I) and (15.3 ± 0.5) 9 10 -6 K -1 (region II), and at 0.1 bar in the range of (13.0 ± 0.2) 9 10 -6 K -1 (region I) and (15.7 ± 0.2) 9 10 -6 K -1 (region II). At lower oxygen partial pressure, i.e. pO 2 = 0.01 bar, a values are (13.4 ± 0.3) 9 10 -6 K -1 (region I) and (17.4 ± 0.9) 9 10 -6 K -1 (region II), and at 0.001 bar (13.2 ± 0.3) 9 10 -6 K -1 (region I) and (18.4 ± 0.4) 9 10 -6 K -1 (region II). The increase in a with decreasing pO 2 (esp. at 1 9 10 -2 and 1 9 10 -3 bar) and increasing temperature (esp. in region II) is presumably due to the additional contribution of chemical expansion caused by the formation of oxygen vacancies as observed with similar perovskites [7,22,23].
As previously reported in the literature, the Ndfree compound La 0.8 Ca 0.2 FeO 3-d shows a reversible transition from the orthorhombic (SG: Pnma) to the trigonal (SG: R-3c) structural modification at approximately 740°C [7,25]. According to DSC measurements, substitution of La with Nd seems to suppress the structural transition in La 0.8-x Nd x Ca 0.2-

Summary and conclusions
Single-phase perovskites from the series La 0.8-x Nd x Ca 0.2 FeO 3-d with 0.1 B x B 0.6 were synthesized and characterized with respect to crystal structure, chemical composition, thermal expansion behaviour and phase stability. All the investigated compounds of the La 0.8- x Nd x Ca 0.2 FeO 3-d series crystallize in the orthorhombic space group Pnma. High-resolution transmission electron microscopy with energy-dispersive X-ray spectroscopy confirmed the homogeneous distribution of the A-site ions (La 3? , Nd 3? and Ca 2? ) on the example of La 0.6 Nd 0.2 Ca 0.2 FeO 3-d . The thermal expansion coefficients of the materials from the LNCF series are in the range of (13-18) 9 10 -6 K -1 at 40-990°C and 1 9 10 -3 B pO 2 /bar B 1. Differential scanning calorimetry indicated that substitution of La with Nd in La 0.8-x-Nd x Ca 0.2 FeO 3-d (0.1 x 0.6) suppresses the transition from the orthorhombic to the trigonal structural modification, which is observed in the Nd-free compound La 0.8 Ca 0.2 FeO 3-d .
In future work, the materials from the series La 0.8-x Nd x Ca 0.2 FeO 3-d will be characterized with regard to oxygen nonstoichiometry and electronic conductivity as a function of temperature and oxygen partial pressure. It would also be of interest to further investigate selected compositions of La 0.8-x Nd x Ca 0.2 FeO 3-d in terms of long-time stability, especially in comparison with the Nd-free perovskite La 0.8 Ca 0.2 FeO 3-d and state-of-the-art SOC air electrode materials from the La 1-x Sr x Co 1-y Fe y O 3-d series.

Compliance with ethical standards
Conflicts of interest There are no conflicts of interest.
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