Fundamental material property trends in the La _0.8-xNd _xCa _0.2FeO _3-δ series: crystal structure and thermal expansion

Compounds from the series La _0.8-xNd _xCa _0.2FeO _3-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 Nd ³⁺ compared to La ³⁺ 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 (ABO ₃) lattice. The thermal expansion behaviour of La _0.8-xNd _xCa _0.2FeO _3-δ (0 ≤ x ≤ 0.6) was characterized by dilatometry at 30 ≤ T/°C ≤ 1000 and 1 × 10 ^–3 ≤ pO ₂/bar ≤ 1. The thermal expansion coefficients of La _0.8-xNd _xCa _0.2FeO _3-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 pO ₂. The transition from orthorhombic to trigonal modification, which is observed for La _0.8Ca _0.2FeO _3-δ at approx. 740 °C, is suppressed for all Nd-substituted compounds with x ≥ 0.1.