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

Abstract Compounds from the series La 0.8-x Nd x Ca 0.2 FeO 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-x Nd x Ca 0.2 FeO 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-x Nd x Ca 0.2 FeO 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 0.8 Ca 0.2 FeO 3-δ at approx. 740 °C, is suppressed for all Nd-substituted compounds with x ≥ 0.1.