Crystal structure and thermal expansion of LaCr1−xMgxO3, 0 < x ≤ 0.25

Solid solution LaCr1−xMgxO3, 0 < х ≤ 0.25 was prepared by heating stoichiometric amounts of appropriate oxides in air at 1400 °C, 48 h. At room temperature it crystallizes in orthorhombically distorted GdFeO3-type structure (a ≈ √2 × aper; b ≈ √2 × ape; c ≈ 2 × aper, where aper – perovskite subcell parameter). High-temperature X-ray powder diffraction (HT XRPD) and dilatometry revealed first order phase transition to rhombohedral perovskite phase (R-3c, a ≈ √2 × aper, c ≈ 2√3 × aper) at 260–311 °C (OR phase transition). Crystal structures of room-temperature orthorhombic and high-temperature rhombohedral phases for LaCr0.75Mg0.25O3 were refined using HT XRPD data. Temperature of OR phase transition increases gradually with increasing of magnesium content. Low-temperature orthorhombic phase exhibits TEC lower in comparison with high-temperature rhombohedral one (e.g. for LaCr0.85Mg0.15O3 TEC(O) = 8.8 ppm K−1; TEC(R) = 11.6 ppm K−1). TEC for rhombohedral phase increases with increasing magnesium content from 10.4 ppm K−1 for LaCr0.95Mg0.05O3 to 12.1 ppm K−1 for LaCr0.75Mg0.25O3.