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

Abstract

Solid solution LaCr1−xMgxO3, 0<х≤0.25 was prepared by heating stoichiometric amounts of appropriate oxides in air at 1400°C, 48h. 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)=8.8ppmK−1; TEC(R)=11.6ppmK−1). TEC for rhombohedral phase increases with increasing magnesium content from 10.4ppmK−1 for LaCr0.95Mg0.05O3 to 12.1ppmK−1 for LaCr0.75Mg0.25O3.