Crystal structure of A-site deficient La0.2Sr0.7−xCaxTiO3 perovskite at ambient conditions and high temperatures: a neutron powder diffraction study

Abstract

The crystal structures of several members of the solid solution perovskite La_0.2Sr_0.7−xCa_xTiO₃ were investigated using the Rietveld analysis of neutron powder diffraction patterns collected in ambient conditions and high temperatures. At room temperature, samples showed a tetragonal I4/mcm symmetry for compositions with 0.1 ≤ x ≤ 0.35 followed by a phase transition to the orthorhombic Pbnm symmetry for compositions with 0.4 ≤ x ≤ 0.7. Samples with the orthorhombic symmetry showed two reversible phase transitions in the temperature range 20 °C–900 °C. The first phase transition was a discontinuous Pbnm–I4/mcm around 300 °C and the second was a continuous I4/mcm–Pmm transition around 900 °C. The lower symmetries resulted from very small distortions and changes in tilts of the BO₆ octahedra of this perovskite material; which was a direct result from the A-site ionic radius mismatch.