Crystal structure of A-site deficient La_0.2Sr_0.2-xCa_xTiO₃ perovskite at ambient conditions and high temperatures: a neutron powder diffraction study

The crystal structures of several solid solutions of the perovskite La_0.2Sr_0.7-xCa_xTiO₃ were determined using the Rietveld analysis of neutron powder diffraction patterns collected at room temperature and at 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 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 20oC – 900oC. The first transition was a discontinuous Pbnm – I4/mcm around 300oC and the second was a continuous I4/mcm - Pm3 ̅m transition below 900oC. The lower symmetries resulted from very small distortions and changes in tilts of the BO6 octahedra of this perovskite; which was a direct result from the A-site ionic radius mismatch.