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

摘要

The crystal structures of several members of the solid solution perovskite La0.2Sr0.7-xCaxTiO3 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 degrees C-900 degrees C. The first phase transition was a discontinuous Pbnm-I4/mcm around 300 degrees C and the second was a continuous I4/mcm-Pm (3) over barm transition around 900 degrees 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.