Cation-size control of structural phase transitions in tin perovskites

The structural evolution at 300 K of the series Sr_xBa_1−xSnO₃ and Sr_xCa_1−xSnO₃ at x = 0.2 intervals has been determined by powder neutron diffraction. All Sr_xCa_1−xSnO₃ samples (x = 0–1) have the Pbnm superstructure. In the series Sr_xBa_1−xSnO₃, the x = 1.0 and 0.8 samples have the Pbnm superstructure. The x = 0.6 sample has a second orthorhombic structure with space group Imma. There is a tetragonal I4/mcm phase at x = 0.4 and the undistorted cubic structure for x = 0.2 and BaSnO₃. The octahedral tilt angles show a smooth variation with average A cation radius over the range of superstructures described by mean field theory close to the transition. The transitions are also analysed via the variation of spontaneous strains.