Structural Phase Transitions in CsPbBr₃

Structural phase transitions in perovskite-type CsPbBr 3 have been investigated by neutron diffraction method. Phase transitions occur at 88°C and 130°C, which are respectively second and first order. The phase transition at 130°C is caused by condensation of the M 3 mode at the M point of the cubic Brillouin zone, while the one at 88°C results from condensation of the doubly degenerate R 25 -like mode ( Z 9 mode) at the Z point of the tetragonal Brillouin zone. Group theoretical considerations based on these results reveal that the crystal trans-forms from cubic perovskite structure (O h 1 - P m 3 m ) to tetragonal D 4h 5 - P 4/ m b m at 130°C and further to orthorhombic D 2h 16 - P m b n at 88°C. Possible atomic displacements induced at the phase transitions are obtained from the eigenvectors of the condensing modes.