Neutron-scattering study on phase transitions of CsPbCl3
Abstract
Neutron-scattering experiments have elucidated the mechanism of the successive phase transitions of CsPbCl3 at 47, 42, and 37°C. These phase transitions are basically associated with the condensation of rotational modes of PbCl6 octahedra around the three principal axes. The phase transition at 47°C is caused by the condensation of the nondegenerate M3 phonon mode at the zone boundary along the [110] direction of the cubic lattice. The axis of rotation of the PbCl6 octahedra is along the [001] axis. The second phase transition at 42°C is associated with the condensation of one of doubly degenerate modes Zx5 and Zy5 at the zone boundary along the [001] direction of the tetragonal lattice, which are derived from the triply degenerate R25 mode in the cubic phase. The direction of the rotation axis is along the [100] axis when we assign Zx5 as the condensing mode. The third phase transition at 37°C is caused by the condensation of the remaining Zy5 mode. The crystal systems and the space groups of three low-temperature phases were determined from the eigenvectors of the condensing modes as tetragonal D54h, orthorhombic D172h, and monoclinic C22h in the sequence of decreasing temperature. These are consistent with the previous experimental results on nuclear-quadrupole-resonance spectra of Cl- ion. A phenomenological theory has been developed to explain these successive phase transitions caused by the condensation of the M3 and R25 zone-boundary phonons. The observed phase-transition schemes can be qualitatively explained by a suitable choice of the parameters in the free-energy expansion with respect to order parameters.
- Publication:
-
Physical Review B
- Pub Date:
- May 1974
- DOI:
- 10.1103/PhysRevB.9.4549
- Bibcode:
- 1974PhRvB...9.4549F