Neutronscattering study on phase transitions of CsPbCl_{3}
Abstract
Neutronscattering experiments have elucidated the mechanism of the successive phase transitions of CsPbCl_{3} at 47, 42, and 37°C. These phase transitions are basically associated with the condensation of rotational modes of PbCl_{6} octahedra around the three principal axes. The phase transition at 47°C is caused by the condensation of the nondegenerate M_{3} phonon mode at the zone boundary along the [110] direction of the cubic lattice. The axis of rotation of the PbCl_{6} octahedra is along the [001] axis. The second phase transition at 42°C is associated with the condensation of one of doubly degenerate modes Z^{x}_{5} and Z^{y}_{5} at the zone boundary along the [001] direction of the tetragonal lattice, which are derived from the triply degenerate R_{25} mode in the cubic phase. The direction of the rotation axis is along the [100] axis when we assign Z^{x}_{5} as the condensing mode. The third phase transition at 37°C is caused by the condensation of the remaining Z^{y}_{5} mode. The crystal systems and the space groups of three lowtemperature phases were determined from the eigenvectors of the condensing modes as tetragonal D^{5}_{4h}, orthorhombic D^{17}_{2h}, and monoclinic C^{2}_{2h} in the sequence of decreasing temperature. These are consistent with the previous experimental results on nuclearquadrupoleresonance spectra of Cl^{} ion. A phenomenological theory has been developed to explain these successive phase transitions caused by the condensation of the M_{3} and R_{25} zoneboundary phonons. The observed phasetransition schemes can be qualitatively explained by a suitable choice of the parameters in the freeenergy expansion with respect to order parameters.
 Publication:

Physical Review B
 Pub Date:
 May 1974
 DOI:
 10.1103/PhysRevB.9.4549
 Bibcode:
 1974PhRvB...9.4549F