High-Pressure Structural and Optical Studies of Pure Low-Dimensional Cesium Lead Chlorides CsPbCl and CsPbCl.

We report high-pressure single-crystal X-ray diffraction, optical absorption, and photoluminescence investigations of all-inorganic perovskite-related materials CsPbCl and CsPbCl. The crystal structure of CsPbCl, composed of alternate layers of Cs cations and Pb-Cl frameworks, is stable under pressure up to at least 4.2 GPa. Because external stress is mainly absorbed by the Cs layers, the optical absorption edge of the crystal only slightly red-shifts with increasing pressure, which correlates well with a moderate shortening of the Pb-Cl bonds. A quite different response to pressure shows CsPbCl, the crystal built of isolated PbCl octahedra and Cs cations. During the compression at around 3.4 GPa, the trigonal phase I, space group 3̅, transforms to the orthorhombic phase II, space group , which at around 4 GPa transforms into phase III. On decompression, phase II is not restored, but phase III converts through a diffuse phase transition into another high-pressure phase IV, which is stable in a wide pressure range and transforms to the initial phase I only around atmospheric pressure. The red shift of the absorption edge and the profound modification of the absorption spectrum in phase II were ascribed to the deformation of the PbCl octahedra. The transition to phase III induces a blue shift of the absorption edge, while the transition to phase IV is associated with a large red shift. Photoluminescence was detected in phases I and II with the intensity quenched with increasing pressure.