High-Pressure Structural and Optical Studies of Pure Low-Dimensional Cesium Lead Chlorides CsPb₂Cl₅ and Cs₄PbCl₆

We report high-pressure single-crystal X-ray diffraction, optical absorption, and photoluminescence investigations of all-inorganic perovskite-related materials CsPb₂Cl₅ and Cs₄PbCl₆. The crystal structure of CsPb₂Cl₅, 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 Cs₄PbCl₆, the crystal built of isolated PbCl₆^4- octahedra and Cs⁺ cations. During the compression at around 3.4 GPa, the trigonal phase I, space group R3̅c, transforms to the orthorhombic phase II, space group Cmce, 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 PbCl₆^4- 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.