Temperature and Pressure Effects on Phase Transitions and Structural Stability in CsPb₂Br₅ and CsPb₂Br₄I Perovskite-Derived Halides

All-inorganic perovskites exhibit outstanding light absorption properties in the visible range suitable for solar energy applications. We focus on the synthesis of CsPb₂(Br,I)₅ using mechanochemical procedures. Synchrotron X-ray diffraction (SXRD) data are essential for determining the crystallographic evolution in the 295-753 K temperature range. From room temperature to 573 K, the crystal structure is refined in a two-dimensional (2D) tetragonal framework (space group: <i>I</i>4/<i>mcm</i>) consisting of layers of face sharing [Pb(Br,I)₈] polyhedra. Above a transition temperature of <i>T</i>_t = 630 and 621 K, identified from differential scanning calorimetry (DSC) curves for CsPb₂Br₅ and CsPb₂Br₄I, respectively, a cubic three-dimensional (3D) corner-sharing perovskite structure is identified, showing substantial Cs and (Br,I) deficiency. A more ionic character for Cs-halide versus Pb-halide is derived from the evolution of the anisotropic atomic displacements. An ultralow thermal conductivity of 0.35-0.18 W m^-1 K^-1 is related to the low Debye temperatures. From high-pressure SXRD studies, the change in <i>B</i>₀ can be attributed to a basic expansion of the unit-cell volume caused by the chemical pressure from iodine within the CsPb₂Br₅ framework. UV-vis-NIR spectroscopy revealed optical gaps of approximately 2.93 and 2.50 eV, respectively, which agrees with ab initio calculations.