Phase transitions, phonon behavior and elastic constants in CsPbX₃ (X = Br, Cl) single crystals: Experimental and theoretical insights

In this work, we present a detailed investigation of the structural, electronic, vibrational and elastic properties of CsPbBr₃ and CsPbCl₃ single crystals using combined experimental and theoretical approaches. High-quality single crystals were synthesized and structurally characterized by X-ray diffraction, confirming orthorhombic (Pnma) symmetry at room temperature. Electronic band structures calculated using density functional theory (DFT) revealed direct band gaps of approximately 1.78 eV (CsPbBr₃) and 2.21 eV (CsPbCl₃). Temperature-dependent Raman spectroscopy identified distinct anomalies at ∼323 K, 365 K, and 403 K in CsPbBr₃, corresponding to structural transitions (Pnma → cmcm → P4/mbm → Pm3¯m). Raman and Brillouin spectroscopy studies of CsPbCl₃ revealed closely spaced transitions in a similar sequence near room temperature (∼296 K, ∼303 K and ∼309 K), accompanied by critical slowing-down phenomena evidenced by the central peak dynamics. Elastic constants derived from Brillouin scattering and confirmed by DFT calculations demonstrated that CsPbCl₃ exhibits significantly higher mechanical rigidity compared to CsPbBr₃, attributed to stronger Pb–Cl bonds and a smaller halide ionic radius. C ₁₁​ exhibited a gradual increase with temperature in the cubic phase of CsPbCl₃. This comprehensive investigation provides valuable insights into the structural and dynamic origins of phase transitions, lattice stability, and elastic properties in cesium-lead-halide perovskites, essential for their development in stable optoelectronic applications.