Phonopy Calculations of Thermodynamic Properties and Phase Transitions of CsSnI<sub>3</sub> Based on SCAN-relaxed Structures

In this work, the structural stability of cubic (α), tetragonal (β), and orthorhombic (γ) phases of perovskite CsSnI3 and their phase transitions have been studied by density functional theory (DFT) calculations, and the results obtained have been compared with the characteristics of nonperovskite orthorhombic (δ) modification of CsSnI3 compounds. The relaxed structures of the CsSnI3 phases were produced and their geometrical properties were assessed using the strictly constrained normalized potential (SCAN) functional. According to the results, the energetic hierarchy of CsSnI3 polymorphs is Eβ>Eα>Eγ>Eδ. The phonon and thermodynamic characteristics as well as the temperatures of phase transitions of CsSnI3 have been estimated using the Phonopy tool based on SCAN relaxed structures. The nature of the change in the total energy of the four phases of CsSnI3 from VASP package calculations justifies the trends of free energy, entropy, enthalpy, and heat capacity. In contrast, the β- phase, which has the highest energy among the perovskite phases, is extremely unstable. It was discovered that the tetragonal phase becomes stable at 450K and transitions to the cubic phase at lowering temperatures. CsSnI3 undergoes a phase transition between γ- and β-phases at 300-320K. At temperatures below 320K, a black-yellow transformation of CsSnI3 occurs, in which the black perovskite transforms into a yellow non-perovskite conformation. It was found that temperature phase transitions occur between two orthorhombic phases of CsSnI3 at 360K, although direct transitions of the α⟷γ and γ⟷δ types have not yet been reported in the literature, with the exception of γ→δ transitions under the influence of moisture.