Thermodynamic study of CsCaCl₃ using neutron diffraction

The pressure and temperature phase diagram of the halide-based perovskite, CsCaCl₃ is investigated using neutron diffraction. At ambient pressure, it undergoes a cubic to tetragonal phase transition at approximately 95 K. The structural evolution is characterised by changes in Cs and Ca coordination, bond lengths, and polyhedral volumes. Heat capacity measurements reveal a discontinuity at the phase transition, where we use a two-term Debye model to show the presence of two distinct contributions. Under pressure, CsCaCl₃ exhibits phase transitions from cubic to tetragonal to rhombohedral symmetry, which can be understood when comparing the relative compressibilites of the CaCl₆ and CsCl_n (where n = 12 for the cubic and 8 for the tetragonal and rhombohedral phases). Overall, this study provides experimentally derived thermodynamic properties and a tentative phase diagram of CsCaCl₃ as a function of pressure and temperature.