Polymorphism of CsGaS2 – structural characterization of a new two-dimensional polymorph and study of the phase-transition kinetics

Abstract

CsGaS₂-mC64 was obtained by reaction of CsN₃ with stoichiometric amounts of Ga₂S₃ and S at elevated temperatures. The crystal structure of the air- and moisture stable compound was determined from single-crystal X-ray diffraction data. The colourless solid crystallizes in the monoclinic space group C2/c (no. 15) with the lattice parameters a = 10.5718(6) Å, b = 10.5708(6) Å, c = 16.0847(8) Å, β = 99.445(4)°, V = 1773.1(2) ų, and Z = 16. The compound crystallizes in the TlGaSe₂ structure type and features anionic layers ²_∞[Ga₄S₈⁴⁻] consisting of corner-sharing Ga₄S₁₀ supertetrahedra. At temperatures above 600 °C an irreversible phase-transition to CsGaS₂-mC16 occurs. The phase-transition kinetics were studied using in situ high-temperature X-ray powder diffraction techniques. This transition can only be reversed by using high pressures (>5 GPa at 500 °C). The compound was further characterized using Raman- and diffuse reflectance spectroscopy. Chemical bonding was analysed by DFT calculations.