A long sub-bandgap absorption (Urbach) edge is present in the Tauc plot of CuBiSeCl2, indicative of either structural (static) or thermal (dynamic) disorder present in the material. More specifically, the size of this Urbach edge is directly influenced by lattice vibrations in the material, and in lead halide perovskites it has been shown that increased electron-phonon scattering enlarges the Urbach edge. Modelling of heat capacity data from CuBiSeCl2 (see Thermal Properties section) confirms the presence of several localised phonon modes, suggesting that a complex landscape of lattice vibrations in the material influence band edge disorder, resulting in the extended Urbach edge.