CuBiSeCl2 255 62 28 11 0.00018 Three Einstein parameters, in addition to a linear term, are required to fit the heat capacity data, indicating the presence of several highly localised phonon modes within the structure. The Debye temperature obtained from the heat capacity modelling enables the speed of sound in CuBiSeCl2 to be calculated, which can provide information on the low thermal conductivity in the material. The speed of sound calculated from θD1 is 2438 ms -1 . Using this value, the minimum thermal conductivity under the assumptions of the Cahill model (κmin) for CuBiSeCl2 was calculated (shown by the dashed line in Fig. (a)). Unusually, the calculated minimum thermal conductivity (0.39 W K -1 m -1 ) is greater than the measured data (0.27 (4) W K -1 m -1 ) at 300 K. However, a diffuson model (κdiff) of the thermal conductivity accurately captures the high temperature plateau in the measured data (shown in Fig. (a) in red). This indicates that the thermal conductivity of the material must likely be understood through a twochannel model where propagons dominate at low temperatures (resulting in the peak at 35 K)