This suggests that the origin of the large Seebeck coefficient in CuBiSeCl2 is at least partially due to anisotropic charge carrier effective masses, though further work is suggested to calculate the carrier concentration in CuBiSeCl2 and understand its contribution to the measured Seebeck coefficient. Again, this implies that depositing As well as the resistivity and Seebeck coefficient, the thermal conductivity was simultaneously measured on the same dense pellet of CuBiSeCl2, parallel to the pressing direction, between 2 and 300 K (Figure (a)). The behaviour of the thermal conductivity of CuBiSeCl2 between 2 and 300 K is consistent with that of a crystalline material in which heat is transported by propagating phonons. At low temperatures, the thermal conductivity increases rapidly and peaks at 0.646(1) W K -1 m -1 at ~35 K, before decreasing to 0.27(4) W K -1 m -1 at 225 K where it plateaus and is almost temperature independent up to 300 K. Observation of the peak at ~35 K in κ confirms the phonon crystal characteristics of CuBiSeCl2.