The very light electron effective masses in the a direction result from the highly connected face-sharing Bi 3+ polyhedra and corner-sharing Cu + octahedra, creating 'channels' for the charge carriers. These channels are visible in structural Figures and. In a similar way, the moderate effective masses in the b direction result from the edge-sharing Cu + octahedra and face-sharing Bi 3+ polyhedra providing pathways for conductivity. Electrons are more mobile than holes in both the a and b directions. The heavy electrons in the c direction may be a result of the displacement of Cu + away from the octahedral centre, with the reduced orbital overlap between Cu + and Cl - weakening the pathway for charge carriers and therefore reducing the conductivity through the network. Therefore, these anisotropic effective masses are likely to result from the anisotropic bonding in CuBiSeCl2 that arises due to the inclusion of mixed anions in the structure.