Comparing the structure of CuBiSeCl2 with the previously reported material, CuBiSCl2, which adopts the higher symmetry Cmcm space group, indicates that increasing the chalcogen anion size is the primary driving force for the transition to lower symmetry observed. A group-subgroup relation scheme detailing the symmetry lowering that occurs between CuBiSCl2 and CuBiSeCl2 is included in the Supporting Information (S11). Figure CuBiSCl2 exists as a 'post-perovskite' structure, first observed in a high-pressure polymorph of MgSiO3. The structure of CuBiSCl2 is made up of alternating layers of CuS2Cl4 octahedra and BiS2Cl6 polyhedra, and shows the same packing of polyhedra as the selenide compound, but the polyhedra are much less distorted in the sulphide analogue. Cu is octahedrally coordinated to four chloride ions and two chalcogen ions in both CuBiSeCl2 and CuBiSCl2, but is displaced off the octahedral centre in the selenide material. The Cu-Se bond lengths also show some anisotropy, though these differences are less pronounced than with the Cu-Cl bond lengths. As the Cu + is displaced towards the Cl - at the octahedral vertex, it is also displaced in the a direction towards one of the Se 2- species. Figure Bi 3+ also exhibits the same coordination of six chloride ions and two chalcogen ions in both materials, however the Cu + displacements in CuBiSeCl2 also affect Bi 3+ , which is displaced from the centre of the polyhedra, introducing differences in the Bi-Cl and Bi-Se bond lengths. As shown in Figure Bi shifts towards the under-bonded Cl of the CuSe2Cl4 octahedra, which induces lengthening of the Bi1-Cl2 bond (3.0172(7) Å) opposite. Again, the Bi 3+ displacement happens only in the a-c plane. The variation in structural displacements achieved in these materials by using different coordination environments in three similar structures suggests that using a mixed anion approach provides a route to establishing control over specific structural features.