Density of states (DOS) plots were produced for the lowest energy SrxBi4-xO4Se1-xCl2+x and CaBi3O4Cl3 structures using a higher plane wave cut off of 600 eV and a gamma centred kpoint mesh with a density of ≥56 Å. All DOS plots in this work were visualised in the Sumo package . The formation energy (𝐸 , ) per formula unit of the x=0.25 and 0.75 phases relative to the Bi4O4SeCl2 and SrBi3O4Cl3 end members was calculated via the equation: