A clear shift towards higher energies can also be observed as a function of x. Figures ) and (b) show the extracted band gap as a function of x. There is a monotonic, linear increase in both the direct and indirect gaps from x = 0 to x = 0.75, with the direct gap increasing by 0.25 eV and the indirect gap increasing by 0.20 eV. Above x = 0.75, this linear relationship breaks down, with the indirect gap decreasing to lower values for x = 0.95 and x = 0.99. At the same time, the band gap associated with the second transition approaches the value observed for x = 1.0 (SrBi3O4Cl3). For x = 0.99, the higher energy band gaps are within error of that of SrBi3O4Cl3, and that the indirect and direct gaps for the smaller energy transition are within error of each other. This supports a picture of a localised defect-like Se state 1.36 (8) eV below the conduction band minimum for x = 0.99. The overall trends are consistent with the introduction of a semi-isolated Se p band upon introduction of Se into SrBi3O4Cl3, which has poor orbital mixing with the O and Cl p states, as corroborated by density of states calculations (Figure ). The linear relationship in gap between from x = 0 and x = 0.75 is evidence of some orbital mixng, however the very modest change (0.2 eV) given the large difference in band gap between x = 0 and x = 1.0 is evidence that this orbital mixing is weak. This is because the Se p states sit at considerably higher energy than the Cl or O p states.