Fresnoites (A2BC2O8) and melilites (A2BC2O7) are layered oxides whose basic structures are modified by crystallographic modulation arising from the need to accommodate pentagonal oxygen rings that cannot fill space without distortion. The modulations are systematized using higher dimensional space groups ( > 3D). Interest in these materials has focused on chemically and/or thermally induced commensurate-incommensurate phase transitions, and provided insights into the nature of interlayer bonding and polyhedral rigidity. Exploitation of this understanding to develop and tune functionality is, as yet, hardly explored. In particular, the application of these oxides as heterogeneous semiconductor photocatalysts has not been considered. This thesis examines the drivers of crystal chemically induced commensurate-incommensurate phase transitions, their impact on defect structure at nano- and micro- scales, the influence of these changes on the band gap energy, and the possibility of moving the photocatalytic response towards visible light energies.