Investigation of factors affecting the stability of compounds formed by isovalent substitution in layered oxychalcogenides, leading to identification of Ba 3 Sc 2 O 5 Cu 2 Se 2 , Ba 3 Y 2 O 5 Cu 2 S 2 , Ba 3 Sc 2 O 5 Ag 2 Se 2 and Ba 3 In 2 O 5 Ag 2 Se 2

Four novel compositions containing chalcogenide layers, adopting the Ba M O M' Ch layered structure have been identified: Ba Sc O Cu Se , Ba Y O Cu S , Ba Sc O Ag Se and Ba In O Ag Se . A comprehensive comparison of experimental and computational results providing the crystallographic and electronic structure of the compounds under investigation has been conducted. Materials were synthesised at 800 °C under vacuum using a conventional ceramic synthesis route with combination of binary oxide and chalcogenide precursors. We report their structures determined by Rietveld refinement of X-ray powder diffraction patterns, and band gaps determined from optical measurements, which range from 1.44 eV to 3.04 eV. Through computational modelling we can also present detailed band structures and propose that, based on their predicted transport properties, Ba Sc O Ag Se has potential as a visible light photocatalyst and Ba Sc O Cu Se is of interest as a p-type transparent conductor. These four novel compounds are part of a larger series of sixteen compounds adopting the Ba M O M' Ch structure that we have considered, of which approximately half are stable and can be synthesized. Analysis of the compounds that cannot be synthesized from this group allows us to identify why compounds containing either M = La, or silver sulfide chalcogenide layers, cannot be formed in this structure type.