Reflectance spectra of some FePS3-type layer compounds in the vacuum ultraviolet

Near-normal-incidence reflectivity spectra of single crystals of MnP${\mathrm{S}}_3$, FeP${\mathrm{S}}_3$, and NiP${\mathrm{S}}_3$ have been obtained in the vacuum ultraviolet region of the optical spectrum. The spectra are interpreted using molecular-orbital ideas as well as simple physical and chemical arguments. The metal $d$ orbitals in these materials are envisaged as localized discrete levels (rather than bands) in the energy-band models proposed. The most illuminating result of this work is that the reflectivity features on the three spectra look identical. These similarities in the three spectra have led us to suggest that, in a broad view, the band structures (as yet unknown) of the first-row transition-metal chalcogenophosphates (archetype FeP${\mathrm{S}}_3$) are closely similar. Furthermore, we conclude that for photon energies greater than the absorption edge, the localized transition-metal $d$ orbitals do not seem to take part in the optical transitions (only the energy bands of the ${\mathrm{P}}_2$ ${\mathrm{S}}_6$ complex take part), thus causing the observed similarities in the reflectivity spectra of $M{\mathrm{PS}}_3$ systems.