Optical transitions, XPS, and electronic states in NiPS3

We have measured the optical absorption below the fundamental threshold, the normal-incidence reflectivity between 1.5 and 30 eV and the X-ray photoemission spectra of NiPS3. Shake-up satellites present at the Ni 2p and 3p core levels are strong evidence for the ionicity of the Ni---S bonds. We have also derived a qualitative molecular orbital model of NiPS3 in which the trigonal crystal field splits the P and S 3pxpy-3pa states, and strong covalent hybridization between P and S pxpy orbitals leads to covalent electronic bonding. Ni is envisaged as a divalent ion which plays little role in the electronic bonding and its 3d levels are localized, lying near the top both of the valence states. This model accounts well for both the valence band XPS data and the low energy optical transitions. Our model should represent, at the center of the Brillouin zone but not at the boundaries, the energy level sequence in NiPS3 and other related MPX3 layer-type compounds where Mtriple bond; length as m-dash Co2+, Mn2+, Fe2+, Zn2+ and X is sulfur or selenium. The XPS spectra and optical properties of NiPS3 have been obtained and interpreted on a qualitative molecular orbital model in which the Ni is a divalent positive ion which plays little role in the bonding. Evidence for such ionicity appears in the optical properties and XPS satellite structures, as well as in the magnetic properties. The model should represent qualitatively the band structure at the center of the Brillouin zone, but not at the boundaries. It should also be valid for other compounds similar to NiPS3, i.e. those with other metals in place of Ni and those with Se in place of S.