Resolving Conflicts in Rhenium Dichalcogenides (ReS₂ and ReSe₂)

The absence of consensus regarding the correct structure of two-dimensional rhenium dichalcogenides (ReS₂ and ReSe₂) with distorted 1T′ structures is the source of numerous conflicts between their theoretical and experimental results. Maximal prior studies have dealt with the structures predicted by Murray et al. and Wildervanck et al. for ReS₂ and ReSe₂ respectively and have overlooked the structural rectification suggested by Lamfers et al. With the help of first principles calculations, we have extensively investigated the comparative energetics and electronic properties of all available structures to conclude about the correct structures, capable of resolving all conflicts about their layer-dependent anisotropic optical properties. The non-iso-structural and non-iso-electronic nature of these two compounds indicate that they differ in structural parameters, polyhedral coordination and across-plane electronic hybridization. The pressure-induced metallic transitions are analyzed from detailed electronic band structures and orbital projection analysis implying a 3D and 2D-like behavior for ReS₂ and ReSe₂ respectively. Next, with the help of a combined full-potential density functional theory and multiplet ligand field theory (DFT+MLFT), the X-ray spectral properties of these new structures are analyzed under both ambient and high-pressure conditions in the light of their intricate differences of non-relaxed versus relaxed optimized structures.