Identifying the Origins of Vacancies in the Crystal Structures of Rock Salt-type Chalcogenide Superconductors

The tailored (computational) design of materials addressing future challenges requires a thorough understanding of their electronic structures. This becomes very apparent for a given material existing in a certain homogeneity range, as its particular composition influences its electronic structure and, eventually, its physical properties. This led us to explore the influence and, furthermore, the origin of vacancies in the crystal structures of rock salt-type superconductors by means of quantum-chemical techniques. In doing so, we examined the vibrational properties, electronic band structures, and nature of bonding for a series of superconducting transition-metal sulfides, i.e., MS (M = Sc, Y, Zr, Lu), which were identified to exist over certain homogeneity ranges. The outcome of our research indicates that the subtle competing interplay between two electronically unfavorable situations at the Fermi levels, i.e., the occupations of flat bands and the populations of antibonding states, appears to control the presence of vacancies in the crystal structures of the sulfides.