Role of Partial Vacancy and Structural Distortion in the Stability of Nonstoichiometric Phases Ni 7-δ InSe 2- x S x (1.26 ≥ δ ≥ 0.94; 0 ≤ x ≤ 1.33).

This study explores the structure and stability of partly disordered sulfur-substituted NiInSe (4/, = 3.6766(1) Å, = 18.8178(10) Å, = 2). The structure of NiInSeS ( = 0.2, 0.36, 0.66, 0.80, 0.94) compounds is isotypic to their parent NiInSe and can be viewed as alternating heterometallic CuAu-type [] slabs and defective CuSb-type [] slabs along the [001]-axis. Similar to the parent Se-compound, the Ni- ( = chalcogen) fragment is non-stoichiometric and possesses a partially occupied Ni-site. It was observed that with sulfur insertion at the selenium site of NiInSe, the interatomic distance between the partially occupied nickel and mixed (S/Se) sites decreases from ∼2.24 to ∼1.95 Å, and the occupancy of the disordered nickel site simultaneously increases. The limiting composition NiInSeS ( = 1.33, = 0.94) is formed in the sulfur-rich region. Its average structure resembles the NiSnS-type and has a similar motif to NiInSe; the only difference is that CuAu-type [] alternates with two types of Ni- fragments (CuSb or LiO type units). By using first-principles electronic structure calculations, we explained the presence of partially disordered nickel sites in the Ni- fragment and rationalized why the nickel site occupancy increases with sulfur insertion.