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 Ni 5.74 InSe 2 ( I 4/ mmm , a = 3.6766(1) Å, c = 18.8178(10) Å, Z = 2). The structure of Ni 7- δ InSe 2- x S x ( x = 0.2, 0.36, 0.66, 0.80, 0.94) compounds is isotypic to their parent Ni 5.74 InSe 2 and can be viewed as alternating heterometallic Cu 3 Au-type ∞ 2 [ Ni 3 In ] slabs and defective Cu 2 Sb-type ∞ 2 [ Ni 4-δ (Se/S) 2 ] slabs along the [001]-axis. Similar to the parent Se-compound, the Ni- Ch ( Ch = chalcogen) fragment is non-stoichiometric and possesses a partially occupied Ni-site. It was observed that with sulfur insertion at the selenium site of Ni 5.74 InSe 2 , 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 Ni 6.06 InSe 0.67 S 1.33 ( x = 1.33, δ = 0.94) is formed in the sulfur-rich region. Its average structure resembles the Ni 6 SnS 2 -type and has a similar motif to Ni 5.74 InSe 2 ; the only difference is that Cu 3 Au-type ∞ 2 [ Ni 3 In ] alternates with two types of Ni- Ch fragments (Cu 2 Sb or Li 2 O type units). By using first-principles electronic structure calculations, we explained the presence of partially disordered nickel sites in the Ni- Ch fragment and rationalized why the nickel site occupancy increases with sulfur insertion.