Electronic Structure Progression across the ACu₂Q₂(MQ₂)_n Semiconductor Series

Moving beyond the engineering of known materials and elemental substitution within common structure types is critical for designing unique properties. Here, we present a new homologous series, ACu₂Q₂(MQ₂)_n (A = Sr, Ba, 2Na; MQ₂ = ZrS₂, HfSe₂), establishing 11 new members. In β-BaCu₂Q₂ and Na₂Cu₂Se₂ (n = 0), the [Cu₂Q₂]^2- motifs extend in two dimensions, whereas those in α-BaCu₂Q₂ extend in three dimensions. Hence, there are two structural evolutions within the ACu₂Q₂(MQ₂)_n family driven by the polymorphism of the host structures. MQ₂ (n → ∞) displays 2D layers of edge-sharing [MQ₆]^8- octahedra that are 1-octahedron-thick and connected via van der Waals bonding. Each insertion of MQ₂ into ACu₂Q₂ incorporates [MQ₆]^8- octahedra extending infinitely in one direction, confined to being 1-octahedron-thick in the second direction, with n controlling the number of [MQ₆]^8- octahedra in the third direction. Therefore, increasing n predictively expands the [Cu₂M_nQ_2n+2]^2- network in the direction controlled by n and relative to the A⁺/A²⁺ ions. We demonstrate that for a given set of elements, one can enforce a "Host(Insertion)_n" formula to systematically evolve both crystal and electronic structures from the host (n = 0) to the insertion (n → ∞) materials through intermediate values of n. Specifically, the energetic misalignment of the electronic band extrema of the parents predictively determines the band extrema and the resulting band gaps of all intermediate n members.