Dimensional Reduction Guides Electronic Structure Evolution in the AnCu4–nSnS4 Semiconductor Series

The search for new functional materials with tunable properties remains a central challenge in chemistry, particularly for applications in energy and electronics. In this work, we present a framework for predictive crystal design in alkali metal chalcogenides that enables controlled dimensional reduction of a parent covalent motif, yielding a broad range of electronic structures, which systematically evolve from one parent to the other. We present 11 new members of the A_nCu_4–nSnS₄ family (A = alkali metal; n = 0–4), which reduce the three-dimensional (3D) covalent network of Cu₄SnS₄ into various 3D, 2D, 1D, and 0D [Cu_4–nSnS₄]ⁿ⁻ motifs through the substitution of Cu with alkali metals of various radii. The end members of the family set the range in achievable band gaps at 0.99 eV for fully covalent Cu₄SnS₄ (n = 0) and 3.38 eV for K₄SnS₄ (n = 4) with 0D [SnS₄]ⁿ⁻ tetrahedra. As the dimensionality of [Cu_4–nSnS₄]ⁿ⁻ systematically reduces within A_nCu_4–nSnS₄ (n = 1–3), a stepwise increase in band gap energy occurs through a gradual decrease in the energy of the valence band maximum and an increase in the conduction band minimum, with an increase in the effective masses of charge carriers. Furthermore, irrespective of the alkali metal, the thermal stability decreases with decreasing [Cu_4–nSnS₄]ⁿ⁻ dimensionality within the quaternary members. Most importantly, we demonstrate that predictable crystal structure and property evolution for a given composition space is possible by deriving a general formula based on substituting the covalent metals of a parent structure with alkali metals.