Dimensional Evolution Guides Property Control in the A_nCu_4-nTiS₄ Semiconductor Series

Through progressive reduction of the three-dimensional (3D) covalent network of Cu₄TiS₄, we isolate seven new members of the A_nCu_4-nTiS₄ family (A = alkali metal; n = 0-4), spanning 3D, 2D, 1D, and 0D structural fragments. The dimensional reduction is rational, as it preserves the edge-sharing connectivity between [CuS₄]^7- and [TiS₄]^4- tetrahedra across the series. This structural evolution is driven by the stepwise substitution of Cu with alkali metals, guiding the formation of fragments with reduced dimensionality. The effects of "n" and "A" on the crystal structures, stabilities, electronic structures, and optoelectronic properties are profound, demonstrating that the manipulation of alkali metal size and A_nCu_4-nTiS₄ stoichiometry enables predictable variations in structure and properties. For example, the n = 0 and n = 4 end members of the A_nCu_4-nTiS₄ family set the range of achievable band gaps with 2.00 eV for Cu₄TiS₄, 2.60 eV for Na₄TiS₄, and intermediate values for the n = 1-3 members. Notably, CsCu₃TiS₄ exhibits exceptional air stability and congruent melting, with density functional theory (DFT) calculating moderate hole and electron effective masses in specific crystallographic directions (m_h = 1.24m₀, m_e = 0.87m₀). Additionally, A₃CuTiS₄ (A = Na, K, Rb) displays direct band gap behavior and long photoluminescence lifetimes of 2.3-8.6 μs, and K₃CuTiS₄ has a PLQY of 5.19%. These findings underscore the potential of the A_nCu_4-nTiS₄ family for applications in optoelectronics and demonstrate widely applicable design concepts that unveil rational stoichiometries within a given composition space to generate a series of crystal structures related through an evolving covalent dimensionality that corresponds to a predictable electronic structure and property progression.