Dimensional Evolution Guides Property Control in the A_<i>n</i>Cu_4–<i>n</i>TiS₄ Semiconductor Series

Through progressive reduction of the three-dimensional (3D) covalent network of Cu₄TiS₄, we isolate seven new members of the A_<i>n</i>Cu_4-<i>n</i>TiS₄ family (A = alkali metal; <i>n</i> = 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 "<i>n</i>" and "A" on the crystal structures, stabilities, electronic structures, and optoelectronic properties are profound, demonstrating that the manipulation of alkali metal size and A_<i>n</i>Cu_4-<i>n</i>TiS₄ stoichiometry enables predictable variations in structure and properties. For example, the <i>n</i> = 0 and <i>n</i> = 4 end members of the A_<i>n</i>Cu_4-<i>n</i>TiS₄ family set the range of achievable band gaps with 2.00 eV for Cu₄TiS₄, 2.60 eV for Na₄TiS₄, and intermediate values for the <i>n</i> = 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 (<i>m</i>_h = 1.24<i>m</i>₀, <i>m</i>_e = 0.87<i>m</i>₀). 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_<i>n</i>Cu_4-<i>n</i>TiS₄ 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.