Structure-Optical Property Relationships in AMM'Q₃ Chalcogenides

Ten ACuHfQ₃ (A = Na, K, Rb, Cs; Q = S, Se, Te) chalcogenides were prepared via high-temperature solid-state methods and structurally characterized using single-crystal X-ray diffraction. The type-I AMM'Q₃ structures are defined by the arrangement of HfQ₆ octahedra (O) and CuQ₄ tetrahedra (T) in their layers, which adopt either OTOT (Cmcm, Pnma) or OOTT (Pnma, P2₁/m) connectivity. UV-vis absorption and photoluminescence spectra indicate optical band gaps ranging from 1.2 to 1.7 eV and reveal that OTOT and OOTT variants have direct and indirect band gaps, respectively. Density functional theory calculations corroborate these experimental observations, demonstrating that this structure-property relationship extends across the broader type-I AMM'Q₃ family. A survey of reported compounds reveals trends linking ionic radii to structure, wherein systems with larger alkali (A⁺) cations and small chalcogen (Q^2-) anions preferentially adopt OTOT connectivity. To formalize this trend, a machine learning model was trained on the expanded data set and used to classify compounds as either OTOT or OOTT based on ionic radii descriptors. From the model, we construct a two-dimensional structure-property map that provides a semiquantitative framework for predicting the phase and optical features in related quaternary systems. Together, these results establish a structure-property relationship in an important class of chalcogenides and demonstrate a strategy for targeting optical properties through integrated experiment and theory.