Strain-engineered indirect-to-direct band gap transition, photocatalytic water splitting activity, and optical responses in ACuGa6S10 (A = Rb and Cs).

Chalcogenide semiconductors have emerged as promising candidates for photocatalytic (PC) hydrogen evolution and optoelectronic applications, owing to their tunable band gaps (E g) and favorable band edge alignments, rendering them indispensable for clean and carbon-free energy technologies. This study presents the first comprehensive first-principles investigation of the ACuGa6S10 (A = Cs and Rb) chalcogenide and examines its electronic, optical, and PC water-splitting (WS) traits under the application of biaxial strain through density functional theory calculations. The ab initio molecular dynamics simulations confirm the thermal stability, while the quasi-harmonic Debye model analysis establishes the thermophysical response across a wide range of temperature and pressure conditions for both structures. The modified Becke-Johnson potential parametrization yields an E g of 2.48/2.53 eV for the Cs/Rb-based systems, in close agreement with the experimental values of 2.40/2.48 eV, with deviations of only 3.3%/2.02%, respectively, validating the reliability of the adopted computational framework. Compressive strain induces a widening of the E g alongside a notable indirect-to-direct band gap transition at critical strain levels of -1%/-3% for the Cs-based and -1% for the Rb-based system, which is highly favorable for optoelectronic device applications. In contrast, tensile strain progressively narrows the E g. Crucially, Mulliken electronegativity-derived band edge calculations reveal that the conduction band minima and valence band maxima of both unstrained systems intrinsically straddle the H+/H2 reduction and O2/H2O oxidation potentials, confirming their suitability for full WS and solar-based hydrogen production. Additionally, the solar-to-hydrogen (STH) efficiency analysis reveals a maximum thermodynamically viable efficiency of 7.86% for the unstrained Cs-based system. Although tensile strain increases the optical upper-limit STH efficiency (reaching 47.37% at +5% strain), these strained configurations no longer satisfy the thermodynamic band-edge requirement for the oxygen evolution reaction (OER). Consequently, they are unsuitable for overall photocatalytic water splitting, highlighting the trade-off between enhanced optical absorption and redox thermodynamic feasibility. Furthermore, the optical responses of the direct-E g strained levels demonstrate strong ultraviolet-region absorption with peak absorption coefficients exceeding 100 × 104 cm-1 above 6.0 eV, while maintaining high optical transparency across the infrared and visible spectral ranges. Thus, these findings collectively establish ACuGa6S10 (A = Rb and Cs) as a highly promising class of strain-tunable multifunctional chalcogenides with prospective applications in PC hydrogen generation, optoelectronic devices, and nonlinear optical systems.