Abstract
Cd1-xZnxS alloys are currently explored as one class of the most promising for producing hydrogen energy under visible-light irradiation. Here, using an ab intio evolutionary algorithm, we reveal that CdS–ZnS systems can form nearly thermodynamically stable crystal phases of Cd3ZnS4 (P-43m), CdZnS2 (P-4m2), and CdZn3S4 (P-43m), with low miscibility temperatures of 130 °C. By decomposing the formation enthalpy of the alloys into multiple contributions, it is found that electronic charge exchange plays a key role in controlling the weak thermodynamic instability of Cd1-xZnxS alloys. Phonon dispersion and elastic constants calculations were carried out to demonstrate the dynamical and mechanical stabilities of these structures. The bandgap in CdZnS alloys is tunable from wide direct bandgaps of 2.54eV (CdS) → 2.70eV (Cd3ZnS4) →2.93eV (CdZnS2) → 3.33eV (CdZn3S4) → 3.86eV (ZnS), with a nonlinear dependence on the composition. We found a medium-sized optical bandgap bowing of ∼1eV and ∼0.7eV; while considering respectively the wurtzite (WZ) and zinc-blend (ZB) crystal phases as reference of the compounds parents CdS and ZnS. The bowing has a remarkable contribution due to the structural volume-deformation effect; whereas charge exchange contribution is found relatively weak.
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