Out-of-Plane Rashba Spin-Splitting and Structure-Property Relationships in Layered Cu2AMVSe4 Chalcogenides (A = K, Rb, Cs; MV = V, Nb, Ta)

Abstract

Recent research targeting functional multinary chalcogenide semiconductors (MCSs) has successfully demonstrated the tailoring of structure-property-application relationships within the I₂-II-IV-X₄ material family (Roman numerals I, II, and IV refer to the oxidation states of the constituent elements; X refers to a chalcogen). To apply and expand upon the experience gained from these systems, we utilize a combined computational and experimental approach to investigate nine members of a compositionally analogous I₂-I′-V-X₄ family, incorporating a coupled substitution of an alkali (I′ = K, Rb, Cs) and a pentavalent transition metal (V = V, Nb, Ta) for the II and IV sites, respectively, while I = Cu and X = Se. Four previously unreported compounds in this set adopt a layered noncentrosymmetric (Ama2 space group) structure, analogous to that of existing family members. One compound, Cu₂CsVSe₄, instead forms a related Pna2₁ lattice. All nine compounds show strong absorption with direct bandgaps ranging from 1.2–2.5 eV, appropriate for potential applications involving optoelectronics and (due to inversion asymmetry) spin-optoelectronics. The density functional theory (DFT) study concludes that the heavy-element (e.g., Ta) containing members exhibit significant out-of-plane Rashba spin splitting of up to 96 meV at the conduction band minimum, suggesting promise for further examination of spin behavior and control. Spin splitting parameters progressively increase across the series V→Nb→Ta, consistent with increased spin–orbit coupling. Optoelectronic properties (e.g., the bandgap) in this series depend primarily upon the identity of the 5+ transition metal ion, while the size of the alkali spacer ions primarily impacts spin-splitting behavior. Finally, thermal analysis studies highlight stability for the considered compounds up to ∼650 °C.