Sr₃Zr₂Cu₄Q₉ (Q = S and Se): two novel layered quaternary mixed transition metal chalcogenides

Depending on their bandgaps, mixed metal layered chalcogenides are potential candidates for thermoelectric and photovoltaic applications. Herein, we reported the exploratory synthesis of Sr-Zr-Cu-Q (Q = S/Se) systems, resulting in the identification of two novel quaternary chalcogenides: Sr₃Zr₂Cu₄S₉ and Sr₃Zr₂Cu₄Se₉. These isoelectronic compounds (Sr₃Zr₂Cu₄Q₉) crystallized in two different structural types. The Sr₃Zr₂Cu₄S₉ structure (space group: P1̄) adopted the Ba₃Zr₂Cu₄S₉ structure type with eighteen unique atomic sites: 3 × Sr, 2 × Zr, 4 × Cu, and 9 × S. In contrast, the Sr₃Zr₂Cu₄Se₉ structure (P1̄) represented a unique structure type with nineteen unique atomic positions including one additional Cu site compared to the Sr₃Zr₂Cu₄S₉ structure. The sulfide structure was stoichiometric, whereas the selenide structure was found to be non-stoichiometric with three partially occupied Cu positions. The Sr₃Zr₂Cu₄Q₉ structures consisted of layers with the Sr²⁺ cations occupying the interstitial spaces. In both structures, the Zr atoms occupied distorted octahedral positions. A striking difference between the two structures resulted from the distinct bonding interactions between the Cu and Q atoms. The optical bandgap of polycrystalline Sr₃Zr₂Cu₄S₉ was 1.7(1) eV. Interestingly, resistivity measurements of polycrystalline Sr₃Zr₂Cu₄Se₉ revealed metallic/degenerate semiconducting behavior at low temperatures. The photovoltaic performance of semiconducting Sr₃Zr₂Cu₄S₉ demonstrated ∼24% increment in power conversion efficiency when incorporated into a TiO₂/CdS photoanode due to its narrower bandgap, which increased the light-harvesting ability of the cell. We also explored the theoretical electronic structures, COHP, and Bader charges of the Sr₃Zr₂Cu₄Q₉ structures using DFT calculations.