Sodium-Stuffed Open-Framework Quaternary Chalcogenide Built with (Cu₂Ga₆S₁₈)^16- Ribbons Cross-Linked by Unusual...

A quaternary compound, Na₁₅Cu₃Ga₆S₁₈, the first member in the A-Cu-Ga-S (A = alkali metal) series, has been synthesized from a solid-state metathesis reaction between Na₆Ga₂S₆ and CuCl as well as from a combination of Na₂S, Ga, Cu, and S. The compound crystallizes in a monoclinic crystal system, space group C2/c, and represents a unique open-framework structure with channels filled with eight crystallographically distinct Na ions. The anionic framework is built up of infinite chains of corner-shared GaS₄ tetrahedra fused together by an edge-shared dimer of CuS₄ tetrahedra forming one-dimensional ribbons of (Cu₂Ga₆S₁₈)^16-, which are cross-linked by linearly coordinated S-Cu-S linkages resulting in a three-dimensional network with tunnels filled with Na atoms. Optical band gap measurements show that the compound has a direct band gap of 3.00 eV that is in good agreement with the theoretical band gap derived from density functional theory calculations. Band structure calculations further indicate that the states near the Fermi level are dominated by tetrahedral Cu⁺(d) and S(p) states resulting from the antibonding interactions, while s-d hybridization is prevalent in linear Cu⁺ coordination. Ionic conductivity measurements show that the compound has a room-temperature Na ion conductivity of 2.72 × 10^-5 mS/cm with an activation energy of 0.68 eV, which corroborates well the nudged elastic band calculations.