Na4Ga8S14: a Ga-enriched wide band gap ternary alkali-metal sulfide with unique [Ga12S42] 12-membered rings

A Ga-enriched ternary alkali-metal sulfide Na4Ga8S14 has been synthesized by a high temperature solid-state reaction. It crystallizes in the centrosymmetric Pbca (no. 61) space group with cell parameters a = 13.5260(4) Å, b = 11.4979(3) Å, c = 29.9592(9) Å, and Z = 8, and exhibits a three-dimensional (3D) network structure constructed from unique [Ga12S42] 12-membered rings, one-dimensional ∞[Ga4S11] chains, individual [GaS4] units and Na+ ions. The experimental band gap of Na4Ga8S14 was measured as ∼3.57 eV. Theoretical calculations indicate that the title compound is a direct band gap compound and the band gap is mainly determined by [GaS4] units. Meanwhile, statistical analysis shows that the atomic ratio N (N = AIAII/Ga, where AI = alkali-metal, AII = alkaline earth-metal) can be used to regulate the connection of [GaS4] units from zero-dimensional (0D) isolated groups, one-dimensional (1D) chains, and two-dimensional (2D) layers to 3D frameworks in Ga-containing alkali- and/or alkaline earth-metal chalcogenides. The results enrich the diversity of alkali-metal sulfides and give an insight into the structural regulation of alkali- and/or alkaline earth-metal chalcogenides.