Synthesis, Crystal Structures, Optical Properties and Theoretical Calculations of Two Metal Chalcogenides Ba2AlSbS5 and Ba2GaBiSe5

Two quaternary metal chalcogenides, Ba2AlSbS5 and Ba2GaBiSe5, were successfully synthesized by solid-state reaction in sealed silica tubes. Both of them crystallize in the same orthorhombic space group Pnma, but they appear with obviously different construction features. For example, Ba2AlSbS5 exhibits [SbS3]3− units and zero-dimensional (0D) [AlSbS5]4− clusters, which is different from those ([BiSe6]3− units and 1D ∞[GaBiSe5]4− chains) of Ba2GaBiSe5. We also systematically investigated the entire series of Ba2MIIIMIII’Q5 (MIII = Al, Ga, In; MIII’ = As, Sb, Bi; Q = S, Se, Te) compounds, and the results showed that the interconnection of MIIIQ4 and MIII’Qn (n = 3, 5, 6) units can form three different structural types, including 0D [MIIIMIII’Q5] clusters, single [MIII’Q3] chains and isolated [MIIIQ4] units, or [MIIIQ3]n and [MIII’Q3]n double chains, which may be induced by the flexible coordination and on-link modes of MIII’ atoms. Spectral investigation shows that their bandgaps are about 2.57 eV for Ba2AlSbS5 and 2.14 eV for Ba2GaBiSe5. Theoretical calculation was also used to analyze their structure-property relationships, and the results indicate that the title compounds exhibit larger birefringences (Δn > 0.10), thus having potential as the IR birefringent materials.