Mixed-anion chalcogenide-halide semiconductors A₂BaTaS₄Cl (A = K, Rb, Cs) and K₂BaNbS₄Cl

Four mixed-anion materials, K₂BaTaS₄Cl, Rb₂BaTaS₄Cl, Cs₂BaTaS₄Cl and K₂BaNbS₄Cl were synthesized via low temperature solid state synthesis at 848 K. All materials degrade upon melting to a mixture of the binary ionic salts, ACl, and ABaMS₄, two of which (KBaTaS₄ and CsBaTaS₄) are reported here for the first time. Single crystal X-ray diffraction revealed that K₂BaTaS₄Cl, Rb₂BaTaS₄Cl, and K₂BaNbS₄Cl crystallize in the tetragonal space group I4/mcm, adopting the Cs₃CoCl₅ structure type. In this phase, isolated [MS₄]^3- tetrahedra are encapsulated within a (K, Rb)/Ba-Cl ionic cage. Although structurally similar, Cs₂BaTaS₄Cl crystallizes in the orthorhombic space group Ibam, arising from a supercell with a √2 expansion along the a-axis and a 2√2 expansion along the b-axis due to extended Cs and Ba ordering. Consistent with electronic structure calculations, the measured bandgaps of the Ta-based compounds are ∼3.0 eV while the Nb-based compounds exhibit a reduced bandgap of ∼2.5 eV, reflecting the lower energy of the Nb d orbitals.