New Layered Tin(II) Thiophosphates ASnPS4 (A = K, Rb, Cs): Synthesis, Structure, Glass Formation, and the Modulated CsSnPS4

The layered compounds KSnPS4 (1), RbSnPS4 (2), and CsSnPS4 (3) were synthesized using the chalcophosphate flux technique at high temperature and are rare examples of divalent Sn­(II) thiophosphates. Orange polyhedral crystals of compound 1 crystallize in the monoclinic space group P21/c with a = 6.6673(13) Å, b = 11.9697(24) Å, c = 8.7604(18), and β=127.347(8)° in a 2-dimensional layered structure. Compound 2 is isostructural to 1. Yellow block shaped crystals of compound 3 crystallize in the monoclinic superspace group P21(αβ0)0 with a commensurate q-vector at 1/4a* + 1/4c* with a = 18.0477(14) Å, b = 6.2021(5) Å, and c = 6.8415(5) Å. The structure of all three compounds contains SnS3 pyramids, which is an extremely rare solid state chalcogenide coordination environment. All three compounds are semiconductors having well-defined band-gaps between 2.0 and 2.2 eV. The compounds are congruently melting and can be obtained as glasses by rapid quenching of the melt, which subsequently crystallize upon heating.