Syntheses and structural characterization of non-centrosymmetric Na2M2M′S6 (M, M′ = Ga, In, Si, Ge, Sn, Zn, Cd) sulfides

Seven new non-centrosymmetric Na2M2M'S-6 sulfides, namely, Na2Sn2ZnS6(1), Na2Ga2GeS6(2), Na2Ga2SnS6(3-alpha), Na2Ga2SnS6(3-beta), Na2Ge2ZnS6(4), Ha(2)Ge(2)CdS(6)(5), Na2In2SiS6(6) and Na2In2GeS6(7), were synthesized by high temperature solid state reactions and structurally characterized by single crystal X-ray diffraction. They crystallize in non-centrosymmetric Fdd2 and Cc space groups and their three-dimensional [M2M'S-6](2-)framework structures consist of MS4 and M'S-4 tetrahedra corner-connected to one another in either orderly or disordered fashion. Sodium ions reside in the tunnels of the anionic framework. Compounds 1, 2 and 3-alpha have the structure of known Li2Ga2GeS6, whereas compounds 6 and 7 are isostructural with known Li2In2GeS6 compound. Isostructural compounds 4 and 5 represent a new structural variant. Compounds 3-alpha and its new monoclinic structural variant 3-beta have disordered structural framework. All of them are wide band gap semiconductors. Na2Ga2GeS6(2), Na2Ga2SnS6(3), Na2Ge2ZnS6(4) and Na2In2GeS6(7) compounds are found to be second-harmonic generation (SHG) active. Compounds 1, 2 and 3-alpha melt congruently. (C) 2016 Elsevier Inc. All rights reserved.