Centrosymmetric to noncentrosymmetric structural transformation of new quaternary selenides induced by isolated dimeric [Sn2Se4] units: from Ba8Ga2Sn7Se18 to Ba10Ga2Sn9Se22

A noncentrosymmetric structure is a prerequisite of many important properties, such as ferroelectric, piezoelectric and nonlinear optical properties. The design synthesis of a noncentrosymmetric compound is highly desired but it is a great challenge. Herein, two new quaternary selenides, Ba8Ga2Sn7Se18, 1 and Ba10Ga2Sn9Se22, 2, have been discovered by solid-state reactions. Compound 1 crystallizes in the centrosymmetric space group Pnma (no. 62) with a = 12.466 (9) angstrom, b = 9.358 (6) angstrom, c = 18.14 (2) angstrom, V = 2116 (3) angstrom(3), Z = 2; and 2 crystallizes in the noncentrosymmetric space group Cmc2(1) (no. 36) with a = 9.384 (2) angstrom, b = 44.834 (6) angstrom, c = 12.416 (2) angstrom, V = 5224 (2) angstrom(3), Z = 4. The major motif of both 1 and 2 is the [Ga2Sn7Se18](infinity)(16-) ladder chain built by alternately jointed GaSe4 tetrahedra and SnSe4 tetragonal pyramids, and to each of these polyhedra, an accessory (SnSe3 pyramid or Sn2Se4 dimer) is attached. Interestingly, the noncentrosymmetric structure of 2 is the combination of centrosymmetric 1 and the imbedded isolated dimeric [Sn2Se4] with a C-2v polar symmetry that separates every two such ladder chains. 1 and 2 directly illustrate that a simple combination reaction can drive the centrosymmetric to noncentrosymmetric structural transformation.