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

Abstract

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) Å, b = 9.358 (6) Å, c = 18.14 (2) Å, V = 2116 (3) ų, Z = 2; and 2 crystallizes in the noncentrosymmetric space group Cmc2₁ (no. 36) with a = 9.384 (2) Å, b = 44.834 (6) Å, c = 12.416 (2) Å, V = 5224 (2) ų, Z = 4. The major motif of both 1 and 2 is the [Ga₂Sn₇Se₁₈]_∞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.