Sulfides with Strong Nonlinear Optical Activity and Thermochromism: ACd4Ga5S12 (A = K, Rb, Cs)

Noncentrosymmetric quaternary sulfides, ACd4Ga5S12 (A = K, Rb, Cs) have been synthesized by a modified reactive flux method with high yields. Single-crystal X-ray diffraction analyses revealed they adopted a known KCd4Ga5S12-structure type in the trigonal space group R3 (No. 146) with cell parameters of a = 13.7905(5)–13.8639(5) Å, c = 9.3304(6)–9.3974(6) Å and Z = 3. Each asymmetric MS4 tetrahedral building unit (centered by disordered Cd and Ga) is aligned in the same direction along the c axis and connected via vertexes forming a chiral 3D framework, inside which the cuboctahedral S12 cavities accommodate A cations. These compounds have been discovered to exhibit the second largest powder second harmonic generation (SHG) intensities among sulfides to date, about 12, 11, and 10 times commercial AgGaS2 in the particle size of 46–74 at 2.05 μm incident laser with comparable wide transparency ranges (0.42–25 μm). Besides, their intense luminescence emissions around 590 nm shift to about 520 nm from 298 to 10K indicating thermochromism properties. The Vienna ab initio theoretical studies have aided the understanding of electronic structures and linear and nonlinear optical properties.