Crystal structure of piezoelectric nonlinear-optic AgGaS2

Abstract

Silver thiogallate, AgGaS₂, is a representative member of the ${\mathrm{A}}^{\mathrm{I}}{\mathrm{B}}^{\mathrm{III}}{\mathrm{C}}_2^{\mathrm{VI}}$ family with chalcopyrite structure. AgGaS₂ is strongly piezoelectric and is phase matchable for second harmonic generation. The lattice constants of this tetragonal crystal are a = 5.75722 ± 0.00003 and c = 10.3036 ± 0.0002 Å at 298 °K. The space group is I 42d, with four formulas per unit cell. The integrated intensities of 1978 reflections were measured within a hemisphere of reciprocal space having radius (sin θ)/λ = 1.02 Å⁻¹ using PEXRAD, resulting in 268 symmetry‐independent structure factors. The x coordinate of sulfur (0.2908 ± 0.0004) is 0.088 Å less than the value calculated on the assumption of a geometrically regular tetrahedral bond distribution about the B atom. By contrast, four ${\mathrm{A}}^{\mathrm{II}}{\mathrm{B}}^{\mathrm{IV}}{\mathrm{C}}_2^{\mathrm{V}}$ compounds previously reported have x coordinates for the C^v atom either slightly greater than (for ZnSiP₂, CdSiP₂, and ZnSiAs₂), or not significantly different from (for ZnGeP₂) that calculated. The experimental Ag–S distance is 2.556 ± 0.001 Å, and Ga–S is 2.276 ± 0.001 Å: the S–Ga–S angle bisected by the c axis is 111.1 ± 0.1°.