Piezoelectric nonlinear optic CuGaSe2 and CdGeAs2: Crystal structure, chalcopyrite microhardness, and sublattice distortion

Abstract

CuGaSe₂ and CdGeAs₂ crystallize in the chalcopyrite‐type structure. The lattice constants of tetragonal CuGaSe₂ are a = 5.5963 ± 0.0001 and c = 11.0036 ± 0.0002 Å at 298 °K, those of CdGeAs₂ are a = 5.9432 ± 0.0001 and c = 11.2163 ± 0.0003 Å at 298°K. The space group is I4̄2d with four formulas in a unit cell. 1747 reflections were measured with PEXRAD for CuGaSe₂ and 1892 for CdGeAs₂ using MoKα x radiation. Least squares refinement of the structural parameters using 149 symmetry‐independent CuGaSe₂ and 300 CdGeAs₂ structure factors led to final agreement factors of 0.035 and 0.038, respectively. CuGaSe₂ is slightly Cu deficient: the value of the x coordinate in the final model is 0.2431 ± 0.0002, with considerable vibrational anisotropy. CdGeAs₂ is stoichiometric: the final model has x = 0.2785 ± 0.0002, also with significant vibrational anisotropy, but less than that in CuGaSe₂. The two compounds confirm the pattern of sublattice distortion in ${\mathrm{A}}^{\mathrm{I}}{\mathrm{B}}^{\mathrm{III}}{\mathrm{C}}_2^{\mathrm{VI}}$ and ${\mathrm{A}}^{\mathrm{II}}{\mathrm{B}}^{\mathrm{IV}}{\mathrm{C}}_2^{\mathrm{V}}$ chalcopyrites previously found. The closest Cu–Se distance is 2.385 Å, Ga–Se is 2.435 Å, Cd–As is 2.629 Å and Ge–As is 2.430 Å, all ± 0.001 Å. The effective Debye characteristic temperatures are simply related to the reported microhardnesses of the chalcopyrites.