X-ray diffraction and optical properties of a noncentrosymmetric borate CaBiGaB2O7

Single crystals of the noncentrosymmetric borate CaBiGaB2O7 were synthesized by conventional solid state reaction. The purity of the crystal was checked by x-ray powder diffraction. The optical properties were measured by analyzing the diffuse reflectance data obtained with a Shimadzu UV-3101PC double-beam, double-monochromator spectrophotometer. We find a steep absorption edge confirming its semiconducting nature. The optical band gap obtained by extrapolation of a linearlike absorption edge was roughly 2.9 eV consistent with the observed pale yellow color of the sample. Theoretical calculations based on the structural model built from our measured atomic parameters have been performed using the all-electron full potential linearized augmented plane wave method. The generalized gradient approximation (GGA) of the exchange correlation potential as given by Engel-Vosko GGA is used. The frequency-dependent complex dielectric function was calculated and the origin of some of the spectral peaks is discussed. The linear optical properties show strong uniaxial anisotropy and birefringence that favors large second order susceptibility. Our calculations show that the complex second order nonlinear optical susceptibility tensor chi((2))(322)(omega) is the dominant component having the largest total Re chi((2))(322)(0) value of about 1.8 pm/V.