Electronic structure and optical properties of BiSeI were investigated with ab initio calculations based on density-functional theory (DFT). The full potential linearized augmented plane wave (FP-LAPW) method was used with the generalized gradient approximation (GGA). The complex dielectric function and optical constants, such as optical absorption coefficient, refractive index, extinction coefficient, energy-loss spectrum and reflectivity, were calculated. It is shown that the p-states of Bi, Se and I play an important role in absorption and reflectivity constants. The calculated optical constants of BiSeI exhibit anisotropy in two directions (x- and c-axes). The experimental optical dielectric functions for single-crystal BiSeI were determined using spectroscopic ellipsometry. The ellipsometry results were analyzed using the approximation of a pseudodielectric function (PDF). This method shows that experimental spectra consist of a four Lorentzian lines sum. The Lorentzian line parameters are obtained by minimization using the mean-square method of the difference between the calculated and measured spectra of the dielectric constants. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
