Comparative Analysis of the Electronic Structure and Nonlinear Optical Susceptibility of α-TeO2 and β-TeO3 Crystals

The hypothesis of the huge optical nonlinearity of the crystalline TeO3, recently advanced on the basis of the quantum mechanical simulations, is tested. Electronic band gaps of alpha-TeO2 and beta-TeO3 crystals are determined by diffuse reflectance measurements. The DFT+U method is applied to calculate electronic band gap E-g and the third-order nonlinear dielectric susceptibility chi((3))). The chi((3))(TeO3) is about two times lower than the chi(3)(TeO2) in spite of the fact that the E-g(TeO3) is narrower than E-g(TeO2). It is shown that this peculiarity is related to the 5s(Te) electronic states which are occupied in TeO2 and are vacant in TeO3. This distinction is due to the specific electronic state related to the electron lone pairs localized on the Te(IV) atoms.