THEORETICAL-STUDY OF THE NONLINEAR-OPTICAL PROPERTIES OF KTIOPO4 - EFFECTS OF TI-O-TI BOND ANGLES AND OXYGEN ELECTRONEGATIVITY

Second harmonic generation in the model system [TiO5--TiO6--TiO5]20- is studied using a perturbative sum-over-states formalism together with extended Huckel wave functions. The nonlinear response of the central TiO6 group, a distorted octahedron with its fourfold axis defined by the atoms 01--Ti--02, improves both with increasing Ti--O--Ti angle and with decreasing electronegativity of the chain oxygens. A small shift of charge toward the distant oxygen, O1, takes place as the Ti--O--Ti angle is increased, thereby enlarging the electronic third moment and with it the local hyperpolarizability. Reducing the electronegativity of the oxygens, by contrast, brings about a near-resonant enhancement of the hyperpolarizability, which occurs as energies of the highest occupied molecular orbitals are raised. These results are placed in the context of recent experimental measurements involving compounds isomorphic to KTiOPO4.