Electronic structure of LiNbO3: densities of states, optical anisotropy and spontaneous polarisation calculated from the Xα molecular orbital method

Abstract

The electronic structure of ferroelectric LiNbO3 at room temperature has been investigated using the X alpha molecular orbital method applied to a NbO6 cluster of C3v symmetry. The total and partial densities of states associated to the O 2p valence band built up from the X alpha eigenvalues are compared with XPS and UPS data as well as the calculated binding energies of the inner states. The band gaps E/sub /// and Eperpendicular to relevant to light polarised along or parallel to the spontaneous polarisation Ps, are obtained from the allowed O 2p-Nb 4d transitions. Moreover, the imaginary part of the dielectric constants epsilon 2// and epsilon 2 perpendicular to are estimated from these transitions and used to explain the strong anisotropy observed in the reflectivity data. A population analysis, performed from the electronic distributions in the cluster, leads to the cluster ionic charges. These latter are compared with the effective charges obtained from IR measurements and from point-charge model calculations. They set up the covalency of Nb-O bonds and allow the authors to estimate Ps.