Origin of giant anisotropy in synthesized Ba pentaborates

Abstract

The electronic structure, charge density distribution, and linear-optical properties of the highly anisotropic crystal $\mathrm{Ba}[{\mathrm{B}}_{5}{\mathrm{O}}_{8}(\mathrm{OH}{)}_{3}]$ (BaBOH) are calculated using a self-consistent norm-conserving pseudopotential method within a framework of local-density approximation theory. Large anisotropies of the band energy gap (5.22 eV for the $\mathbf{E}\ensuremath{\parallel}\mathbf{b},$ 5.67 eV for the $\mathbf{E}\ensuremath{\parallel}\mathbf{c})$ and giant birefringence (up to 0.17) are obtained. Comparison of the theoretically calculated and experimentally measured polarized spectra of the imaginary part of dielectric susceptibility ${\ensuremath{\varepsilon}}_{2}$ shows good agreement. It is shown that the anisotropy of the charge density distribution and the optical spectra is caused prevailingly by the huge anisotropy between ${2p}_{z}$ $\mathrm{B}--{2p}_{z}$ O and ${2p}_{y,x}$ $\mathrm{B}--{2p}_{y,x}$ O bonding orbitals. The observed anisotropy in BaBOH is principally different from the $\ensuremath{\beta}\ensuremath{-}{\mathrm{BaB}}_{2}{\mathrm{O}}_{4}$ (BBO) single crystals. In the BaBOH single crystals the anisotropic optical and charge density distribution is caused by a different projection of the orbitals originating from particular borate clusters on the particular crystallographic axes, contrary to the BBO, where the anisotropy is originated mainly due to different local site symmetry of the oxygen within the borate planes. The observed anisotropy is analyzed within the band energy approach and space charge density distribution.