Electronic structures and optical properties of low- and high-pressure phases of crystalline B2O3

The high- and low-pressure phases of crystalline boron oxide B2O3 are investigated by first-principle calculations using a local-density approximation (LDA). Both phases are insulators with wide LDA band gaps of 6.20 eV for B2O3-I (low pressure) and 8.85-eV for B2O3-II (high pressure). The total density of states are calculated and resolved into atomic and orbital components. The bond strength and charge transfers in these two crystals are investigated by calculating the overlap populations and Mulliken effective charges. B2O3-II is found to be more ionic than B2O3-I It is also concluded that the sp(2) planar bonding in B2O3-I is stronger than the sp(3) tetrahedral bonding in B2O3-II. The optical conductivities, the dielectric functions, and the energy-loss functions are also calculated using the wave functions at a large number of k points in the Brillouin zone. The calculated static dielectric constants for B2O3-I and B2O3-II are 2.32 and 2.35, respectively. The gross features of the optical spectra for the two crystals are quite similar with one marked difference. The B2O3-II crystal shows substantial optical anisotropy while B2O3-I is optically more isotropic.