The high-and low-pressure phases of crystalline boron oxide B 2 O 3 are investigated by first-principle calculations using a local-density approximation ͑LDA͒. Both phases are insulators with wide LDA band gaps of 6.20 eV for B 2 O 3 -I ͑low pressure͒ and 8.85-eV for B 2 O 3 -II ͑high pressure͒. 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. B 2 O 3 -II is found to be more ionic than B 2 O 3 -I. It is also concluded that the sp 2 planar bonding in B 2 O 3 -I is stronger than the sp 3 tetrahedral bonding in B 2 O 3 -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 B 2 O 3 -I and B 2 O 3 -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 B 2 O 3 -II crystal shows substantial optical anisotropy while B 2 O 3 -I is optically more isotropic. ͓S0163-1829͑96͒03143-8͔

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