Electronic structure ofB2O3glass studied by one- and two-dimensional electron-spin-echo envelope modulation spectroscopy

Abstract

${\mathrm{B}}_{2}{\mathrm{O}}_{3}$ in glass and crystalline states have been subjected to \ensuremath{\gamma} irradiation at room temperature and subsequently studied by continuous-wave electron paramagnetic resonance and electron-spin-echo envelope modulation (ESEEM) spectroscopy at liquid-helium temperature. The ESEEM study of ${\mathrm{B}}_{2}{\mathrm{O}}_{3}$ revealed the existence of weak magnetic couplings between the irradiation-induced paramagnetic centers and nearby ${}^{10}\mathrm{B},$ ${}^{11}\mathrm{B}$ nuclei. The assignment of the observed couplings was achieved by the use of the two-dimensional hyperfine sublevel correlation spectroscopy. A detailed theoretical analysis of the ESEEM spectra and a $S=\frac{1}{2},$ $I=\frac{3}{2}$ system in both time and frequency domain is also presented. Computer simulation of the spectra, Hartree-Fock self-consistent field, and modified neglect of differential overlap calculations revealed that the unpaired electron in the ${\mathrm{B}}_{2}{\mathrm{O}}_{3}$ glass is associated with a dangling bond of oxygen attached to a boron of a boroxol ring. The paramagnetic centers of the ${\mathrm{B}}_{2}{\mathrm{O}}_{3}$ crystal are associated with oxygen dangling bonds in boron trigonal units. Pertinent structural models for the glass ${\mathrm{B}}_{2}{\mathrm{O}}_{3}$ are examined at a microscopic level.