ESR of X-ray irradiated lithium boracites

ESR spectra of paramagnetic defects induced by X-ray irradiation in lithium chloroboracites and bromoboracites have been measured for the first time, at 9 GHz and room temperature. ESR parameters are deduced by computer simulations of the spectra for polycrystalline powders and a single crystal. The paramagnetic center is characterized by an isotropic g — factor and an anisotropic hyperfine interaction with a boron nucleus. It is shown to be an unpaired electron trapped in a vacant site of the network and localized on a sp³ boron orbital. The defect is similar to the boron electron center (BEC) detected in borate glasses.