Electron spin resonance studies of defect centers containing boron atoms in natural danburite crystals

X-band ESR studies at liquid nitrogen temperature of a naturally occurring danburite single crystal are presented. The prinipal values of the g˜- and A˜-tensors for the various complexes, as well as their direction cosines are evaluated from the data fitting simultaneously resonance line positions obtained in three mutually perpendicular planes. The spectra are due to a hole trapped on the oxygens surrounding a boron atom substituting for silicon in the eight possible substitutional sites of the unit cell with almost equal probability. An interpretation, based on molecular-orbital theory, is provided.