Radiation-induced defects in apophyllites. II. An O centre and related OO pairs in hydroxylapophyllite

A hole-like centre present in natural hydroxylapophyllite without any artificial irradiation has been investigated by single-crystal and powder electron paramagnetic resonance (EPR) spectroscopy at 290 K and 90 K, and by three-pulse electron spin echo envelope modulation (ESEEM) spectroscopy at 25 K. Calculated matrices g , A ( 29 Si), A ( 1 H), A ( 39 K) and P ( 39 K) suggest it to be an O − centre at the hydroxyl oxygen site and the unpaired electron in the 2 P z orbital. A series of weak satellite peaks accompanying the main absorption line in single-crystal EPR spectra have been shown to arise from four geometrically distinct pairs of neighbouring O − centres ( i.e. , biradicals). The spin Hamiltonian parameters of these O − −O − pairs provide further support for the O − model and its location at the hydroxyl oxygen site. The O − centre in hydroxylapophyllite most likely formed from natural radiation and can be enhanced by gamma-ray irradiation. It is annealed out at 300°C but can be restored readily by gamma-ray irradiation. The presence of these O − −O − pairs in the hydroxylapophyllite is probably attributable to a high abundance of the O − centre in this sample.