Radiation-induced defects in apophyllites; II, An O (super -) centre and related O (super -) -O (super -) 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( (super 29) Si), A( (super 1) H), A( (super 39) K) and P( (super 39) K) suggest it to be an O (super -) centre at the hydroxyl oxygen site and the unpaired electron in the 2P (sub 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 (super -) centres (i.e., biradicals). The spin Hamiltonian parameters of these O (super -) -O (super -) pairs provide further support for the O (super -) model and its location at the hydroxyl oxygen site. The O (super -) centre in hydroxylapophyllite most likely formed from natural radiation and can be enhanced by gamma-ray irradiation. It is annealed out at 300 degrees C but can be restored readily by gamma-ray irradiation. The presence of these O (super -) -O (super -) pairs in the hydroxylapophyllite is probably attributable to a high abundance of the O (super -) centre in this sample.