[TIO4]- IN ALPHA-QUARTZ STUDIED BY LOW-TEMPERATURE ELECTRON-PARAMAGNETIC RESONANCE

The centre [TiO4]-, formed by x-irradiation at 77 K in naturally occurring crystalline rose quartz, has been studied by single-crystal electron paramagnetic resonance (EPR) spectroscopy. Here the central 3d1Ti3+ ion exists within a slightly distorted oxygen tetrahedron. The hyperfine matrix of the low-abundance isotope Ti-49 was measured at 8 K and the g matrix was measured at several temperatures from 30 K down to 4 K. The analysis of the spin-population distribution within the centre showed that the unpaired spin is almost entirely in the Ti 3d orbital \3z2-r2>. Estimates of the spin-lattice relaxation time T1 and of the EPR linewidth in the temperature range 4-30 K were also obtained. T1 decreases exponentially with increasing temperature, presumably owing to phonon coupling to the low-lying excited state \xy> which is 8.1 meV (65 cm-1) above the ground state, via an Orbach process. At temperatures above approximately 15 K, the abnormally large EPR linewidth is predominantly caused by lifetime broadening through T1. The near degeneracy of \3z2 - r2> and \xy> indicate that [TiO4]- has almost tetrahedral symmetry.