B-11 NMR AND RELAXATION STUDY OF BORON-NITRIDE

The different allotropic forms of boron nitride have been investigated by B-11 NMR and spin-lattice relaxation measurements. In the material with hexagonal symmetry the central transition is broadened by second-order quadrupole perturbation and the related quadrupole frequency nu(Q) = e(2)qQ/2h is found to be 1.50 +/- 0.05 MHz, a result interpreted on the basis of charge transfer from B to N. The study of the B-11 spin-lattice relaxation shows that the dominant mechanism is related to the presence of paramagnetic centers, i.e., nitrogen vacancies, previously investigated by electron paramagnetic resonance. The relaxation mechanism due to lattice vibrations is relevant only in samples with a very low concentration of paramagnetic centers. An analysis of temperature and field dependences of the magnetic relaxation rates is carried out in terms of the competing processes of spin-diffusion-driven and electronic-relaxation-driven mechanisms. Enlightening information on the relaxation regimes and the spin dynamics of the paramagnetic centers is obtained from a comparison of the experimental findings with the theoretically expected behaviors.