BONDING AND DYNAMICAL PHENOMENA IN MGO - A HIGH-TEMPERATURE O-17 AND MG-25 NMR-STUDY

We have measured the isotropic chemical shifts (delta(iso)) and the spin-lattice relaxation times (T1) for O-17 and Mg-25 in MgO from room temperature up to 1300-degrees-C. The O-17 chemical shifts increase linearly from 47 ppm at room temperature to 57 ppm at 1300-degrees-C, and over the same temperature range the Mg-25 chemical shift increases linearly from 25 to 27 ppm. These changes are not the result of changes in the bulk magnetic susceptibility of the samples, but may be due to increased orbital overlap which is the result of the increase in thermal vibration of the ions with temperature. In the case of Mg-25, the shift to lower shielding with increasing temperature is opposite to that expected from simple bond length versus chemical shift trends established for the oxides at room temperature. If this is a general phenomenon, high-temperature NMR data may be biased to lower shielding. Spin-lattice relaxation times (T1) were measured in order to study the energetics of defect motion. T1's for O-17 and Mg-25 exhibit similar behavior over the range of temperatures studied. Up to 800-degrees-C, T1's decrease gradually, but above 800-degrees-C, T1's drop rapidly, with slopes corresponding to apparent activation energies of 192 +/- 9 kJ/mol (2.0 +/- 0.1 eV) for O-17 and 151 +/- 6 kJ/mol (1.56 +/- 0.06 eV) for Mg-25. While direct comparison of these activation energies to those derived from diffusion or conductivity measurements is complicated, the similar behavior for both nuclei suggests their relaxation phenomena are related.