ISSN:
1432-2021
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
Notes:
Abstract We have measured the isotropic chemical shifts (δiso) and the spin-lattice relaxation times (T1) for 17O and 25Mg in MgO from room temperature up to 1300° C. The 17O chemical shifts increase linearly from 47 ppm at room temperature to 57 ppm at 1300° C, and over the same temperature range the 25Mg 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 25Mg, 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 17O and 25Mg exhibit similar behavior over the range of temperatures studied. Up to 800° C, T1's decrease gradually, but above 800° C, T1's drop rapidly, with slopes corresponding to apparent activation energies of 192±9 kJ/mol (2.0±0.1 eV) for 17O and 151±6 kJ/mol (1.56±0.06 eV) for 25Mg. 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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00211854
Permalink