Nuclear Magnetic Resonance Study of Dislocation Effects and Elastic Axial Compression Effects in Sodium Iodide Single Crystals

Abstract

The nuclear magnetic resonance (NMR) intensity and line shape for 127I in NaI single crystals have been examined as a function of the orientation of the crystals relative to the external magnetic field. The intensity of the 127I NMR varied as the crystals were rotated about 〈110〉 and 〈100〉 directions perpendicular to the magnetic field. Full intensity of the NMR line is observed when H0 is along a 〈100〉 direction. This variation in intensity cannot be explained by nuclear magnetic dipole-dipole interaction alone and is thus termed anomalous. The intensity anomaly has been found to be due to the changing amount of first-order quadrupolar broadening of the 127I NMR line as a crystal is rotated in the magnetic field; the quadrupole effects are attributed to oriented dislocations in these previously unstrained crystals. A theoretical treatment of the effects of oriented dislocations has been carried out. The angular dependence of the frequency splittings was calculated for the cases of H0 varying in a (100) or (110) plane, assuming that the dislocations were oriented preferentially along either 〈100〉 or 〈110〉 directions. A comparison of the calculated angular dependence and the experimental angular dependence was made to determine which was the preferred orientation of both screw and edge dislocations. It was found that the best agreement with experiment is obtained if the screw dislocations are oriented with their axis along 〈100〉 directions and edge dislocation lines are oriented along 〈110〉 directions. Since the total intensity of the 127I NMR line is observed when H0 is along a 〈100〉 direction, it was possible to compress NaI crystals while a 〈100〉 direction was parallel to H0 and introduce quadrupolar broadening due to the strain-induced electric field gradients. The NMR lines were examined as the crystals were compressed up to about 3.5 × 107 dyn/cm2. With small pressure the NMR lines broadened and then returned to their original width when the weights were removed. Such compression experiments were continued to higher pressures at which the crystals obtained a permanent change as measured by the fact that the NMR line did not return to its original height or width. No width change was noticed if the magnetic field was along any crystal direction departing significantly from a 〈100〉 direction. From these compression experiments, the value (∼4) of the electrical polarizability of the iodine ion has been estimated.