Nuclear Magnetic Resonance Studies of Imperfect Ionic Crystals

Abstract

The method of nuclear magnetic resonance is applied to the study of ionic crystals containing point defects, i.e., vacancy, interstitial, or impurity ions. Experiments are described which show that the presence of such defects can have marked effects on the resonance line and can lead to a complicated temperature dependence of its width. A brief theoretical discussion is given of two effects important if the nucleus whose resonance is observed has a large electric quadrupole moment: (1) line width due to static second-order quadrupole interaction with defects, and (2) contribution to the nuclear relaxation time ${T}_{1}$ by quadrupole interaction with fluctuating electric fields caused by diffusing defects. Experimental results on the Br resonances in AgBr are analyzed in detail in terms of these effects. It is shown that the quadrupolar relaxation time due to diffusing defects in this salt can become sufficiently short to cause lifetime broadening of the resonance line, in particular that the motion of Ag vacancies seems to lead to a characteristic minimum of ${T}_{1}$ at about 0\ifmmode^\circ\else\textdegree\fi{}C. The data also give some evidence for the association of defects in AgBr below 200\ifmmode^\circ\else\textdegree\fi{}K. Cases of motional narrowing of the resonance line caused by the diffusion of defects are illustrated by experimental results on NaCl and LiBr. The existence of the various effects discussed in connection with these experiments indicates that nuclear resonance techniques can provide a means of studying the behavior of point defects in solids, their motion and in some cases their association, from a rather microscopic point of view. The desirability of careful relaxation time measurements is pointed out.