NMR Techniques for Studying Ionic Diffusion in Solids
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NMR TECHNIQUES FOR STUDYING IONIC DIFFUSION IN SOLIDS DAVID C. AILION Department of Physics, University of Utah, Salt Lake City, UT 84112 ABSTRACT A survey of NMR relaxation time techniques for studying ionic diffusion in solids is presented. Particular emphasis is placed on discussing the kinds of information obtainable from T1, Tlp, TlD, and TlD' measurements. Applications to the study of local and nonlocal diffusion, diffusion in weakly magnetic systems, and motions between unequal potential wells are described. Relaxation due to fluctuating dipolar, quadrupolar, and chemical shift anisotropy interactions is discussed. INTRODUCTION Nuclear magnetic resonance (NMR) has long been used in the study of atomic diffusion and molecular reorientations in solids. These investigations have been applied to ionic diffusion in insulators [1], metals [2], superionic materials [3], and amorphous materials [4]. NMR is not restricted to cases where there is mass transport as in translational diffusion, but has been extensively applied to the study of reorienting molecules [5] and to localized (bound) diffusion [6]. In this paper, the principal NMR techniques for studying atomic motions will be discussed. Examples will be given which illustrate the potential and limitations of these techniques. The great strength of NMRis that the resonance signal is characteristic of the particular nucleus being studied. This features arises from the fact that the nuclei of many elements are characterized by intrinsic magnetic moments; accordingly, in a magnetic field Ho, the resonance frequency fo equals y Ho/2w [7], where the gyromagnetic ratio y is a constant which is different for different nuclei [8]. For this reason, NMR can normally distinguish the diffusion of a specific nuclear species from that of other nuclei. In contrast, techniques like dielectric loss and ionic conductivity measure properties which do not belong exclusively to the nuclei whose motions are of interest. Furthermore, the immediate surroundings of a nucleus may affect measurable NMR properties like linewidth, relaxation time, and even the resonance frequency, Accordingly, NMRtechniques can be used to study the environment of the nuclei and thereby elucidate the microscopic details governing atomic motions. There are two problems facing a researcher who wishes to use magnetic resonance to study ionic diffusion in solids. The first is that NMR is a bulk technique whose sensitivity is proportional to the number of identical nuclei being studied and to their y. Accordingly, its application to the direct study of nuclei of low concentration (e.g., impurities and surface atoms) and to nuclei of low y is more limited. As a result, these nuclei are usually studied indirectly, by observing their effect on the resonance of
56 other atoms [9]. A second problem which occurs to some extent in all solids is that the linewidth is usually quite large (- 1-100 gauss), in contrast to the narrow line characteristic of liquids. This large linewidth arises in solids from magnetic dip
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