Nuclear Magnetic Resonance in Alloys
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NUCLEAR MAGNETIC RESONANCE IN ALLOYS L. H. Bennett National Bureau of Standards, Metallurgy Division, Washington, D.C.
20234
ABSTRACT Many papers on NMRin alloys are addressed to NMR specialists rather than to metal or alloy specialists, and talk about the "potential" of NMRfor application to alloys. This presentation emphasizes a review of some useful results of NMRexperiments in alloys, including applications in diffusion, phase diagrams, magnetic materials, ordering in intermetallic compounds, liquid alloys and amorphous alloys. INTRODUCTION There are three distinct "branches" of nuclear maqnetic resonance (NMR). Roughly speaking, these are: molecular chemistry, nonmetallic solids, and alloys. The most developed of these from the point of view of commercial, well-engineered equipment is the first -- molecular chemistry, growing out of the early days of high-resolution NMR. The application of NMRto chemical molecules is advanced and sophisticated and its use is widespread, with multipulse techniques routine, and the operation of the equipment "black-box". NMRthus has joined infra-red and other spectrocopies as a tool in chemical manufacturing as well as in research. The second branch -- non-metallic solids, is rapidly catching up. In this area, I include new techniques such as NMRimaging (or zeugmatography or NMR"diffraction") and applications to relaxation processes in polymers and a host of other such developments. In this branch, as in the first, 1H and 13 C resonances predominate. The third branch -- the application of NMRto the study of alloys, is the theme of this talk. Here the situation is quite different. Although there is available high-quality, commercial equipment, most laboratories doing NMR in alloys have at least partially home-made apparatus, designed to perform some specialized function. Most papers on NMRin alloys are written for the NMR specialist, not for the user. Many papers still talk, after about thirty years, of the "potential" of NMR for application to alloys. In this talk, I will highlight some particular applications of NMRto alloys which are important to the material scientist who has interests other than NMR. But first I want to say a few words about what is, in some sense, the most important application of NMRin alloys, namely, to the understanding of the theory of alloys. NMRis most powerful when it tests detailed, careful, first principle theories of alloys. In my opinion, for a theory to be finally accepted, it must be able to reproduce the NMRresults. For example, consider an energy-band calculation for an intermetallic compound. Many such calculations give controversial results for the charge transfer from atom A to atom B, or vice-versa. Often, there is charge left over, in the muffin-tin region. How can such a theory be tested? It should be required to predict NMRmeasurables such as the Knight shifts, relaxation times, and quadrupole interactions for both atoms A and B. Such calculations are feasible but they require more care and cost to perform, including taking proper
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