Hydrides Examined by Nuclear Magnetic Resonance
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R. G. BARNES Ames Laboratory-USDOE and Department of Physics, University, Ames, Iowa 50011
Iowa State
ABSTRACT Nuclear magnetic resonance (NMR) methods offer many opportunities for studying both crystal and electronic structure in hydrides, as well as the changes that occur in these at phase transitions. NMR also affords an almost uniquely powerful approach to the study of hydrogen diffusion in hydrides. The NMR of all three hydrogen isotopes can be utilized, as well as of a range of technolgically significant metals. Single crystals are not required, samples need not be single-phase, and measurements can be conveniently made over a wide range of temperatures.
INTRODUCTION Nuclear magnetic resonance (NMR) methods offer many opportunities for obtaining information on crystal structure and atom locations and on electronic structure in hydrides, as well as on the changes that occur in these properties at structural and electronic phase transitions. In addition, NMR methods afford an almost uniquely powerful approach to the study of hydrogen motion (diffusion) in hydrides. Nuclei interact with the microscopic local environment via their magnetic dipole and electric quadrupole moments, and since these moment properties are invariable for a given nuclear species, the interactions serve to probe the magnitudes and symmetries of the electric and magnetic fields at the nuclear sites. A variety of experimental techniques may be utilized, including These may be both steady-state (wide-line) and transient (pulsed measurements. 3 applied to study the NMR of all three hydrogen isotopes (IH, 2D, T) as well as the NMR of a considerable number of transition metals. Important practical features of NMR methods relevant to the case of metal hydrides include the fact that single crystal specimens are not required, that samples need not be single phase, that measurements can be conveniently made over a wide range of temperatures, and that information pertaining to both the metal and hydrogen sites in the structure can frequently be obtained in a given hydride. In this brief survey we illustrate with examples the capabilities of NMR in studying metal hydride systems. Recent review of these topics should be consulted for greater detail and thorouqhness [1,2]. HYDROGEN LOCATIONS AND STRUCTURAL PHASE TRANSITIONS A problem fundamental to most other metal-hydrogen system studies is that of determining the locations of the hydrogen in the metal lattice. Whether one is concerned with hydrogen solubility in the a-phase, with a simple binary hydride phase, or with a more complex intermetallic (ternary) hydride, accurate knowledge of hydrogen locations is frequently essential to the interpretation of other measurements (e.g., optical) and to formulating correct theoretical descriptions (e.g., band structure). Since hydrogen is essentially undetected by x-rays, the principal experimental methods available for its location are neutron diffraction and nuclear magnetic resonance. NMR provides information on
20 hydrogen locations and structure in
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