Microscopic Study of Metal Hydrides Using Electron Spin Resonance
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MICROSCOPIC STUDY OF METAL HYDRIDES USING ELECTRON SPIN RESONANCEt
E. L. VENTURINI Sandia National Laboratories*,
Albuquerque, NM 87185
ABSTRACT Electron spin resonance (ESR) of dilute paramagnetic ions in nonmagnetic metallic hydrides provides microscopic information about the hydrogen ions in the immediate vicinity of the impurity. By comparing ESR spectra for different host metals and several hydrogen/metal ratios, one can determine material properties including host lattice symmetry, phase boundaries and occupation of available sites by hydrogen. Examples are presented of ESR of dilute Er in group IIIB and IVB metal hydrides, demonstrating the sensitivity and versatility of ESR as a spectroscopic technique.
INTRODUCTION Microscopic information concerning lattice symmetry, phase boundaries and the location, net charge and site energy of hydrogen ions in metals and metal hydrides is of fundamental importance in understanding these materials and Several optimizing their macroscopic properties for specific applications. spectroscopic techniques have been applied to this problem including nuclear magnetic resonance [1], Mbssbauer resonance [2], inelastic neutron scattering In this paper several features of electron spin [3] and photoemission [4]. resonance (ESR) of dilute paramagnetic ions in nonmagnetic metallic hydrides are discussed to demonstrate the sensitivity and versatility of this method in answering the questions posed above. In a typical ESR experiment one measures the derivative of the absorbed microwave power at a fixed frequency versus the applied magnetic field strength. The position of the maximum absorbed power is termed the resonance field, and is usually specified by the ESR g-factor g = 0.7145x(frequency in MHz)/(resonance All samples were prepared by arc-melting 0.1 atomic % Er into field in Oe). purified Sc, Y or Zr, followed by surface cleaning and then hydriding in a The polycrystalline samples were then ground into modified Sieverts' apparatus. a fine powder, and their derivative ESR absorption spectra recorded at 9.8 GHz The combination of dilute Er and low temperature and helium temperatures. assures narrow absorption linewidths, facilitating the detailed spectral analysis presented in the next three sections. HOST LATTICE SYMMETRY The ideal metal dihydride MH2 has the face-centered-cubic
(fcc) fluorite
structure, where the metal atom is surrounded by eight nearest-neighbor (nn) hydrogen ions forming a simple cube as shown by the solid circles in Fig. 1. These hydrogen positions are termed tetrahedral (T) sites. The six [5,6,7] next-nearest-neighbor (nnn) octahedral (0) sites shown as open circles are vacant in the ideal MH2 lattice, and fully occupied in the trihydride MH3 . When a paramagnetic Er ion is randomly substituted for a host metal atom, its ESR spectrum reflects the local crystal field arising primarily from the nn and Our spectra show clearly the effect of small distortions in nnn hydrogen ions. the nn cube or the partial occupation of nnn O-sites by hydrogen.
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