A 207 Pb NMR Study of Beta''-Alumina
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207Pb
NMR STUDY OF BETA"-ALUMINA
BRUCE M. SASS*, BRYAN H. SUITSt AND DAVID WHITE* *Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323 tDepartment of Physics, Michigan Technological University, Houghton, MI 49931 ABSTRACT We report results of a nuclear magnetic resonance (NMR) study of lead(II) cations in P"-alumina. Measurements of line positions and spin-lattice relaxation times (T 1 ) as a The results show that Pb ions possess both function of temperature are presented. static and dynamic disorder in low and intermediate temperature regimes, respectively. Also, a comparison is made between Lorentzian and logarithmic spectral density functions, from which we conclude that the latter provides a better fit to the data at short correlation times (wojel). The logarithmic form has been proposed to describe diffusive motion in 2-dimensional conductors. INTRODUCTION In the past decade ionic conductivities have been measured in a variety of P3"-alumina isomorphs, although the remarkable ability of this material to conduct electric current is best known in the sodium form. The ease with which ions diffuse through the two-dimensional structure is necessarily related to activation energies of site hopping and interactions between the diffusing ions. It is not surprising that the conductivities of divalent and trivalent isomorphs are generally several orders of magnitude lower than that of the sodium form [1]: increased Coulombic forces produce the expected increase in activation energies, and although the competition for sites is reduced, their availability alone is insufficient to make up for the potential energy increase. There is, however, one interesting nonconformity in the divalent series. The conductivity of lead(H) P3"-alumina differs only by a factor of three from that of the sodium isomorph below room temperature. To understand why this anomaly exists, one must have more detailed information about lead dynamics in the conduction plane. Conductivity in P"-alumina is controlled by vacancy diffusion, and the large number Assuming that all ions are associated with of vacancies present is noteworthy. Beevers-Ross (BR) sites, 58% of these are vacant in the divalent form, compared with 17% vacant in the monovalent form. Other things being equal, it would seem that site hopping in divalent P3"-alumina is much less restricted than in most other materials. Several groups of researchers have investigated disorder in the Pb 0"-alumina conduction plane. Some recent x-ray diffuse scattering studies show surprisingly dissimilar results. For example, room temperature coherence lengths for lead were reported to be 5A within the conduction plane and 10A between planes in one case [2], and much greater than 200A in plane and 50A between planes in another [3]. In a far IR study [4], ordering of Pb ions was found to exist due to clustering, which is different from the kind of ordering found in Ba P"-alumina, where Ba ions preferentially condense into one of the equivalent sublattices. Unfortunately, n
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