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CHARACTERIZATION METHODS. 65 Nuclear Spectroscopy. With Mössbauer spectroscopy, the local atomic environments of probe atoms are characterized in terms of hyperfine parameters. Herr and coworkers (1987) used this method as a further test of the proposed two- component model of nanocrystalline Fe deduced from their x-ray analysis (Zhu et al., 1987). The Mössbauer spectrum of a nanocrystalline pellet of Fe with 6-nm grain size compacted to 5 GPa could be fitted with two components in accord with their two-component (crystal plus interface) structural model. It was found that one subspectrum can be described by the Mössbauer parameters of crystalline body-centered-cubic Fe. The second subspectrum is characterized by an enhanced hyperfine magnetic field, a larger line width, and an increased isomer shift in comparison to the crystalline component. Positron annihilation spectroscopy (PAS) is very sensitive to small defects with increased free volume relative to crystal interstices (like vacancies, vacancy clusters, grain boundaries, voids, etc.) and can therefore be used to investigate the structure of nanocrystalline materials in a regime that is not easily accessible to other techniques (e.g., electron microscopy). PAS can be used to investigate the sintering behavior of nanophase materials as a function of compaction pressure or temperature by following the shrinkage and disappearance of internal voids, as a complement to small-angle x-ray or neutron scattering measurements. Positron-lifetime measurements on a number of nanocrystalline metals by Schaefer and coworkers (1987) and on nanocrystalline TiO 2 (Siegel et al., 1988) have clearly indicated the presence of voids in as-consolidated samples. Furthermore, reduction of this porosity in nanophase TiO2 as a function of increasing sintering temperature could be followed by PAS (Siegel et al., 1988). Such results along with those from complementary SANS experiments (e.g., Epperson et al., 1989) should help to elucidate the sintering behavior of nanophase materials. Improvements in the sensitivity of nuclear magnetic resonance (NMR) have made it possible to use this technique to study simple molecules, especially those containing C and H adsorbed on the surfaces of supported metal particles in 1-to 5-nm diameter clusters (Wang et al., 1986). NMR gives both structural information and kinetic parameters and is suitable for examining high-surface-area catalysts. Studies reported to date have provided information on the bonding and structure of molecules such as CO, ethylene, and acetylene adsorbed on the metal. In addition, NMR gives information on the motion of adsorbed species, adsorption parameters, and the structure and composition of adsorbed intermediates. NMR is proving to be a popular tool for studies of support materials, especially zeolites. Most of these studies have dealt with the nuclei 27Al and 29Si as well as 17O, 13C, 31P, 11B, and 7Li (Stucky and Dugen, 1984; Hanson et al., 1984).
