Single-crystal systems

A possible modification of this expression is presented elsewhere (82). The value of t, can be related to a diffusion coefficient (e.g., tj = l2/6D, where / is the jump distance), thereby making the Ar expressions qualitatively similar for continuous and jump diffusion. A point of major contrast, however, is the inclusion of anisotropic effects in the jump diffusion model (85). That is, jumps perpendicular to the y-ray direction do not broaden the y-ray resonance. This diffusive anisotropy will be reflected in the Mossbauer effect in a manner analogous to that for the anisotropic recoil-free fraction, i.e., for single-crystal systems and for randomly oriented samples through the angular dependence of the nuclear transition probabilities (78). In this case, the various components of the Mossbauer spectrum are broadened to different extents, while for an anisotropic recoil-free fraction the relative intensities of these peaks were affected. 

Each adsorbate section commences with a summary of the surface species that have been identified or proposed on the basis of work on the simplified single-crystal systems. There may be sites, and associated adsorbed species, on the more complex and partially ordered surfaces of metal particles that are not covered by the collected results from single crystals. However, it is anticipated that many, and often the principal, surface species will be identified by this means. An extensive review of work on hydrocarbons adsorbed on metal single crystals was published in 1989 (17), and this review is updated here. 

Spectra obtained by hydrogenation will also be reviewed, although in that area relatively few results are available from single-crystal systems because the prevalent VEELS method requires the use of high vacuum over the sample. This restriction does not apply to RAIRS studies. 

Further progress in structural interpretations probably requires spectra on simplified single-crystal systems. Such low- and room-temperature spectra using the higher resolution of RAIRS would be particularly valuable on three of the metals, Pt, Ni or Pd, and Rh, which give different results at room temperature on the finely divided metals. 

In the case of Pt3Ni(M0 surfaces UPS results (Fig. 3.2) show that the d-band density of states (DOS) shifts from -2.70 eV on Pt3Ni(110) to -3.10 eV on Pt3Ni(lll) to -3.14 eV on Pt3Ni(100). Furthermore, the DOS of the alloy surfaces are quite different from corresponding pure Pt single crystals the d-band center is downshifted by approximately 0.16, 0.24, and 0.33 eV, respectively. We have had six different single-crystal systems that could be studied specifically for the elusive electronic effect in electrocatalysis. 

The single-crystallizer system has the benefit of simplicity but requires periodic harvesting of the dissolver. 

Photosensitization.—The photosensitization of semiconductor electrodes is still the subject of a number of papers every year. Although the chance is small that single-crystal systems will benefit appreciably from photosensitization, the possibilities for high surface area systems, such as dispersions, are more exciting. Key references on photosensitization have been included in this review, although they do little to change the currently accepted view of the mechanism or efficiency of photosensitization. 

Most adaptronic structures are used, or are intended to be used in macroscopic devices. In a single domain crystal system, macroscopic properties are simply the statistical average of microscopic properties of each miit cell. For most functional materials, however, such a simple average fails due to nonlocal interactions and the additional mesoscopic structures created at the intermediate length scale, such as domain patterns in single crystal systems and grain microstructures in ceramics. These nonlocal interactions and mesoscale structures often produce very strong extra enhancement to the functional proper-... 

Fig. 3.15. Illustration of misorientational poling in PZN-PT single crystal system. The field is applied along [001] and the dipoles in each unit cell are pointing to the four upper corners along body diagonals...

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