Characterizing Single Crystals

Rutherford Backscattering Spectroscopy (RBS) can also give compositional information. It is a highly surface sensitive technique requiring specialized equipment and is best for samples containing elements with Z s that differ by several atomic numbers. The accuracy of RBS is controlled by counting statistics but is usually about 5%. 

X-ray Diffraction Structural characterization of single crystal samples can reveal both the atomic arrangement and the composition of a sample. The unit cell size and symmetry alone can quickly establish whether a crystal is a known material or a new phase. A detailed discussion of X-ray diffraction studies of the known superconductors is the topic of another chapter in this book. 

The shape anisotropy also complicates efforts to determine the resistivity anisotropy. An extension of the Montgomery method must be employed for anisotropic materials (93). Such characterization has been carried out for Bi22Sr2 Ca 0 8Cu2O8+fi (94). 

In favorable cases, superconductivity is a sensitive and useful screen for desired crystals. Near zero field microwave absorption (95) can be used to examine very small samples of only a few micrograms. This technique has value in the search for new superconducting phases. In early attempts to identify the superconducting phase in the Pb-Sr-Y-Cu-O system, both superconducting and non-supercon-ducting crystals were obtained. Individual crystals were examined for superconductivity using near-zero field microwave absorption. Then X-ray diffraction was used to establish the structure and stoichiometry of the superconducting phase. 

However, there are limitations to the use of superconductivity as a screen for new superconductors. Experience with various of the known high Tc phases has taught us the sensitivity of superconductivity to doping. For, example Tc depends critically on oxygen stoichiometry in Ba2YCus07. Thus,interesting phases might easily be overlooked. 

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There is a very rich literature and a comprehensive book6 on the role of promoters in heterogeneous catalysis. The vast majority of studies refers to the adsorption of promoters and to the effect of promoters on the chemisorptive state of coadsorbed species on well characterized single crystal surfaces. A... 

The use of single crystal electrodes. While the polycrystalline electrodes that were used in older works gave reproducible results in studies of reactions, they did not possess a definite structure. Only recently have electrochemists learned to prepare and characterize single crystal electrodes. This has greatly extended the study of the structure of the electrochemical interface. 

The underpotentlal deposition of lead has been examined on LEED-characterized single crystal silver surfaces with 0.1 M HF as the electrolyte using a special ultra-high vacuum-electrolyte transfer system. Each of the low index surfaces has a characteristic voltammetry curve with multiple adsorption and desorption UPD peaks. 

The structures of the anhydrous vanadiumfiii) sulphates, 2(804)3, yellow (monoclinic) and green (rhombohedral), have been investigated, and a new double sulphate, (NH4)3V(S04)3, has been prepared and extensively characterized. Single-crystal magnetic susceptibility studies (1.5—20 K) of guanidi-nium vanadium(iii) sulphate have been reported, and the zero-field splitting has been estimated as 3.74 cm . ... 

In principle it is possible to verify these findings experimentally. Diffusion measurements characterizing single-crystal samples, free from defects and grain boundaries, should be able to demonstrate that smaller guests do not always diffuse faster. However, there are a number of problems in trying to find suitable experimental results with which to corroborate the claim of the theoretical simulations. One of these problems is the rather ideal distribution of cations that is assumed in the simulations of zeolites A and... 

In the Nb-Cr system a precipitated phase with formula NbCrj and an MnCuj-type lattice was characterized. Single crystals of the ternary species NbCrSi and NbCrGe have been prepared by chemical transport. These phases adopt the FejP structural type. ... 

This chapter focuses on adsorption and desorption of molecular hydrogen, the best understood reactions on Si, though the discussion will also refer to related reactions such as surface diffusion. The discussion is also restricted to results from well-characterized single-crystal surfaces, with special emphasis on the (100) surface. This is the surface on which electronic devices are primarily grown. It reconstmcts to a 2 x 1 surface unit cell, but this remains simple enough for accurate first principles calculations. The Si(lll) surface is also important, but because of the complexity of the 7 x 7 re-constmcted surface, the quality of feasible calculations remains limited. Reactions on the (111) surface are discussed here for comparison against the analogous properties of the (100) surface. 

Since the early 1980s, cyclic voltammetry has been used to characterize single-crystal electrodes in terms of surface order, presence or absence of defects contaminations, and so on. In some cases the voltammo-grams have also been used for identification of adsorbates, mostly on the basis of electric charge calculation. However, since the double-layer charging and surface reactions such as UPD and anion adsorption may occur in parallel, it is difficult to break down the total voltammetric charge into all of its individual components. Therefore, interpretation from voltammetry alone, used as a tool for the adsorbed species identification, may be ambiguous. This has been, for instance, the case with the interpretation of the unusual adsorption states on the Pt(lll) electrode (73-82). 

Since the early 1980s, cyclic voltammetry has been used to characterize single-crystal electrodes in terms of surface order, presence or absence of defects, contaminations, and so on. In some cases, the... 

The aim of many surface science experiments is to provide the fundamental detail of a reaction over a well-characterized single crystal surface in order to establish structure-reactivity relationships. Supported catalytic particles, on the other hand, may have various exposed surface facets along with defect sites. A choice then has to be made as to which single crystal surface will provide the most accurate representation of the active surface facets of the support particles. In order to address the similarities or differences in the rate over ideal surfaces and those over supported particles, Kelly and Goodmanl l compared the rate of methane formation from CO and H2 catalyzed by an Ni(lOO) single crystal with that over a supported catalyst taken under the same conditions (see Fig. 2.12). 

Despite the multitude of applications of these materials, it is true to say that most of the interest shown in their hardness properties is purely theoretical because of the controlled way in which experiments can be carried out involving them, as well as the ready availability of well-characterized single crystals. Thus data on diamond and single-crystal silicon is reasonably easily found while information on other polymorphs is frequently not available. This latter point brings us to a consideration of bonding, polymorphism, and microstructure of these materials. 

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