Disordered phases, structural sample preparation

Toraya s WPPD approach is quite similar to the Rietveld method it requires knowledge of the chemical composition of the individual phases (mass absorption coefficients of phases of the sample), and their unit cell parameters from indexing. The benefit of this method is that it does not require the structural model required by the Rietveld method. Furthermore, if the quality of the crystallographic structure is poor and contains disordered pharmaceutical or poorly refined solvent molecules, quantification by the WPPD approach will be unbiased by an inadequate structural model, in contrast to the Rietveld method. If an appropriate internal standard of known quantity is introduced to the sample, the method can be applied to determine the amorphous phase composition as well as the crystalline components.9 The Rietveld method uses structural-based parameters such as atomic coordinates and atomic site occupancies are required for the calculation of the structure factor, in addition to the parameters refined by the WPPD method of Toraya. The additional complexity of the Rietveld method affords a greater amount of information to be extracted from the data set, due to the increased number of refinable parameters. Furthermore, the method is commonly referred to as a standardless method, since the structural model serves the role of a standard crystalline phase. It is generally best to minimize the effect of preferred orientation through sample preparation. In certain instances models of its influence on the powder pattern can be used to improve the refinement.12... 

The sensitivity of the shape of the low field absorption under various experimental conditions Is thought to be an extremely sensitive diagnostic tool for the local structure o a superconducting phase present as a disordered powder or glassy material on a microscopic scale. Exact reproducibility of the shape and Intensity of this signal under nominally analogous experimental conditions for different preparations Is hard to achieve. However semlquantltatlve reproducibility Is good. This may be an experimental problem at present, but we feel that It Is more likely due to subtle differences In the solid state compound formation process in different sample preparations. Additional work and other locally microscopic techniques are needed to clarify these aspects. 

Ise and coworkers [102 -104] studied solutions of charged latex particles by SAXS and ultramicroscopy. Their micrographs showed a particifiarly interesting phenomenon, the formation of a two-state structure. Within hours after the sample preparation, a dense, ordered phase is formed coexistent with a dilute disordered phase. Using video-techniques, Ise and coworkers were able to study the Brownian motion of the particles in the ordered and disordered phase, thus obtaining information about the dynamics of lattice defects, motion of single particles around their lattice position, and the dynamics of particles at the interphase between the ordered and disordered regime. 

Hardacre el al. (7 75, 174) investigated the properties, structure, and composition of cerium oxide films prepared by cerium deposition on Pt(lll), finding that the activity for CO oxidation is enhanced on Pt(lll) that is partially covered by ceria. It was suggested that new sites at the Pt-oxide interface become available for reaction. A remarkable observation is the high activity for CO oxidation when the Pt(lll) sample is fully encapsulated by ceria (Pt was undetectable by XPS and AES). It was proposed that an ultrathin, disordered ceria film becomes the active catalyst. It was also demonstrated by XPS and AES that Pt dramatically increases the reducibility of cerium oxide that is in intimate contact with Pt. This result suggests that intimate contact between the noble metal and oxide phases is indeed crucial to facile oxygen release from ceria. High-resolution electron microscopy demonstrated the presence of direct contact between ceria and noble metal for supported Pt-Rh catalysts (775). Hardacre et al. (173,174) related the catalytic activity of the ceria phase to partially reduced cerium oxide. 

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