INDEX crystallographic phases

Recently, the In situ Raman scattering from Fe-TsPc adsorbed onto the low Index crystallographic faces of Ag was examined and the results obtained are shown In Fig. 5 (15). On the basis of the similarities of these spectra with those obtained for the macrocycle In solution phase, as well as the polarization behavior characteristics, It has been concluded that the most likely configuration Is that with the macrocycle edge-on with respect to the surface. This Is In agreement with conclusions reached from the UV-vlslble reflectance spectra. The preferred configuration, however, may depend on the particular macrocycle, as well as on the nature of the adsorption site. 

Much of the early work was done before the need for high resolution diffraction equipment was generally recognized, and the Debye-Scherrer techniques available at that time may well have failed to reveal the fine detail at present being recorded. The very ease with which powder patterns can be recorded, and indexed for phases of high symmetry in terms of a particular unit cell, does not in itself mean that a structure is satisfactorily determined by analogy with others of similar composition and crystallographic constants. It is of equal importance... 

The last problem is relative to the crystallographie phases. They are often deduced from electron diffraction patterns (EDP s). Various phenomena whieh oeeurred in the course of the study of thin samples are sometimes badly known, or else the patterns can be misinterpreted and the indexation becomes wholly wrong. Reeently Z. Li et al. (1988) have claimed to the formation of new polymorphic erbium oxide phases. These were in fact the well-known ErH2, C- and B-Er203 compounds (Gasgnier 1980, 1990). Other misinterpretations result from decided opinions on chemical reactivity, phase transitions, compound formation (as Lu(OH)4 for example) (Gasgnier 1991). .. and/or on disorder between two crystallographic phases. The rare earth series display basie chemical and physical properties which are now well established. Moreover, the new micro- (and even nano-) analysis apparatus should be used in a systematic way to insure accurate determination of the specific properties of the materials. 

Immediately after the isolation of macroscopic quantities of Cgo solid [298], highly conducting [299] and superconducting [141] behaviors were verified for the K-doped compounds prepared by a vapor-solid reaction (Haddon, Hebard, et al.). Crystallographic study based on the powder X-ray diffraction profile revealed that the composition of the superconducting phase is KsCeo and the diffraction pattern can be indexed to be a face-centered cubic (fee) structure with a three-dimensional electronic pathway [300]. The lattice parameter (a = 14.24 A) is apparently expanded relative to the undoped cubic Ceo = 14.17 A). The superconductivity has been observed for many A3C60 (A alkali metal), e.g., RbsCeo (Tc = 29 K... 

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... 

Titania most often appears in the form of one of two predominant crystallographic polymorphs, rutile and anatase. At least seven other distinct phases have been characterized by x-ray diffraction [3], but none are sufficiently commonplace to play a role in pigmentation using titanias. Rutile possesses a higher refractive index than anatase. It is also inherently considerably less photoactive than anatase. For these reasons, rutile has become the predominant polymorph of titania for pigmentary uses in polymers. 

The crystallographic data will indicate the proper index k, i.e., into which site thejth is sent by the operator 0. The (+1) is a definition of phase. For example, 0 may be chosen as a two-fold screw axis, or a glide plane, and so on. Having chosen the nature of 03, the remaining operators in the factor group, the y3, behave as follows ... 

Due to the rhombic shape of the unit cell, it should also be possible to index the scattering pattern of the C0I2 phase on the basis of the plane crystallographic group cm. However, as the group cm possess mirror planes this would not be conform to the chirality of the molecules. 

Ti02 exists in three crystallographic forms, namely anatase, rutile, and brookite, but the first two are the most important. However, the optical and electrical properties of anatase and rutile are different. Rutile is the most thermodynamically stable, and exhibits the highest refractive index. Nevertheless, the lattice energies of the other phases are similar and hence metastable over long periods of time. Rutile and anatase are produced industrially in large quantities and used as pigments and catalysts, and in the production of ceramic materials [125]. 

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