Crystal structure types

Crystal structure (type of cellulose and defects) Supramolecular structure (e.g., degree of crystallinity)... 

Among the different families of tantalum and niobium complex fluorides and oxyfluorides, the family of compounds with an X Me ratio equal to 6 is the largest. Table 22 presents the main structural characteristics of hexafluoroniobates and hexafluorotantalates. All known cases of niobium- and tantalum-containing formulary analogs have the same crystal structure type, at least at ambient temperature. 

Thus, the formulated classification allows for the definition of the compound s crystal structure type based on its composition. In this process, the most important stage is the correct definition of the X Me ratio. 

S Tantalum and niobium are present in the crystal structure in the form of complex ions. The lowest coordination number, 6, corresponds to the formation of slightly distorted octahedrons. The linking and packaging of the octahedrons depends on the X Me ratio, where X is the total number of oxygen and fluorine atoms, and Me is the total number of tantalum or niobium ions as well as other metals that can replace tantalum or niobium in the octahedral polyhedron. The crystal structure type can be defined based on the X Me ratio, as follows ... 

Thermal properties and decomposition mechanisms depend on the crystal structure type. Compounds with a crystal structure that includes shared octahedrons decompose forming tantalum- or niobium-containing gaseous components, while island-type compounds release light atoms and molecules into the gaseous phase. 

Crystal Structure Type d calculated d observed Reference and remarks... 

Crystal Structure Type d calculated dobs dobs -(B) Para- meter calc Para- meter obs Reference... 

Crystal structure type AICI3 UClj AICI3 ... 

The next two figures show that crystal structure type and ionicity also play a role in determining dislocation mobility, and therefore hardness. First, if data for the III-N compounds are plotted on Figure 5.2 they do not fall on the regression line. The reason is that they have hexagonal rather than cubic crystal structures. However, when plotted by themselves as in Figure 5.3 their hardnesses are proportional to their bond moduli. 

Four simple crystal structural types encompass the majority of elemental or binary semiconductors. The high symmetry of the structures has important consequences for the NMR spectra in several respects ... 

The fourth and final crystal structure type common in binary semiconductors is the rock salt structure, named after NaCl but occurring in many divalent metal oxides, sulfides, selenides, and tellurides. It consists of two atom types forming separate face-centered cubic lattices. The trend from WZ or ZB structures to the rock salt structure takes place as covalent bonds become increasingly ionic [24]. 

Table 5.1 lists some values of the correlation factor for a variety of diffusion mechanisms in some common crystal structure types. 

Daams, J.L.C., Villars, P. and van Vucht, J.H.N. (1991) Atlas of Crystal Structure Types for Intermetallic Phases (Materials Park, Oh 44073 American Society for Metals), Vol. 1 1. 

To calculate the space-filling value for a specific compound, one has to know the radii of the atoms and the lattice constant. Neither of these is needed for the construction of a space-filling curve of a crystal structure type it is sufficient to know the point positions of the atoms and the axial ratios. The curve is based on a hard sphere... 

Remarks on the crystal chemistry of the alloys of the 3rd group metals. A large number of intermediate phases have been identified in the binary alloys formed by the rare earth metals and actinides with several elements. A short illustrative list is shown in Tables 5.19 and 5.20. Compounds of a few selected rare earth metals and actinides have been considered in order to show some frequent stoichiometries and crystal structure types. The existence of a number of analogies among the different metals considered and the formation of some isostructural series of compounds may be noticed. 

This is not a trivial problem, and has important implications for the mechanism of the reaction. However, the bulk of the evidence is for centrosymmetric rings, which would be in keeping with our experience in small-molecule systems. For the present purposes we assume this to be the case. On this basis DSP is one of a class of monomers of crystal structural type 100 that polymerize to polymers 101. Note that, as is typical of topochemical reactions, there are cases of polymorphism of the monomers, in which only those of structure 100 are reactive. Also small changes in the substitution of this molecule frequently result in changes in crystal structure and reactivity. 

In a third type of crystal structure (Type B2, Fig. 13) exhibited by the larger pericondensed hydrocarbons (e.g., coronene and ovalene), the characteristic excimer fluorescence band may originate from a small concentration of... 

Observed correlation between compound formation mode (congruent vs. incongruent melting) against crystal structure type of AB2 compounds. The underlined compounds are suspect either with the liquidus curve determination or the crystal structure type. 

The melting temperatures for the thirty-one elements, whose inelastic neutron scattering data are available, were calculated and compared against the observed melting temperatures as shown in Table 1. In this calculation, the thirty-one elements are sorted into three groups according to their crystal structure type. Different correction factor, A,... 

The correlation as summarized in Fig.9 is remarkable in the sense that it supersedes the following factors that heretofore were invoked in one way or another to correlate to the phenomenon of superconductivity (a) transition or non-transition elements, (b) crystal structure type, (c) atomic number, (d) valence electron concentration. 

Knapp et al. (144) show that for oxides containing 3d elements in spinel, perovskite, rocksalt, or zircon-type structures, the K-edge XANES spectra are quite independent of 3d electron occupation but instead nicely correlate with the crystal structure type. Various studies of Ti K edges of titanium oxides and other titanium compounds have been reported (40,158,172,177, 297). 

The compounds [M(bipy)2(N03)2] guest have also been studied extensively by us and, interestingly, we have only observed three basic crystal structure types (Figure 15). 

For the Classification of Minerals , crystal-structure determinations improved the definitions of mineral species and varieties, assisted in the development of the concept of crystal structure types, helped to establish isotypic series and homeotypic and heterotypic groups, and pointed to the recognition of much broader crystallochemical relationships. The X-ray method appreciably simplified the generally unique characterization of a mineral species and led to a reduction in varieties and the discreditation of many minerals accepted up to that time, thereby eliminating countless superfluous mineral names . 

To build up a theory of metallic phase stability the conventional arguments for the discussion of crystal structures have to be completed by the concept of spatial correlation of electrons. The parameters of the spatial correlations may be analyzed from the ample empirical material of determined crystal structures by means of several evident rules. The surprising result is that for many crystal structures two correlations are essential for understanding the special features of the crystal structures. This two-correlations model makes possible an easy survey of metallic structures. Two examples of crystal structure-type families are considered, the Cu-family and the W-family. 

Tlie aim of this chapter is to provide an overview of materials where fast transport of alkali metal cations and protons is observed. A general discussion of factors affecting conductivity and techniques used to study ion migration paths is followed by a review of the large number of cation conductors. Materials with large alkali ions (Na-Cs) are often isostructural and therefore examined as a group. Tire lithium conductors with unique crystal structure types and proton conductors with unique conduction mechanisms are also discussed. 

Substance r, i< Crystal structure type Substance r,K Crystal structure t... 

---