Crystallographic lattices

Crystals of (BEDT-TTF)2Re04 are lustrous metallic black in color and are superconducting at a temperature of 2 K at a pressure of 5 k bar.5 In the absence of applied pressure this material shows metallic behavior (increased electrical conductivity as the temperature is decreased) from room temperature to 81 K at which temperature a metal-insulator transition occurs. The crystallographic lattice parameters, for the triclinic unit cell, are5 a = 7.78 A, b = 12.59 A, c — 16.97 A, a = 73.0°, 0 = 79.89°, y = 89.06°, and unit cell volume, Vc = 1565 A3. 

Three of the four presumed types of chemical bond that occurs in the solid state have been reduced to the common basis of interaction between opposite charges localized at crystallographic lattice sites, apparently at variance with the pairwise covalency described before. 

Some of the defect equilibria which we have deduced by this type of analysis were not surprising—a parent lattice may dissociate into interstitials and vacancies in conformity with appropriate equilibrium constants defects may associate, again consistent with an equilibrium constant or the lattice may dissolve excess atoms in simple solubility. (When we speak of a solvent or parent lattice we mean the crystallographic lattice, as it would be determined by x-ray analysis, stoichiometri-cally perfect, and free of vacancies or interstitials. We call the process of vacancy and interstitial formation lattice dissociation. Simple solution adds interstitials or fills voids in the parent lattice). 

This article is the third in the current series on the bibliography of crystal structures of polysaccharides.1,2 In the past decade, many advances in the methods for building theoretical models were witnessed, not only for the conformational analysis of isolated chains, but also for the subsequent or simultaneous packing of the chains in the crystallographic lattice. Many complex polysaccharides, having disaccharide to hexasaccharide repeating units, are now being studied. 

Since every unit cell in the crystal lattice is identical to all others, it is said that the lattice can be primitive or centered. We already mentioned (Eq. 1.1) that a crystallographic lattice is based on three non-coplanar translations (vectors), thus the presence of lattice centering introduces additional translations that are different from the three basis translations. Properties of various lattices are summarized in Table 1.13 along with the international symbols adopted to differentiate between different lattice types. In a base-centered lattice, there are three different possibilities to select a pair of opposite faces, which is also reflected in Table 1.13. 

All of the compounds in Table 13 contain either calcium or sulfate ions, in common with gypsum. The crystallographic lattice dimensions and axial ratios of each of these compounds were compared to those of gypsum. Data were obtained from the Powder Diffraction Standards Data File 1741. The conventions... 

TABLE I Crystallographic lattice parameters and superconducting critical temperatures (T for the La185Sr0.i5Cii1 xMxO4(M=Ni,Zn) and YBa2Cu3 xNix07 y series... 

High reactivity elements (RE e.g., cerium, yttrium, zirconium, hafnium) are sometimes added to the Fe-Cr-Al matrix these help the formation of the alumina protective layer that is, they speed up the transition from the less to the more stable crystallographic lattices [5,6] and increase its adhesion to the substrate. Secondly this action is assisted by the precipitation of "pegs" made up by fhe oxides of fhe reactive elemenfs (RE), partially immersed both in the substrate and in the scale of continuous superficial oxide [6]. However, fhe same authors state that the formation of fhe pegs is nof vital for the resistance to the scaling off of fhe layers of superficial oxide. It is important to note that its crystallographic type is a-Al203, which is much more effective than the 5, y, or 9 types. 

Another useful systematic study has been made of a number of oxides, sulphides, and selenides by comparison of the Eu + chemical isomer shift with the mean Eu +-anion distance as estimated from the crystallographic lattice constants [31]. The correlation for the selenides is illustrated in Fig. 

In the following diagram, we show how these basic shapes are modified in growth as particles (crystals) grow along certain preferred planes of the crystallographic lattice, i.e.- non-cubic ... 

Crystal Growth and Shape as a Function of Crystallographic Lattice ... 

The unusual electrical properties of the 2 1 TMTSF salts are summarized and discussed elsewhere. For additional characterization purposes beyond chemical analyses, the crystallographic lattice constants for the isostructural (triclinic, space group PT) TMTSF derivatives discussed here are given in Table I. 

TABLE I. Crystallographic Lattice Constants for TMTSF Derivatives. 

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