Solid state reactions, correlation with

Based on some interesting reactions in certain inorganic crystalline compounds, Kohlschutter [9,10] proposed that the nature and properties of the products obtained take place on the surface or within the solid state. Indeed, he coined the term topochemistry for such reactions in the solid state. However, systematic investigations of photoinduced reactions in crystals began from 1964 onward by Schmidt and Cohen [11], Their studies on the 2tt + 2tt photoreaction of cinnamic acid derivatives in the crystalline state and correlation with the molecular organization in these crystals led to what are now known as Topochemical Principles. The most important conclusions reached by them are as follows (1) The necessary conditions for the reactions to take place are that the reactive double bonds are parallel to one another and the center-to-center distance be within 4.1 A (2) there is one-to-one correspondence between the stereochemistry of the photoproduct and the symmetry relationship between the reactants. The centrosymmet-ric relationship (called the a-form) leads to centrosymmetric cyclobutane (anti-HT), whereas the mirror symmetric arrangements (called the (5-form) produce mirror symmetric dimer (yy -HH). 

In this paper we have measured the resistance dependence and oxygen content of Er Ba2Cu 0Q and Y.Ba Cu OQ - samples above room temperature under the conditions or a simulated sintering and annealing process. The results provide insight into the importance of specific sample preparation procedures. We will start with a detailed discussion of the two most common preparation procedures for copper oxide superconductors at this moment the solid state reaction method and the coprecipitation method. A discussion of the resistance and thermogravimetric analysis (TGA) data and how it correlates to the sample formation will follow. 

The solid-state reaction of SeOj with V2O5 has been studied by X-ray diffraction methods. In an inert atmosphere the reaction product was the dark red compound Se2V209, which is not isostructural with the corresponding tellurium analogue and which decomposes into its component oxides above 400 The formation of 1 1 complexes between Se02 and SeOCl2 and CP, Br , and I has been reported. The stability constants and nature of the complexes were correlated with those of SO2 and SOCI2, and the role of DMSO as solvent was discussed. ... 

The reaction intermediates of this extraordinary solid state reaction have been correlated to dimer, trimer, tetramer molecules etc. with different chemical structures. The physical properties of the reactive short chain intermediates and final reaction products are best characterized by the approximate structures represented in Figure 2. These structures give the principal contributions to the real structures of the DR , DC and AC intermediates... 

This minimum movement of atoms or molecules in the solid state leading to the reaction is known as topochemical reaction. Since the structure of the product is correlated with the geometry of the reactant solid in the crystal lattice, the stereochemistry of the product obtained in a solid state reaction is, in general, predictable. Most of the photodimerization reactions reported in the literature were found to follow these topochemical postulates of Schmidt. Ai2,is,2i,26-28... 

In the pseudo-binary mixed-oxides the structure assumed will obviously depend on the structures of the pure oxide components, the mole ratio of these, and (most importantly) on the difference between the radii of the two cations. The first thorough study of these systems was carried out by Schneider and Roth (1960). Their specimens were prepared by solid state reaction to equilibrium of individual oxides at 1650°C or sometimes 1900°C. The phase-fields were then determined from room-temperature X-ray diffraction data, and the results correlated with the average cation radius . Full details of the observed and predicted phase relationships are best obtained from the original publication, but a general summary can be given here. 

Models for description of liquids should provide us with an understanding of the dynamic behavior of the molecules, and thus of the routes of chemical reactions in the liquids. While it is often relatively easy to describe the molecular structure and dynamics of the gaseous or the solid state, this is not true for the liquid state. Molecules in liquids can perform vibrations, rotations, and translations. A successful model often used for the description of molecular rotational processes in liquids is the rotational diffusion model, in which it is assumed that the molecules rotate by small angular steps about the molecular rotation axes. One quantity to describe the rotational speed of molecules is the reorientational correlation time T, which is a measure for the average time elapsed when a molecule has rotated through an angle of the order of 1 radian, or approximately 60°. It is indirectly proportional to the velocity of rotational motion. 

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