Chemical crystallographic valence

It is practically impossible to formulate a sensible theory of chemical reaction by treating the physical electron as a point particle. This assumption requires that valence electrons must be either in stationary balance with the positively charged atomic cores, or in rapid motion through the interior of the molecule. Crystallographic analysis rules out the first option. A stationary pair of electrons must scatter X-rays more effectively than a hydrogen atom, which in... 

The ionic charge carriers in ionic crystals are the point defects.1 2 23,24 They represent the ionic excitations in the same way as H30+ and OH-ions are the ionic excitations in water (see Fig. 1). They represent the chemical excitation upon the perfect crystallographic structure in the same way as conduction electrons and holes represent electronic excitations upon the perfect valence situation. The fact that the perfect structure, i.e., ground structure, of ionic solids is composed of charged ions, does not mean that it is ionically conductive. In AgCl regular silver and chloride ions sit in deep Coulomb wells and are hence immobile. The occurrence of ionic conductivity requires ions in interstitial sites, which are mobile, or vacant sites in which neighbors can hop. Hence a superionic dissociation is necessary, as, e.g. established by the Frenkel reaction ... 

Progress in the preparative and structural fields of protactinium chemistry has been rapid during the past 6 years and there is now sufficient information available, particularly in the halide and oxide fields, to permit a more balanced comparison than has previously been possible with the properties of the actinide elements, on the one-hand, and those of niobium and tantalum, on the other. In this connection one must, of course, bear in mind the fact that the ionic radii of protactinium in its various valence states [Pa(V), 0.90 A and Pa(IV), 0.96 A] are appreciably larger than those of niobium or tantalum and this itself will have a considerable influence on the chemical and crystallographic properties of the elements. 

The recent electron spectrometer provides highly resolved spectrum for valence state XPS, which supplies us with very useful information for discussion on chemical bonding, when combined with an appropriate theoretical analysis as mentioned above. Therefore, an accurate calculation of electronic state is required for such a purpose. The DOS calculated by DV-Xa method has been demonstrated to reproduce well the valence state XPS for some oxyanions compared with other theoretical calculations. The calculation was made using a simple model cluster XO4 " with T symmetry, thus the theoretical analysis was insufficient for the valence structure in details. This has significantly been improved by a careful analysis with more realistic model clusters which are determined from crystallographic data for those oxyanions. The comparison of the theoretical and experimental spectra for PO4 ions is shown in Fig.8. The agreement is very good even for the fine structure in the valence band. 

The radial deformation of the valence density is accounted for by the expansion-contraction variables (k and k ). The ED parameters P, Pim , k, and k are optimized, along with conventional crystallographic variables (Ra and Ua for each atom), in an LS refinement against a set of measured structure factor amplitudes. The use of individual atomic coordinate systems provides a convenient way to constrain multipole populations according to chemical and local symmetries. Superposition of pseudoatoms (15) yields an efficient and relatively simple analytic representation of the molecular and crystalline ED. Density-related properties, such as electric moments electrostatic potential and energy, can readily be obtained from the pseudoatomic properties [53]. 

Several investigators have used combined approaches, particularly in the in situ precipitation of active material in the pores of sintered substrates, using cathodic polarization and caustic precipitation in simultaneous or nearly simultaneous steps. A considerable amount of the reported information on the chemistry, electrochemistry, and crystal structure of the nickel electrode has been obtained on thin films (qv) made by the anodic corrosion of nickel surfaces. However, such films do not necessarily duplicate the chemical and/or crystallographic condition of active material in practical electrodes. In particular, the high surface area, space charge region, and lattice defect structure are different. Some of the higher (3.5+) valence state electrochemical behavior seen in thin films has rarely been reproduced in practical electrodes. 

The search for highly conducting low-dimensional materials has been at the forefront of chemical research in the last decade. There are two criteria which have to be satisfied, and which appear naturally from the theories of mixed valence (i) the metal-ligand units should occupy crystallographically similar sites and (ii) they should be mixed-valence units. 

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