Nonstoichiometric factors

The composition of a slightly nonstoichiometric cobalt oxide is C00.999O. (a) Write a defect formation equation for this phase assuming that electronic compensation occurs, (b) If conductivity takes place by hopping, what is the value of the factor cp in Eq. (7.1) ... 

A second classification of hydrates is obtained through consideration of the guest molecules. Such a classification is a function of two factors (1) the chemical nature of the guest molecule and (2) the size and shape (particularly in sH) of the guest. The size of the guest molecule is directly related to the hydration number and, in most cases, to its nonstoichiometric value. 

It is not certain that all examples of nonstoichiometric compounds can be regarded from a single standpoint or represented by a single model. In principle, and depending on the relative importance of essentially metallic or essentially heteropolar properties, the basis of phase limits might be sought in electronic or in structural factors. 

Other variables refined in the LS procedure are the scale factor K (Section 3.3.5) always, atomic site occupancies in the cases of disorder and/or nonstoichiometric composition and the absolute structure parameter for noncentrosymmetric structures. If the sample contains atoms with sufficiently strong anomalous scattering, the latter parameter indicates whether the assumed absolute structure of the crystal is right or wrong. 

Occupancy factor A parameter that defines the partial occupancy of a given site by a particular atom. It is most frequently used to describe disorder in a portion of a molecule, or for describing nonstoichiometric situations, such as when a solvent molecule is being lost to the atmosphere. 

The fraction of electrons of the valence band that are raised to the conduction band by thermal energy corresponds to the Boltzmann factor exp( — e/2kT). The /-type semiconductors play only a minor role in catalysis the n- and p-type semiconductors are far more important. Nonstoichiometric oxides and sulfides are of industrial importance. The conductivity of these materials is low but can be considerably increased by doping with foreign atoms. 

The fraction of N with nonstoichiometric bonds with Ti may be one of the principal factors for determining film quality. It was shown that coatings with the... 

During crystal structure analysis, the site occupancy factor (SOF) of each atom is ideally assigned a value imposed by the site symmetry of its position in the unit cell. For example, an atom located on a general position is assigned an SOF of one, while an atom situated on an inversion center has SOF = 0.5. However, it should be noted that the SOFs are lower than their symmetry-imposed values in cases where atoms are disordered. Furthermore, inclusion compounds such as solvates are often nonstoichiometric as they can lose solvent molecules during crystal selection and mounting. In such cases, it may then become necessary to model guest positions with lower than ideal SOFs. " ... 

Nowick AS, Du Y, Liang KC (1999) Some factors that determine proton conductivity in nonstoichiometric complex perovskites. Solid State Ionics 125 303-311... 

The calculated defectless surface energy for the (001) using (11.13) equals 2.04 eV. This is smaller by a factor of 3 than that for a similar nonstoichiometric (110) surface. 

Among all catalysts with the iron oxides and their mixtures as precursor studied, Fei xO based catalyst with nonstoichiometric and wiistite structure has the fastest reduction rate and lowest reduction temperature. In a wiistite structure, large amounts of defects are iron ions, which enable the diffusion of Fe in oxide lattices, and will be preferable to electron transferences. This is the structural factor for the easy reduction of Fei xO based catalysts. 

Shannon and Pask have suggested that two types of defects may predominate in nonstoichiometric titania, depending on the method of preparation. They propose that titania reduced in a vacuum contains interstitial Ti+ ions, while titania reduced in a hydrogen atmosphere contains oxygen vacancies. A nonstoichiometric rutile sample was prepared in a CO atmosphere. The defect rutile was reacted with strontium carbonate and the activation energy and frequency factor were calculated. This sample had an activation energy of 65.5 kcal/mole and a frequency factor of 6.0 X 10 , as compared with values of 67.2 kcal/mole and 7.2 X 10 for defect rutile of the same stoichiometry prepared under vacuum. These values are within experimental error. This would indicate either that both types of defect show an equal effect on the kinetics of the reaction or else that the same type of defect is created in both types of preparation of the defect rutile. 

The activation energy and frequency factor of the reaction between SrC03 and nonstoichiometric rutile prepared under vacuum are approximately equal to the activation energy and frequency factor of the reaction between SrCOj and nonstoichiometric rutile prepared by heating in a carbon monoxide atmosphere. This indicates that either both methods of preparation resulted in the same type of defects, either interstitial cations or anion vacancies, or that both types of defects show an equal effect on the kinetics of the reaction. 

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