Binary systems defect structures

The Pi — X diagram is also useful for indicating a nonstoichiometric composition it is usually presented in log x vs. log p, or log Pi vs. x form for constant temperature. A log X diagram for the U-O binary system is shown in Fig. 2. Presentations in log x—log Pj and log electric conductivity) are useful for their permitting estimation of the defect structure of nonstoichiometric compositions from the slopes of the plots, as will be discussed further on. 

Descriptions will be given below of the methods available for determining the defect structure in nonstoichiometric compositions of nuclear ceramic fuels, but the discussions on the defect structure will be limited mainly to binary system. 

The binary systems have been reinvestigated and ScMnj (MgZn2-type), ScjSi3 (MnjSi3-type) and ScSi (CrB-type) were confirmed, but Sc3Sij with the defect AlB2-type could not be obtained at 800°C. Similarly, one of the so-called Nowotny chimney ladder structures of the general formula Mn Si2 , was confirmed... 

In this chapter, we will discuss the generation modes of thermodynamically stable point defects and the defect-chemical logic to calculate the equilibrium defect structure of a given system. As a stereotype of systems, we will consider only a binary oxide MO, but the idea and logic can be readily extended to other binary, ternary and higher systems with minor modifications [2-6]. 

It was Ziman [77] who has noted that there is little hope, at least at present, to develop an experimental technique permitting the direct measurement of these correlation functions. The only exception are the joint densities x / (r> ) information about which could be learned from the diffraction structural factors of inhomogeneous systems. On the other hand, optical spectroscopy allows estimation of concentrations of such aggregate defects in alkali halide crystals as Fn (n = 1,2,3,4) centres, i.e., n nearest anion vacancies trapped n electrons [80]. That is, we can find x mK m = 1 to 4, but at small r only. Along with the difficulties known in interpretating structure factors of binary equilibrium systems (gases or liquids), obvious specific complications arise for a system of recombining particles in condensed media which, in its turn, are characterized by their own structure factors. 

CS in Rutile. The reduced rutile, Ti02 c, based C5 phase systems show behaviour typical of the CS phases in general. In the binary Ti-0 system the slightest reduction of Ti02 results in the formation of CS planes, which lie on 132 planes of the parent structure. Despite claims to the contrary, there is no unequivocal evidence in the literature to show that there is any tolerance of point-defect populations to account for the lowering of stoicheiometry below Ti02.o-... 

Since we have only one non-stoichiometric solid phase, we will approach the study of this system by quasi-chemistiy of structure elements. However, this approach presents some difficulties. Indeed, hydrated salts are relatively conplex solids with at least three principal components the anion (itself often complex), the cation, and water. If salt admits several limiting hydrates, water molecules are not all equivalent. All these complexities require a simplification of the representation of solid. With this intention, we consider hydrated solids as pseudo-binary (see section 2.4.1) of which one of the components is the water concerned with dehydration and the other component is the skeleton of anhydrous salt or incorporates possible n molecules of water not implicated in the equilibrium under study. We will disregard specific defects related to the skeleton and thus take into account the following structure elements ... 

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