Solid nonstoichiometric

In many nonstoichiometric solids high-temperature structures contain disordered defects, but, as in the previous example, these order or form aggregates at lower temperatures. If this aggregation could be suppressed, low-temperature ionic conductivity would be enhanced. An example of this strategy is provided by compounds with the... 

How does composition variation change the electronic properties of a solid Why does the electronic conductivity of a nonstoichiometric solid change as the surrounding partial pressure changes ... 

The discussion of Brouwer diagrams in this and the previous chapter make it clear that nonstoichiometric solids have an ionic and electronic component to the defect structure. In many solids one or the other of these dominates conductivity, so that materials can be loosely classified as insulators and ionic conductors or semiconductors with electronic conductivity. However, from a device point of view, especially for applications in fuel cells, batteries, electrochromic devices, and membranes for gas separation or hydrocarbon oxidation, there is considerable interest in materials in which the ionic and electronic contributions to the total conductivity are roughly equal. 

It is possible to make nonstoichiometric solids that have ionic conductivities as high as 0.1-1000 S m-1 (essentially the same as for liquid electrolytes) yet negligible electronic conductances. Such solid electrolytes are needed for high energy density electrical cells, fuel cells, and advanced batteries (Chapter 15), in which mass transport of ions between electrodes is necessary but internal leakage of electrons intended for the external circuit... 

Nonstoichiometric solid solutions are substances whose composition approximates that of stoichiometric compounds, but which have a range of compositions. The problem of applying thermodynamics to such substances is primarily how to express the composition of the solution. The simplest choice would be to use the mole fractions or atom fractions in terms of the components. In such a case the effects of the formation of the compound from the components would be contained in the values of the activity coefficients or excess chemical potentials. 

As indicated by West in Ref. X, chaps. 9 and 11, a phase can be defined as a physically distinct and mechanically separable (in principle) portion of a system, each phase being itself homogeneous. It is also pointed out by West that this definition is sometimes difficult to apply in nonstoichiometric solids and mixed crystalline systems. 

When the composition of a crystal is defined by a distinct chemical formula e.g., Si02), it is known as a stoichiometric compound. If the composition of the crystal is altered upon doping or thermal treatment, the resulting solid may deviate from the original chemical formula, forming a nonstoichiometric solid. Nonstoichiometry and the existence of point defects in a solid are often closely related. For instance, the formation of x anion vacancies per each quartz unit cell will result in the nonstoichiometric compound Si02-x ... 

Determine the oxidation states present in nonstoichiometric solids (Section 21.5, Problems 41-42). 

Representations of lattice imperfections as individual and randomly-distributed entities within crystalline sohds have provided satisfactory theoretical models for the explanation of a number of phenomena. This approach [81], however, has not so far been shown to be capable of accounting for observed reactivities, structures and thermodynamic properties of a munber of the more highly defective and nonstoichiometric phases. Evidence has accumulated which indicates that properties of nonstoichiometric solids are determined by structures and mechanisms which depend on the interactions of imperfections which are distributed in a regular... 

This group can be supplemented also by amalgam electrodes (or other liquid electrodes) and electrodes fabricated from nonstoichiometric solids capable of changing their composition reversibly (intercalation compounds based on carbons, oxides, sulfides, and multicomponent salts, particularly, Li-intercalating electrodes of batteries). 

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