Non-stoichiometric solid

The thermodynamic functions of non-stoichiometric solids at very high deviations from stoichiometry are strongly affected by defect clusters and molecularities. The detailed theoretical description of the interactions between defects and the lattice as well... 

Note that aZn(s) = 1 since zinc is a pure substance. A more complex relationship is required if one component is a non-stoichiometric solid which can exist over a range of compositions. E % is known as the standard electrode potential of zinc. 

The major products from most metal oxalate decompositions can be predicted from thermodynamic data [46,47] (Chapter 2). Interpretation of observations must allow for the possibility that the identifiable sohd phases may not be those initially formed, but arise as stable products of a secondary process. Secondary reactions may involve adsorption-desorption steps at the surfaces of finely-divided, reactive and perhaps non-stoichiometric solids. The composition of product gases may also vary within the mass of reactant, through chemical interactions between primary products. 

The titanium-oxygen system is particularly complex among the binary metal-oxygen systems. It exhibits many stoichiometric and widely non-stoichiometric solid phases as well as two gaseous compounds, TiO(g) and Ti02(g). The complexity explains the large number of investigations carried out... 

The appearance of superconductivity and the magnitude of the superconducting transition temperature are also closely coimected with the composition of these non-stoichiometric solids, which all exhibit considerable variation in oxygen content. The superconducting transition temperature is also affected by external factors such as pressure and crystal elastic strain. 

Experience leads us to classify these solids into two families stoichiometric solids and non-stoichiometric solids. 

L Vandenbulcke. Thermodynamic approach to the deposition of non-stoichiometric solids from the gas phase Example of titanium carbide at high temperature. J Electrochem Soc 128 1584, 1981. 

In non-stoichiometric solids, there exist variations that can be very low (e.g. of the range of about 10 ) compared with the stoichiometry of the ideal sohd, but the... 

Doping is particularly interesting for the ionic compounds if the doping element has a oxidation number different from that of the normal elements, which constitute the solid. We will study such effects on stoichiometric and non-stoichiometric solids. 

The addition of a foreign element with a valence different from that of the basic con nents will allow controlling the electronic defect in a non-stoichiometric solid. Take as an example a sohd of Wagner with cation vacarKies such as the FeO arrd dope it by lithium, by using the vapor of this metal. We have seen earlier (see section 2.3.2.4) that in FeO, the electric corrqrensation of the anion vacancies was due to electron holes trapped on iron of the lattice. The lithium irrtroduction into the iron vacancies causes a decrease in the number of trivalent ions, that is, the nrrrrrber of electron holes (Figure 2.8). The reaction of introduction is as follows ... 

We examine the equilibrium between a non-stoichiometric solid and a gas likely to react with an element of the solid. We will take the example of hydrogen with an oxide, for example, the ceria, from the following quasi-chemical reaction ... 

We will now approach the quantitative aspect of doping, that is, the effect of the introduction of a foreign element into a solid on the concentrations of its various structure elements. As in Chapter 2 (section 2.6), we will distinguish between stoichiometric solids and non-stoichiometric solids. We will discuss this through an example, in both cases, the effects of the introduction of a cation. 

To quantitatively calculate the effect of doping with cations of a non-stoichiometric solid, we examined the case of potassium-doped zinc oxide. This oxide is at equilibrium with oxygen ... 

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 ... 

The ionic solids often present properties of conduction of electricity. This one can be ensured by ions, that is, ionic conduction. For the non-stoichiometric solids, it is ensured by charge carriers electrons or electron holes. This is the electronic conductivity. 

If we take the case of a non-stoichiometric solid of Wagner (see section 2.3.2) with only one disorder, total conductivity is the snm of the ionic contribution and the electronic contribntion ... 

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