Aliovalent impurities

Nonstoichiometric Compounds Intrinsic defects are stoichiometric defects (i.e., they do not involve any change in overall composition). Defects can also be nonstoichiometric. In the case of extrinsic defects where the host crystal is doped with aliovalent impurities, the solid so formed is a nonstoichiometric compound because the ratio of the atomic components is no longer the simple integer. There is also... 

The composition variation described in the previous chapter has a considerable impact upon the electronic properties of the solid. However, it is often difficult to alter the composition of a phase to order, and stoichiometry ranges arc frequently too narrow to allow desired electronic properties to be achieved. Traditionally, the problem has been circumvented by using selective doping by aliovalent impurities, that is, impurities with a different nominal valence to those present in the parent material. However, it is important to remember that all the effects described in the previous chapter still apply to the materials below. The division into two chapters is a matter of convenience only. 

There are several ways in which a solid doped with an aliovalent impurity can maintain charge balance. It is by no means simple to be sure which compensation mechanism will hold, or even if one mechanism will hold over all of the doping range. However, there are some quantitative guidelines that apply, especially for oxides. The principal mechanism will depend upon how easily the host cation that is being replaced is oxidized or reduced. 

The occurrence of such ion trapping is clearly undesirable since it inevitably leads to a decrease in conductivity. In practice, in materials that contain potential traps such as charged aliovalent impurities/dopants, the conductivity values of a particular sample may actually decrease with time as the mobile ions gradually become trapped. Such ageing effects greatly limit the usefulness of a solid electrolyte in any device that needs to have a long working-life. 

Anion conduction, particularly oxide and fluoride ion conduction, is found in materials with the fluorite structure. Examples are Cap2 and Zr02 which, when doped with aliovalent impurities. Fig. 2.2, schemes 2 and 4, are F and 0 ion conductors, respectively, at high temperature. The 3 polymorph of 61303 has a fluorite-related structure with a large number of oxide vacancies. It has the highest oxide ion conductivity found to date at high temperatures, > 660 °C. 

Figure 5.1 Point defects in ionic solids Schottky defect, vacancy pair, Frenkel defect and aliovalent impurity (for definitions see Section 5.2).

A variety of techniques has been employed to investigate aliovalent impurity-cation vacancy pairs and other point defects in ionic solids. Dielectric relaxation, optical absorption and emission spectroscopy, and ionic thermocurrent measurements have been most valuable ESR studies of Mn " in NaCl have shown the presence of impurity-vacancy pairs of at least five different symmetries. The techniques that have provided a wealth of information on the energies of migration, formation and other defect energies in ionic solids are diffusion and electrical conductivity measurements. Electrical conductivity in ionic solids occurs by the motion of ions through vacancies or of interstitial ions. In the case of motion through vacancies, the conductivity, a, is given by... 

Several isovalent ions form solid solutions with KTP (Table II), showing that this structure is relatively tolerant, with respect to isovalent impurities, as are the traditional nonlinear optical oxide crystal structures. But due to the relatively limited range of nonstoichiometry in KTP, aliovalent impurities, such as divalent Ba, Sr and Ca introduced through ion exchange in nitrate melts, which substitute on the K site, are incorporated at concentrations less than one mole percent.(36) Typical impurity concentrations present in flux and hydrothermally grown KTP are shown in Table ID. 

Point defects are an important part of the work in this paper. There are many reasons for the formation of point defects in minerals and their presence can exert important perturbations on the properties of the material (4). Point defects are formed because of the thermally driven intrinsic disorder in a lattice, the addition of aliovalent impurities or dopants, the presence of metal-nonmetal nonstoichiometry, and the creation of nonideal cation ratios. The first three source of defects are well-known from binary compounds but the last is unique to ternary compounds. Ternary compounds are much more complex than the binary compounds but they also have gained a great deal of attention because of the variety of important behavior they exhibit including now the presence of superconductivity at high temperatures. The point defects can be measured by introducing probe ions into the lattice. 

Extrinsic defects form as a result of the introduction of impurities. The incorporation of aliovalent impurities in any host compound results in the formation of defects on one of the sublattices, in order to preserve the lattice site ratio. 

Hence aliovalent impurity concentrations affect the concentrations of interstitials and vacancies. Dopands can also act as sensitizers (they improve the formation of the latent image) or as efficient traps for defects Pb g for e Cu g for h and Cu g for 7 ,. 

The number of vacancies at low temperatures, called the extrinsic regime, will depend on the number of aliovalent impurities and at high temperatures, the intrinsic regime, the vacancies will be thermally degenerated and hence depend on the free-energy of formation of the vacancies, AG/... 

In the extrinsic domain where the vacancy concentrations are not determined by the Schottky disorder (equation 1) but by the concentration of aliovalent impurities dissolved in the material, the situation is slightly more complex. If, for simplicity, we suppose that there is only one aliovalent impurity M, of valence 2,... 

In the typical situation where the aliovalent impurities have valencies that are stable in the investigated range, the interval where the ionic transport number is noticeably different from zero is larger than that of the pure material (cf. Fig, 4). The variation of Pg, i.e. the shift of the Fermi levels that can be obtained by the electrochemical process, encompasses a larger interval... 

For nonstoichiometric oxides the concentration of electronic defects is determined by the deviation from stoichiometry, the presence of native charged point defects, aliovalent impurities and/or dopants. The concentration of electronic defects can be evaluated from proper defect structure models and equilibria. Various defect structure situations have been described in previous chapters and at this stage only one example - dealing with oxygen deficient oxides with doubly charged oxygen vacancies as the prevalent point defects - will be described to illustrate the electrical conductivity in nonstoichiometric oxides. 

The concentration of the intrinsic point defects can be influenced by aliovalent impurity doping. Assmning that only ionic point defect concentrations are affected, the following lattice reactions exemplify the formation of extrinsic disorder by substitution in MO, e g., MF2 in MO... 

If aliovalent impurities are present, these have to be included into Equation (5.26) because they influence the point defect concentrations. A great number of studies, including extrinsic disorder in nonstoichiometric compounds, have appeared in the literature, and the concepts have been extended to ternary and multinaiy compounds. Before discussing these compounds, the concepts will be applied to transition metal oxides, because these represent an important technological class of materials. 

Krdger F A, The Chemistry of Imperfect Crystals, 1974, Amsterdam North-Holland. Mrowec S, GrzesikZ, The influence of aliovalent impurities on transport properties of nonstoichiometric manganous sulphide . Bull Pol Acad Sci Chem, 2002 50 37-49. 

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