Aliovalent oxides

Ejfects of dissolution of aliovalent oxides in the metal-deficient oxide Mj.yO. 

Bismuth sesquioxide, BijOj, exhibits a high oxide ion conductivity at high temperature without doping of aliovalent cations. The oxide transforms from the monoclinic... 

Four solid oxide electrolyte systems have been studied in detail and used as oxygen sensors. These are based on the oxides zirconia, thoria, ceria and bismuth oxide. In all of these oxides a high oxide ion conductivity could be obtained by the dissolution of aliovalent cations, accompanied by the introduction of oxide ion vacancies. The addition of CaO or Y2O3 to zirconia not only increases the electrical conductivity, but also stabilizes the fluorite structure, which is unstable with respect to the tetragonal structure at temperatures below 1660 K. The tetragonal structure transforms to the low temperature monoclinic structure below about 1400 K and it is because of this transformation that the pure oxide is mechanically unstable, and usually shatters on cooling. The addition of CaO stabilizes the fluorite structure at all temperatures, and because this removes the mechanical instability the material is described as stabilized zirconia (Figure 7.2). 

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. 

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. 

The most well-studied and useful materials to date are those with fluorite-related structures, especially ones based on ZrOj, ThOj, CeOj and Bi203 (Steele, 1989). To achieve high oxide ion conductivity in ZrOj, CeOj and ThOj, aliovalent dopants are required that lead to creation of oxide vacancies. Fig. 2.2, scheme 4. The dopants are usually alkaline earth or trivalent rare earth oxides. 

The activation energy for oxide ion conduction in the various zirconia-, thoria- and ceria-based materials is usually at least 0.8 eV. A significant fraction of this is due to the association of oxide vacancies and aliovalent dopants (ion trapping effects). Calculations have shown that the association enthalpy can be reduced and hence the conductivity optimised, when the ionic radius of the aliovalent substituting ion matches that of the host ion. A good example of this effect is seen in Gd-doped ceria in which Gd is the optimum size to substitute for Ce these materials are amongst the best oxide ion conductors. Fig. 2.11. 

In the fluorides, chlorides and oxides of the Group-A main-group metals and the transition metals zirconium and hafnium, aliovalent cation substitutions are generally charge-compensated by the introduction of native defects (e.g. an oxygen vacancy in Zr, ,Ca 02 x) because the intrinsic is large however, in some oxides neutral oxygen or water may... 

Diffusion in ionically bonded solids is more complicated than in metals because site defects are generally electrically charged. Electric neutrality requires that point defects form as neutral complexes of charged site defects. Therefore, diffusion always involves more than one charged species.9 The point-defect population depends sensitively on stoichiometry for example, the high-temperature oxide semiconductors have diffusivities and conductivities that are strongly regulated by the stoichiometry. The introduction of extrinsic aliovalent solute atoms can be used to fix the low-temperature population of point defects. 

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. 

The discussion draws on the extensive studies by Philips researchers [7] and [8] and by D.M. Smyth and co-workers [4], Several cases of oxide systems in which the conductivity is controlled by the substitution of aliovalent cations are given in Chapter 4. For instance, Sb5+ can replace Sn4+ in SnC>2 and be compensated by an electron in the conduction band conferring n-type conductivity (see Section 4.1.4). However, models for oxide systems are generally more complex than for... 

The effects of deliberately added donors, such as titanium, and acceptors, such as iron and magnesium, on electrical conductivity have been studied. Doping with aliovalent ions affects the concentration of intrinsic defects and, in consequence, the diffusivity of A1 and O. In the case of variable-valency dopants, changes in p0l change the fraction of dopants in the aliovalent state and the nature and concentration of the defects. For example, the dopant Ti substitutes for A1 and, in the fully oxidized state, produces the defect TiA1, compensated by Va", so that... 

Vacancies can also make possible the low-temperature manipulation of a solid s stmcture. For example, aliovalent ion exchange (e.g. Ca for Na ) has been used to create extrinsic vacancies in some transition metal oxides at relatively low temperatures (Lalena et al., 1998 McIntyre et ah, 1998). These vacancies were then used to intercalate new species into the lattice, again at relatively low temperatures. Intercalation refers to the insertion of an extrinsic species into a host without a major rearrangement of the crystal structure. If the new species is a strong reducing agent, mixed-valency of the transition metal may be introduced into the host. 

For any particular solid, the relative activation barriers for the available mechanisms determine whether the anions or cations are responsible for the ionic conduction. For example, in a yttria-stabilized Zr02, with the formula Zri Y ,.02-(x/2). aliovalent substitution of Zr by Y generates a large number of oxygen vacancies, giving rise to a mechanism for oxide ion conduction. Indeed, it is found that the anions diffuse about six orders of magnitude faster than the cations. 

Nonstoichiometry is a pervasive aspect of oxide chemistry, particularly where the cation can assume two or more valences or aliovalent cation substitutions are facile. These can be classified into three rather broad ranges. Class 1 includes systems where the nonstoichiometry approaches or exceeds that which caimot be detected by classical methods of chemical analysis (i.e., less than 1 part in 1000) but may manifest itself in dramatic changes in electrical or optical properties. Class n includes systems where the nonstoichiometry is of the order of several mole % and readily discernible by chemical analysis, density measurements, or X-ray diffraction measurements of unit cell constants. Class 111 are those systems with broad ranges of nonstoichiometry such as the alkah metal tungsten bronzes. 

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