Aliovalent Cations

In dispersed metal-support systems (Fig. 11.2 right), one can vary pe(M) - M-e(S) by varying the support or by doping the support with aliovalent cations. This is known in the literature as dopant-induced metal-support interactions (DIMSI).8,11,41,42 Thus one can again vary the electrochemical potential and thus the coverage of backspillover O2 on the supported catalyst surface. 

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). 

In this section we are concerned with the properties of intrinsic Schottky and Frenkel disorder in pure ionic conducting crystals and with the same systems doped with aliovalent cations. As already remarked in Section I, the properties of uni-univalent crystals, e.g. sodium choride and silver bromide which contain Schottky and cationic Frenkel disorder respectively, doped with divalent cation impurities are of particular interest. At low concentrations the impurity is incorporated substitutionally together with an additional cation vacancy to preserve electrical neutrality. At sufficiently low temperatures the concentration of intrinsic defects in a doped crystal is negligible compared with the concentration of added defects. We shall first mention briefly the theoretical methods used for such systems and then review the use of the cluster formalism. 

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... 

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... 

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. 

Figure 4 Influence of aliovalent cation doping on specific electrical conductivity and activation energy of electron conduction of 0.5% Pt/Ti02(D) at 333 K, in vacuum. (From Ref. 82.)...

The influence of aliovalent cation doping of the support (Ti02) on the catalytic properties of supported Pt and Rh crystallites was also investigated under other reaction conditions, among which the photocatalytic cleavage of water [116] and the reduction of NO by propylene [117] in the presence or absence of oxygen. In the case... 

An alternative interpretation of the phenomenon of metal-support interactions induced by doping of semiconductive carriers with aliovalent cations is based on the theory of electrochemical promotion or the NEMCA effect. According to this interpretation, the charge carriers transported from the carrier to the metal particles are oxygen ions, which diffuse to the surface of the metal particles, thus altering the surface work function and, subsequently, chemisorptive and catalytic parameters. Work is currently in progress to elucidate the mechanism of induction of metal-support interactions by carrier doping. 

A network of aliovalent cations and oxygen ions, actually covalently bonded, in which the dual valence states in both species (copper and oxygen) are sufficiently close energetically to establish conductivity. 

TjxOi- The Kroger-Vink diagram for yttria-doped zirconia is shown in Fig. 7.16a, the construction of which is left as an exercise to the reader. In pure zirconia, the concentration of oxygen vacancies is simply y/Ks-However, as noted earlier, that value can be dramatically increased by doping with aliovalent cations such as Ca or. Based on this diagram. 

Sintering conditions can also have an important effect on the permittivity. The replacement of various aliovalent cations such as La and Nb in BaTi03 has also been shown to inhibit grain growth which, as... 

The microporous crystallite of AlPO-n has the following characteristics (1) molecular-sized pore diameter (2) variety of pore structures, some of which are found in other zeolites (3) no inherent acidity, although both acidity and ion-exchange properties can be altered by the isomorphous substitution by aliovalent cations and (4) high thermostability. Among these properties, most interest has focused on new pore structures and solid acidity. However, the high thermal stability of AlPO-n-based molecular sieves has also prompted new research. 

Tsai et a/. also approached the problem of increasing O2 flux and stability. Their approach was to balance the substitutions on the A and B sites of the ABO3 perovskite. Stability is strongly influenced by a stable BO3 skeletal sublattice, and the choice of Fe with a mild Co substitution gives stability with reasonable oxygen anion conductivity. Then different amounts and types of aliovalent cations (Ca, Sr ", Ba " ) were partially substituted for La in a LaFeo.8Coo.203 s perovskite framework to attain higher electron conductivity. 

In the search for new electrolyte materials, the perovskite based systems (ABO3) have been considered as alternative options, particularly because ABO3 can take on a number of different structures, and can be doped with aliovalent cations on both the A (for example Sr) and B (for example Mg)-sites. They can also accommodate veiy large concentrations of anion vacancies into their structures. 

Doping by aliovalent cations is indispensable for the appearance of protonic conduction in these oxides. It seems that electron holes and oxide ion vacancies formed by doping might play an important role in the formation of protons. For example, substitution of Yb for Ce in SrCeOg may provide oxygen vacancies Vq as a means of charge compensation ... 

Rare-earth fluorides are good fluorine-ion conductors (see sect 5.2). Addition of aliovalent cations significantly increases the fluorine-ion conductivity even fiulher. For example, single crystals of Lap3 doped with Eup2 are widely used in commercial apphcations of ion-selective electrodes (ISE) as specific electrodes (Frant and Ross 1966). In the field of ISE, only the pH-sensing glass electrode is more widely used. Fluoride-ion detection is important in sea water, water minerals, rocks, fossils and minerals, biomedical applications, potable water and plant and animal metabolism. 

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