Nonstoichiometry, oxides

The reductive (and oxidative) nonstoichiometry and the stability in reducing oxygen atmospheres of perovskite-type oxides was reviewed by Tejuca et al. [174]. Data from temperature programmed reduction (TPR) measurements indicate that... 

Actinide chalcogenides can be obtained for instance by reaction of the elements, and thermal stability decreases S > Se > Te. Those of a given actinide differ from those of another in much the same way as do the oxides. Nonstoichiometry is again prevalent and, where the actinide appears to have an uncharacteristically low oxidation state, semimetallic behaviour is usually observed. 

Some Ionic Radii Relevant to Oxidative Nonstoichiometry in Perovskites[Pg.255]

Other manganites LnMn03, where Ln = Nd, Sm, Dy, Y, Er, were found to exhibit oxidative nonstoichiometry (90, 91). Wachowski et al. (92) prepared a perovskite of starting composition LaFe03.22. However, Tofield and Scott (79) did not find nonstoichiometry in LaFe03. The different behavior of these compounds should be related to the significant difference in the temperatures used for the final heat treatment in air (500°C for LaFe03.22 1100°C for the stoichiometric compound). This... 

Voorhoeve et al. (30) reported oxidative nonstoichiometry for the perovskites LaM03+x (M = Cr, Mn, Fe) and reductive nonstoichiometry for LaCo03-x. XPS measurements carried out on a similar series of LaM03 oxides after heat treatment in air at 900°C indicated a surface nonstoichiometry that changes from oxidative (M = Cr, Mn) to reductive (M = Fe, Co, Ni, Rh) (102). This nonstoichiometry was found to be much more marked than that observed in the bulk. The more oxygen deficient perovskites were those that are more easily reducible (M = Co, Ni, Rh). Tabata et al. (103) found also significant differences between the chemical composition of the surface (determined by XPS) and of the bulk (determined by X-ray fluorescence spectroscopy) in a series of Sr Lai- CoOs oxides. These results indicate a very different behavior of the surface with respect to the bulk in these compounds. This is an important factor to be considered when trying to correlate the composition of a perovskite with its catalytic performance. 

Ethanol is also considered a VOC, and literature reported its total oxidation over Lai ,Ca j,Mn03 perovskites prepared by the citrate method [49]. Pure unsubstituted perovskite phases, LaMnOs, presented a certain amount of Mn and cationic vacancies, which is generally called oxidative nonstoichiometry. The electronic unbalance yield by the partial substitution of lanthanum by calcium caused an oxidation state increase of part of the manganese occupying the B sites of the structure. Concurrently, the amount of cationic vacancies decreased with the increase of calcium amount. In spite of these differences, the catalytic activity in the combustion of ethanol showed no evident differences with the value of X and the total oxidation was achieved on all the compositions at temperatures below 220 °C. 

The reductive (and oxidative) nonstoichiometry and the stability in reducing oxygen atmospheres of perovskite-type oxides was reviewed by Tejuca et al. Data from temperature-programmed reduction (TPR) measurements indicate that the stability (or reducibility) of the perovskite oxides increases (decreases) with increasing size of the A ion, which would be consistent with the preferred occupancy of the larger Lrf ion in a 12-fold coordination. The trend is just the reverse of that of the stability of the corresponding binary oxides. The ease of reduction increases by partial substitution of the A ion, e.g., La by Sr. Trends in the thermodynamic stabilities of perovskite oxides have been systematized in terms of the stabilization energy from their constituent binary oxides and the valence stability of the transition metal ions by Yokokawa et al. ... 

Solid State Chemistry (thermodynamic, kinetic, and structural properties of oxides, nonstoichiometry and extended defects in nonmetallic solids, application of high-resolution electron microscopy to solid state chemistry)... 

The chemistry of ceramics plays a role in their behaviour during sintering. Nonstoichiometry of oxides has been found to play a major role in the extent to which a... 

The sulfides have been less thoroughly examined than the oxides but it is clear that a number of stable phases can be produced and nonstoichiometry is again prevalent (p. 679). The most important are the disulfides, which are semiconductors with metallic lustre. TiS2 and ZrS2 have the Cdl2 structure (p. 1211) in which the cations occupy the octahedral sites between alternate layers of hep anions. 

Oxides of the actinides are refractory materials and, in fact, Th02 has the highest mp (3390°C) of any oxide. They have been extensively studied because of their importance as nuclear fuels. However, they are exceedingly complicated because of the prevalence of polymorphism, nonstoichiometry and intermediate phases. The simple stoichiometries quoted in Table 31.5 should therefore be regarded as idealized compositions. 

Metal oxides are not normally stoichiometric, although the nonstoichiometry may be too small to be detected by ordinary methods of... 

Nonstoichiometry is relatively common among mixed metal oxides, in which more than one metal is present. In 1986 it was discovered that certain compounds of this type showed the phenomenon of superconductivity on cooling to about 100 K, their electrical resistance drops to zero (Figure 20.9). A typical formula here is YBa2Cu30 where x varies from 6.5 to 7.2, depending on the method of preparation of the solid. 

Ceria is another type of mixed conducting oxide which has been shown already to induce electrochemical promotion.71 Ceria is a catalyst support of increasing technological importance.73 Due to its nonstoichiometry and significant oxygen storage capacity it is also often used as a promoting additive on other supports (e.g. y-A Cb) in automobile exhaust catalysts.79 It is a fluorite type oxide with predominant n-type semiconductivity. The contribution of its ionic conductivity has been estimated to be 1-3% at 350°C.71... 

P. Kofstad, Nonstoichiometry, Diffusion and Electrical Conductivity in Binary Metal Oxides, Wiley Interscience, New York (1972). 

In order to explain the changing optical properties of AIROFs several models were proposed. The UPS investigations of the valence band of the emersed film support band theory models by Gottesfeld [94] and by Mozota and Conway [79, 88]. The assumption of nonstoichiometry and electron hopping in the model proposed by Burke et al. [87] is not necessary. Recent electroreflectance measurements on anodic iridium oxide films performed by Gutierrez et al. [95] showed a shift of optical absorption bands to lower photon energies with increasing anodic electrode potentials, which is probably due to a shift of the Fermi level with respect to the t2g band [67]. 

Nonstoichiometry of the oxides can be due to a number of reasons, such as hydration,159 incomplete oxidation,158 and the generation of defects at interfaces.157 An important factor affecting the chemical composition of the oxides is the incorporation of electrolyte species into the growing alumina. There have even been suggestions to use this for impurity doping of oxides and modifying their properties.161 Various kinds of anion distributions and mechanisms of anion incorporation and their influence on oxide properties have been reported. The problems attracting attention are ... 

Apart from structures that are built of slabs, modular structures that can be constructed of columns in a jigsawlike assembly are well known. In the complex chemistry of the cuprate superconductors and related inorganic oxides, series of structures that are described as tubular, stairlike, and so on have been characterized. Alloy structures that are built of columns of intersecting structures are also well known. Structures built of linked columns, tunnels, and intersecting slabs are also found in minerals. Only one of these more complex structure types will be described, the niobium oxide block structures, chosen as they played a significant role in the history of nonstoichiometry. 

Figure 7.1 Schematic representation of the electronic consequences of nonstoichiometry in oxides.

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