Nonstoichiometric oxides

Thousands of compounds of the actinide elements have been prepared, and the properties of some of the important binary compounds are summarized in Table 8 (13,17,18,22). The binary compounds with carbon, boron, nitrogen, siUcon, and sulfur are not included these are of interest, however, because of their stabiUty at high temperatures. A large number of ternary compounds, including numerous oxyhaUdes, and more compHcated compounds have been synthesized and characterized. These include many intermediate (nonstoichiometric) oxides, and besides the nitrates, sulfates, peroxides, and carbonates, compounds such as phosphates, arsenates, cyanides, cyanates, thiocyanates, selenocyanates, sulfites, selenates, selenites, teUurates, tellurites, selenides, and teUurides. 

T. Sorenson (eds). Nonstoichiometric Oxides. Academic Piess New York (1981). 

Nonstoichiometric oxide phases are of great importance in semiconductor devices, in heterogeneous catalysis and in understanding photoelectric, thermoelectric, magnetic and diffusional properties of solids. They have been used in thermistors, photoelectric cells, rectifiers, transistors, phosphors, luminescent materials and computer components (ferrites, etc.). They are cmcially implicated in reactions at electrode surfaces, the performance of batteries, the tarnishing and corrosion of metals, and many other reactions of significance in catalysis. ... 

F. A. Kroger, Nonstoichiometric Oxides, O. Toft (Ed.), Acad. Press, Sorensen, 1981... 

The first sample has the greatest deviation from stoichiometry. Samples JV°1 - JV°4 have been found to be nonstoichiometric oxide -hydroxide type. They have a mixed conductivity - ionic (o,) and electronic (ae). The ionic one is due to the presence of OH" - groups. Namely, they stabilize the defects of chemical nature in such compounds. These defects are determined by the presence of Mn4+ and Mn3+ in the same crystallographic position. 

The observed higher rate of mass transfer in nonstoichiometric oxides makes it possible to operate at discharge current densities on the order of 2 - 5 mA/cm2, which is several times greater than that used with the conventional manganese dioxide. Thus, the purposeful disordering of structure of a cathode material of manganese dioxide allows to utilize such power sources in extreme conditions - for element discharge at a low... 

Oxide compounds are widely used as cathodic materials in the power sources and electrochemical generators. Some literature data indicates that cathodic materials based on nonstoichiometric oxide compounds make it possible to increase the solid-phase reduction process. The kinetics of electrochemical reactions and consequently the current density are the higher, the greater the degree of deviation from stoichiometry, and the lager the number of the defects in the compounds structure [1,2]. 

The voltage measured will now appear to drift as the composition range of the nonstoichiometric oxide is crossed. The voltage will become constant above and below the composition range of the oxide. Note that this is closely related to the variation of oxygen partial pressure over a nonstoichiometric oxide (see Sections 7.3, 7.4). 

Taking as an example an ionic oxide MO, this material can be made into a metal-excess nonstoichiometric material by the loss of oxygen. As only neutral oxygen atoms are removed from the crystal, each anion removed will leave two electrons behind, which leads to electronic conductivity. The oxygen loss can be incorporated as oxygen vacancies to give a nonstoichiometric oxide with a formula MOi v, or the structure can assimilate the loss and compensate by the introduction of cation interstitials to give a formula M1+xO. 

Nickel oxide is a classical nonstoichiometric oxide that has been studied intensively over the last 30-40 years. Despite this, there is still uncertainty about the electronic nature of the defects present. It is well accepted that the material is an oxygen-excess phase, and the structural defects present are vacancies on cation sites. Although it is certain that the electronic conductivity is by way of holes, there is still hesitancy about the best description of the location of these charge carriers. 

Figure 7.7 Equilibrium oxygen partial pressure for a nonstoichiometric oxide YBa2Cu3Ox (a) composition, x, versus temperature under an oxygen partial pressure of 1 bar (b) oxygen partial pressure versus temperature for a composition of YBa2Cu306.5 and (c) oxygen partial pressure versus composition, x, for a temperature of 600°C. [Adapted from data in P. Karen, J. Solid State Chem., 179, 3167-3183 (2006).]...

The situation is different in the case of a nonstoichiometric oxide MO, in equilibrium with the vapor phase. In this case, if the composition of the oxide varies slightly, a second solid phase does not appear. Over the composition range of the nonstoichiometric oxide, the number of phases, P, will be two, the nonstoichiometric oxide MO and 02, gas, and Eq. (7.3b) gives... 

This means that two thermodynamic parameters can now vary freely while the system still remains in equilibrium, and the system is said to be bivariant. In this case, the composition becomes an important parameter and must be added to the partial pressure and temperature. Thus, the partial pressure over a nonstoichiometric oxide in a sealed tube will no longer depend solely upon the temperature but on the composition as well (Fig. 7.6) (see also Section 6.8.2). 

This behavior can be illustrated by the nonstoichiometric oxide YBa2Cu3Ov, where x can take values between 6.0 and 7.0. Suppose that some oxide with a composition within the stoichiometry range YBa2Cu306.o-YBa2Cu307.o and 02 gas are... 

The previous sections imply that in the case of nonstoichiometric oxides that favor electronic compensation of composition change a relationship exists between the variation in partial pressure and the electronic conductivity. The form that this relationship takes depends upon the way in which the oxide accommodates composition variation. 

Metal-excess oxides can change composition by way of metal interstitials or oxygen vacancies. The formation of cation interstitials in a nonstoichiometric oxide MO can be represented by... 

An increase of oxygen partial pressure will cause the conductivity of a metal-excess nonstoichiometric oxide to ... 

The slope of the log (conductivity) versus log(oxygen pressure) over a nonstoichiometric oxide has a slope of +1. The material is ... 

In conclusion I should like to consider a few of the chemical investigations which might be accomplished in the rare earth field by Mossbauer spectroscopy. The study of nonstoichiometric oxides has been discussed earlier, but there is the problem of finding an appropriate doping nuclide for the praseodymium oxide system. The element most capable of following the changes in oxidation state of the praseodymium is terbium-159, which does have a Mossbauer state, however, with a rather broad resonance (58,0 k.e.v., = 0.13 nsec.). Nevertheless, with a sufiiciently... 

The most common valence state in solid compounds is -i-3. A +4 valence state is known for the metal in its dioxide, Tb02, and tetrafluoride, TbF4. Terbium also forms several nonstoichiometric oxides of approximate composition Tb407. 

Manes, L. A new method of statistical thermodynamics and its application to oxides of the lanthanide and actinide series, in Nonstoichiometric oxides (Sprensen, O. T. ed.). Academic Press, New York, Chapt. 3, 99 (1981)... 

In addition to the partially oxidized tetracyanoplatinates, bis(oxalato)platinate-(II) can be nonstoichiometrically oxidized by chemical oxidants13 to form highly lustrous needlelike crystals containing platinum in the 2.36 oxidation state. These complexes have not been characterized to the extent of the tetracyano-platinate complexes however, the oxalato complexes are reported to be highly conducting.13 The starting material is bis(oxalato)platinate(II), which can be prepared in 30% yields from hexachloroplatinate(lV) and potassium oxalate.9... 

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