4-2 oxidation state nonstoichiometric oxides

An additional problem is encountered when the isolated solid is non-stoichiometric. For example, precipitating Mn + as Mn(OH)2, followed by heating to produce the oxide, frequently produces a solid with a stoichiometry of MnO ) where x varies between 1 and 2. In this case the nonstoichiometric product results from the formation of a mixture of several oxides that differ in the oxidation state of manganese. Other nonstoichiometric compounds form as a result of lattice defects in the crystal structure. ... 

The known halides of vanadium, niobium and tantalum, are listed in Table 22.6. These are illustrative of the trends within this group which have already been alluded to. Vanadium(V) is only represented at present by the fluoride, and even vanadium(IV) does not form the iodide, though all the halides of vanadium(III) and vanadium(II) are known. Niobium and tantalum, on the other hand, form all the halides in the high oxidation state, and are in fact unique (apart only from protactinium) in forming pentaiodides. However in the -t-4 state, tantalum fails to form a fluoride and neither metal produces a trifluoride. In still lower oxidation states, niobium and tantalum give a number of (frequently nonstoichiometric) cluster compounds which can be considered to involve fragments of the metal lattice. 

A number of metal oxides are known to form nonstoicbiometric compounds, in which the ratios of atoms that make up the compound cannot be expressed in small whole numbers. In the crystal structure of a nonstoichiometric compound, some of the lattice points where one would have expected to find atoms are vacant. Transition metals most easily form nonstoichiometric compounds because of the number of oxidation states that they can have. For example, a titanium oxide with formula TiO, I( is known, (a) Calculate the average oxidation state of titanium in this compound. 

Uranium dioxide, UQ2, can be further oxidized to give a nonstoichiometric compound U024v, where 0 < x < 0.25. See Exercise 5.77 for a description of nonstoichiometric compounds, (a) What is the average oxidation state of uranium in a compound with composition UO,j-> (b) If we assume that the uranium exists in either the +4 or the +5 oxidation state, what is the fraction of uranium ions in each ... 

Uranium oxides are of importance in the nuclear industry, and for this reason considerable effort has been put into understanding their nonstoichiometric behavior. The dioxide, U02 crystallizes with the fluorite structure with an ideal composition MX2 (Fig. 4.7a) but is readily prepared in an oxygen-rich form. In this state it is... 

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).]...

Effect of Oxide Mineralogy on Reductive Dissolution. Oxide/hydrox-ide surface structures and the coordinative environment of metal centers may change substantially throughout the course of a reductive dissolution reaction. Nonstoichiometric and mixed oxidation state surfaces produced during surface redox reactions may exhibit dissolution behavior that is quite different from that observed with more uniform oxide and hydroxide minerals. 

G. L. Frey, A. Rothschild, J. Sloan, R. Rosentsveig, R. Popovitz-Biro, and R. Tenne, Investigations of nonstoichiometric tungsten oxide nanoparticles, J. Solid State Chem. 162(2), 300 (2001). 

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. 

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... 

By applying an outside potential, many metal oxide films are switchable to a nonstoichiometric redox state, creating an intervalence charge transfer with an... 

Notably, SVO can display a variety of phases, both stoichiometric and nonstoichiometric. Thus, variations in reaction conditions, starting materials, and reagent stoichiometries for the preparation of SVO can result in a wealth of products that display different structures and different properties. In addition, the variety of oxidation states available to the silver and especially the vanadium components of SVO, plus the open structure of some of the SVO materials, suggest that these materials are well suited for electron transfer applications. It is thus logical and not surprising that reports of SVO battery applications and SVO redox catalyst applications appear within similar time frames. Some reports involving the structure of SVO solids and the catalysis of organic substrate oxidation by SVO-based catalysts will be described in Section 13.2, due to their possible relevance to the SVO battery chemistry described in Section 13.3. 

The surface of the nanocrystalline nonstoichiometric tungsten oxide conditioned on air (sample NN1) is almost completely formed of crystalline hydrate W03 (H20). Thus on the W4f-spectra (Fig. 2-3) the main component is comp, e with Ep=36.1 eV, on the Ols-spectra (Fig. 3-3) along with the 02 -states, the contributions from OH-groups (comp, g) and from H20 (comp, m) are present. 

After the anneal on air of the nonstoichiometric tungsten oxide nanopowder the signal from crystalline hydrate WO3 (H20) (comp, e) in the spectrum of W4f-level (sample NN2, Fig. 2-4) disappeared. The component d from W6+-states of the oxide dominated in the spectrum of W4f-level after anneal on air and the component c from W5+-states (EpW4f7/2=34.8 eV) appeared in the low energy region. 

Simultaneous appearing of W5+-, W6 -statcs in the W4f-spectrum are connected with the phase of nonstoichiometric tungsten oxide. Taking into consideration the presence of OH-groups in the sample NN2 and knowing the contributions of W5+-, W6+-states (Table 1) it is possible to determine a coefficient x for matrix Wx5+Wi x6+03 x(0H)x, which equals x=0.09. Thus the sample NN2 can be classified as protonated oxide WO2.9i (OH)0.09. 

Because intermetallic systems undoubtedly display certain special features that follow from their metallic binding forces, considerable importance attached to the growing evidence that the chalcogenides, the essentially ionic oxides, the nitrides, and other representative binary compounds of the transition metals were, not infrequently, both variable and irrational in composition. Schenck and Ding-mann s equilibrium study of the iron-oxygen system (39) was notable in this connection They showed that stoichiometric ferrous oxide, FeOi 000, the oxide of an important and typical valence state, did not exist. It lay outside the broad existence field of a nonstoichiometric phase. It is, perhaps, still not certain... 

There are also many oxides that are nonstoichiometric. These commonly consist of arrays of close-packed oxide ions with some of the interstices filled by metal ions. However, if there is variability in the oxidation state of the metal, nonstoichiometric materials result. Thus iron(II) oxide generally has a composition in the range FeO0.90 to FeOo.95, depending on the manner of preparation. There is an extensive chemistry of mixed metal oxides. 

Nonequihbrium concentrations of point defects can be introduced by materials processing (e.g. rapid quenching or irradiation treatment), in which case they are classified as extrinsic. Extrinsic defects can also be introduced chemically. Often times, nonstoichiometry results from extrinsic point defects, and its extent may be measmed by the defect concentration. Many transition metal compounds are nonstoichiometric because the transition metal is present in more than one oxidation state. For example, some of the metal ions may be oxidized to a higher valence state. This requires either the introduction of cation vacancies or the creation of anion interstitials in order to maintain charge neutrality. The possibility for mixed-valency is not a prerequisite for nonstoichiometry, however. In the alkah hahdes, extra alkah metal atoms can diffuse into the lattice, giving (5 metal atoms ionize and force an equal number... 

---