Vanadium nonstoichiometric oxides

Vanadium(II) Oxide. Vanadium(II) oxide is a nonstoichiometric material with a gray-black color, metallic luster, and metallic-type electrical conductivity. Metal—metal bonding increases as the oxygen content decreases, until an essentially metal phase containing dissolved oxygen is obtained (14). 

Like molybdenum and tungsten, vanadium forms a variety of nonstoichiometric oxides with the alkali metals and other cations, for example, Cu+, Ag+, Zn+, or Al+, which have bronze-like properties. " These compounds can be made by heating appropriate mixtures of V2O5 and the desired alkali metal carbonate or oxide in platinum vessels under argon at 700-800 °C and then cooling slowly over... 

The elements of Group 5 are in many ways similar to their predecessors in Group 4. They react with most non-metals, giving products which are frequently interstitial and nonstoichiometric, but they require high temperatures to do so. Their general resistance to corrosion is largely due to the formation of surface films of oxides which are particularly effective in the case of tantalum. Unless heated, tantalum is appreciably attacked only by oleum, hydrofluoric acid or, more particularly, a hydrofluoric/nitric acid mixture. Fused alkalis will also attack it. In addition to these reagents, vanadium and niobium are attacked by other hot concentrated mineral acids but are resistant to fused alkali. 

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. 

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. 

Casalot, A. andM. Pouchard. 1967. New nonstoichiometric phases of the silver oxide-vanadium pentoxide-vanadium dioxide system. I. Chemical and chrystallographic study. Bull. Soc. Chim. 10 3817-3820. 

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