Nonstoichiometric

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. ... 

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. 

Nitrides. Uranium nitrides are weU known and are used in the nuclear fuel cycle. There are three nitrides of exact stoichiometry, uranium nitride [2565843-9], UN U2N3 [12033-85-1/ and U4N2 [12266-20-5]. In addition to these, nonstoichiometric complexes, U2N3, where the N/U ratio ranges... 

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

Carbon forms 2 extremely stable oxides, CO and CO2, 3 oxides of considerably lower stability, C3O2, C5O2 and C]209, and a number of unstable or poorly characterized oxides including C2O, C2O3 and the nonstoichiometric graphite oxide (p. 289). Of these, CO and CO2 are of outstanding importance and their chemistry will be discussed in subsequent paragraphs after a few brief remarks about some of the others. 

The absorption spectrum of this nonstoichiometric phase forms the basis for the formerly much-used qualitative test for zinc oxide yellow when hot, white when cold . Alternatively, anion sites can be left vacant, e.g. ... 

Figure 14.20 Part of the Pr-O phase diagram showing the extended nonstoichiometric a phase Pr02-jt at high temperatures (shaded) and the succession of phases Pr 02 2 at lower temperatures.

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. ... 

Metal sulfides can be prepared in the laboratory or on an industrial scale by a number of reactions pure products are rarely obtained without considerable refinement and nonstoichiometric phases abound (p. 679). The more important preparative routes include ... 

The binaiy hydrides (p. 64), borides (p. 145), carbides (p. 299) and nitrides (p. 417) are hard, refractory, nonstoichiometric materials with metallic conductivities. They have already been discussed in relation to comparable compounds of other metals in earlier chapters. 

High-temperature reduction of Na2Ti03 with hydrogen produces nonstoichiometric materials, Na jTi02 (jr = 0.20-0.25), called titanium bronzes by analogy with the better-known tungsten bronzes (p. 1016). They have a blue-black, metallic appearance with high electrical conductivity and are chemically inert (even hydrofluoric acid does not attack them). 

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. 

A number of nonstoichiometric bronzes are also known which, like the titanium bronzes... 

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. 

X 264 pm = 792 pm) and implies instead the nonstoichiometric composition Hg2.s2(AsF6) or more generally Hg3 3(Asp6) since the composition apparently varies with temperature. Partially filled conduction bands formed by overlap of Hg orbitals produce a conductivity in the a-b plane which approaches that of liquid mercury and the material becomes superconducting at 4 K. 

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