Nonstoichiometric phases

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. ... 

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 ... 

Assume that the anions build up a perfect sodium chloride structure matrix. In this case the formula of the nonstoichiometric phase must be expressed as (Mg, Al)-Oi o- To make sure that this is in agreement with the measured ratio of MgO and A1203, use the following procedure ... 

Even when the composition range of a nonstoichiometric phase remains small, complex defect structures can occur. Both atomistic simulations and quantum mechanical calculations suggest that point defects tend to cluster. In many systems isolated point defects have been replaced by aggregates of point defects with a well-defined structure. These materials therefore contain a population of volume defects. 

One of the most widely explored systems is derived from the interpolation of Li between the TiS2 layers in varying amounts to form nonstoichiometric phases with a general formula LivTiS2. Because the bonding between the layers is weak, this process is easily reversible. The open nature of the structure allows the Li atoms to move readily in and out of the crystals, and these compounds can act as convenient alkali metal reservoirs in batteries and other devices. A battery using lithium intercalated into TiS2 as the cathode was initially developed some 30 years ago. 

Anion-excess fluorite structure nonstoichiometric phases prepared by heating CaF2 and LaF3 contain ... 

If the oxide is a nonstoichiometric phase MO. it will still be able to gain oxygen. The cell will be... 

Figure 7.6 Variation of equilibrium oxygen partial pressure a series of oxides that includes one nonstoichiometric phase MOi x.

The first step is to set out the assumptions concerning the defects that are likely to occur, using physical and chemical intuition about the system in mind. For illustrative purposes, take a nonstoichiometric phase of composition MX, nominally containing M2+ and X2- ions, with a stoichiometric composition, MXl 0. In this example the following is assumed ... 

Silyl manganese pentacarbonyl decomposes in a more complex way to give a mixture of two phases, "MnSi (actually a silicon-rich nonstoichiometric phase of approximate composition MnSii.25) and stoichiometric Mn5Si3, such that the overall metal silicon ratio is 1 1. The analogous rhenium system behaves similarly. 

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. 

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... 

This question must be asked on two grounds. First, because the high concentrations of lattice defects found in some cases (the TiO phase is an extreme example) must raise doubt about the relation of the nonstoichiometric phase to the type structure in terms of which it is described. Secondly, successions of ordered intermediate phases have been discovered during the last few years, in phase diagrams where, previously, nonstoichiometric compounds of broad range had been reported. 

This way of looking at defect interaction could profitably be developed further. There is a useful parallel to be drawn between solvated ions or ion-pair complexes in solution, and defects or defect complexes in their crystal lattice environment. The idea is worth using when considering the growing hints of short range order and defect association in nonstoichiometric phases. 

This is, however, not the whole of the matter. The superstructure ordering of point defects the collection of interstitial ions along certain fines or sheets, as in Magneli s model for the precursor of his shear structures the temperature-dependent adjustment of composition of a nonstoichiometric phase at the boundary of the bivariant range the nucleation of a new phase of different stoichiometry—these depend on accumulating vacancies or interstitials in some regions of the crystal lattice at the expense of others. 

Defect Complexes. The argument this far has reached the following stage. The state of minimum energy of any system at very low temperatures corresponds, without question, to the attainment of order. Third law considerations imply that a nonstoichiometric phase, with its inherent randomness, should be metastable at 0° K. with respect to a mixture of ideal, stoichiometric compounds. The relative status of complex intermediate phases and of stoichiometrically simple compounds,... 

The question then arises whether the free energy curve of a nonstoichiometric phase is to be considered in isolation, unrelated to those of adjacent phases in the equilibrium diagram, or whether the free energy is a continuous function of composition over the entire range. In the former case, for compositions outside the stable existence range, the compound is metastable with respect to a two-phase mixture, but its metastability is contingent on the presence of the second phase there is no inherent restriction to the possible deviation from ideal stoichiometry. The second alternative contains a built-in condition of absolute instability above a certain degree of defectiveness. 

Figure 4. Partial molar enthalpy and entropy of oxygen in nonstoichiometric phases based on fluorite structure...

If the structure can be established in the ordered compounds, there is reason to believe that it will persist as microstructure in the nonstoichiometric phase. This affords a method of classification considered below. 

The ions interpolated in tunnel structures, listed in Table I, are situated in continuous cavities enclosed by the fixed framework of the host (39). Although there is no diffraction evidence that the guest ions are ordered, dielectric absorption in the phase Baa,Ti8 a.Mga.016 favors a model with sequential ordering of barium ions and vacant positions in any one tunnel, but with no relationship to the sequence in any other tunnel (22). This seems to be true in some, at least, of the other nonstoichiometric phases falling into this group, and the application of newer techniques can be expected to amplify the rather limited knowledge of these substances. 

---