Stoichiometric range

Refractory Compounds. Refractory compounds resemble oxides, carbides, nitrides, borides, and sulfides in that they have a very high melting point. In some cases, they form extensive defect stmctures, ie, they exist over a wide stoichiometric range. For example, in TiC, the C Ti ratio can vary from 0.5 to I.O, which demonstrates a wide range of vacant carbon lattice sites. 

Although Fig. 7.10e is similar to that for ionic defects (Fig. 7.9e), there are a number of significant differences. In particular, the stoichiometric range is now far... 

In the Ti-S system, a number of phases between TiS and TiS2 have been identified and they all exhibit wide stoichiometric ranges. For example, TiS and TiS2 have the... 

Iron(II) oxide (wiistite, Section 4.6) also ranges in composition, from Fe0.88O to Fe0.95O stoichiometric FeO is not encountered in practice. As an extreme case, 5-TiO, an NaCl-type solid previously mentioned as exhibiting gross Schottky defects even when stoichiometric, ranges widely in composition ... 

Similarly, the phase equilibria are known with certainty, and free energy measurements are available (although not as closely spaced as one might wish) to define the ferrous oxide phase as having continuity over a wide stoichiometric range at 1000° K. The evidence is incompatible with the idea that these and other compounds owe their existence to ordering processes or are metastable with respect to discrete intermediate phases. 

On this view, interaction between defects determines the concentration at which a phase becomes saturated with defects, and also the manner in which defects aggregate or order. It can be foreseen that stoichiometrically defined intermediate phases are likely to undergo an order-disorder transition at higher temperatures and that, in general, some detectable stoichiometric range is likely before the disordering temperature is reached. 

Russian workers have advanced some apparently related ideas. Ariya and his colleagues (1958, 1961) (3, 17) have pointed out that the standard heat of formation of a compound across its stoichiometric range frequently falls rather smoothly on the curve joining the value for the stoichiometric compound and that of the next phase in the equilibrium diagram (Figure 2). Furthermore, there is... 

DSC Study. From the analysis of the total heats of reaction of the commercial and fractionated TGMDA resin/DDS formulations over a broad stoichiometric range, it is concluded that the total heat of reaction divided by the epoxy concentration has a constant value... 

High mass-transfer coefficients were measured at packing heights between 2 and 5 ft. Limestone stoichiometric feed was varied from 77 to 100%. The calculated mass transfer coefficients and the operating conditions are given in Table HI. Transfer coefficients between 0.31 and 1.26 were computed for the limestone stoichiometric range. The absorption efficiency follows almost directly the limestone feed rate as illustrated in Figure 2. 

The zirconium arsenides have a similar stoichiometric range as the zirconium phosphides. Each of the compounds has also the same structure as its zirconium phosphide analogue. The compounds that have been identified are listed in Table V-34 together with their respective crystallographic data. 

Similarity between displaced weak HA and polar/protic diluents is also seen in their effect on the degree of extraction. Compared to amine + apolar diluent extractants, both of them improve extraction at the substoichiometric region (particularly for relatively weak mineral acids) and reduce it at the above-stoichiometric range. On H3PO4 extraction by T(OD)A -I- LA coupled extractant the species R3NH H2PO is formed. The displaced lauric acid forms H bonds with this ion pair. The ion pair is stabilized and extraction through this mechanism is enhanced. 

As a general rule the nitrides appear to be more stable than the carbides and do not have nonmetal-rich phases, similar to MC2, or any other other nitrogen-rich phases. Once again for all compositions wide stoichiometric ranges are encountered as a result of nitrogen vacancies. It is doubtful if MNi.o exists. 

In the stoichiometric range a vacancy mechanism appears to predominate. The selfdilfusion equation is... 

This method has been extended by Rempp and co-workers [39,40] as a general core-first method to prepare star-branched polymers, as shown in Scheme 4. The plurifimctional metalorganic initiator (A) was prepared by potassium naphthalene-initiated polymerization of DVB in tetrahydrofuran (THF) at -40°C with [DVB]/[K ] ratios of 0.5-3. Microgel formation was reported outside of this stoichiometric range or when m-DVB was used instead of either / -DVB or the commercial mixture. Within the prescribed stoichiometric ratios, star polymers (B) with arm functionalities varying from 8 to 42 were reported. As expected for this type of process, the polydispersities were described as being quite broad and were attributed primarily to a random distribution of core sizes and functionalities. 

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