Stoichiometric phases

While Cordfuncke [997] believes that there are only four stable compounds in the U03—NH3—H20 system, the results of Stuart et al. [998, 999] indicate the existence of a continuous non-stoichiometric phase containing the NH4 ion and possessing zeolitic properties U02(0H)2 x (ONH4 )x yH20. 

At low temperatures magnesium oxide, MgO, which adopts the sodium chloride structure, is virtually a stoichiometric phase, but at high temperatures in the MgO-A1203 system this is not so. At 1800°C the approximate composition range is from pure MgO to 5 mol % A1203 95 mol % MgO. The simplest way to account for this composition range is to assume that point defects are responsible. For this, because both Mg2+ and Al3+ cations in this system have a fixed valence, electronic compensation is unreasonable. There are then three ways to account for the composition range structurally ... 

Silver oxide is a stoichiometric phase that decomposes at about 230°C to silver metal and oxygen gas when heated in air. 

Nonstoichiometric compounds such as FeO, Ni, xO, LaCo03+8 LaCo03+x/2 etc. are indexed under the notionally stoichiometric phase, i.e. FeO, NiO, LaCo03. 

Analyses of the defect chemistry and thermodynamics of non-stoichiometric phases that are predominately ionic in nature (i.e. halides and oxides) are most often made using quasi-chemical reactions. The concentrations of the point defects are considered to be low, and defect-defect interactions as such are most often disregarded, although defect clusters often are incorporated. The resulting mass action equations give the relationship between the concentrations of point defects and partial pressure or chemical activity of the species involved in the defect reactions. 

The configurational entropy term, given by the degeneracy, gc, is included in AfG but not in AfGc. Let us assume the existence of two compounds with different formal oxidation states for the B atom, ABO3 and ABO2.5. The two compounds have the same (perovskite-type) structure and the non-stoichiometric phase... 

Figure 2.18. Examples of binary phase diagrams in each of which one stoichiometric phase is formed. In the Mg-Ge system we have the congruently melting Mg2Ge (33.3 at.% Ge) in Au-Sb, AuSb2 is formed through a peritectic reaction. In the Pt-Ag system one compound at 47 at.%...

Thermodynamically, the composition of any such phase is variable. In a number of cases, however, the possible variation in composition is very small. Invariant composition phases or stoichiometric phases, or compounds proper, also called point compounds in binary alloys, are represented by a point on the composition axis. 

In Fig. 2.19, on the contrary, we observe that intermediate solid phases with a variable composition are formed (non-stoichiometric phases). In the diagrams shown here we see therefore examples both of terminal and intermediate phases. (For instance, the Hf-Ru diagram shows the terminal solid solutions of Ru in a and (3Hf and of Hf in Ru and the intermediate compound containing about 50 at.% Ru). These phases are characterized by homogeneity ranges (solid solubility ranges), which, in the case of the terminal phases, include the pure components and which, generally, have a variable temperature-dependent extension. 

In the case of a ternary system, the formation of several, binary and ternary, stoichiometric phases, and different types of variable composition phases can be observed. One may differentiate between these phases by using terms such as point compounds (or point phases), that is, phases represented in the composition field by points, line phases , field phases , etc. 

The situation in the solid state is generally more complex. Several examples of binary systems were seen in which, in the solid state, a number of phases (intermediate and terminal) are formed. See for instance Figs 2.18-2.21. Both stoichiometric phases (compounds) and variable composition phases (solid solutions) may be considered and, as for their structures, both fully ordered or more or less completely disordered phases. This variety of types is characteristic for the solid alloys. After a few comments on liquid alloys, particular attention will therefore be dedicated in the following paragraphs to the description and classification of solid intermetallic phases. 

A more complex notation is needed for non-stoichiometric phases. Selected simple examples are given below, and more detailed information will be reported when discussing crystal coordination formulae ... 

Pt3Cdo.6Zn4.4 ( Pt5CdZn7) (non-stoichiometric phase with partial substitution of Cd and Zn atoms) Hexagonal, a = 705.0pm, c = 279.2pm space groupP31 m, N. 157. 

Several particularities of phase transfer catalyzed polyetherification are as follows. Stoichiometric phase transfer catalyzed pol)rmerizations do not take place between stoichiometric ratio of monomers, since the nucleophilic monomer is always transferred in a small amount into the organic phase. Consequently, because their reaction is a non-stoichiometric one there is no need for an equimolar ratio between the two monomers to get polymers with high molecular weights. High molecular weight polymers are usually obtained also at low conversions. In several cases, even at 100 percent conversion the polydispersity of the obtained polymers is low, i.e., "Hw/Hh 1.3. At any conversion, the organic phase contains only pol3rmers with electrophilic chain ends, even when the nucleophilic monomer was used in excess. 

The lithium analogues of NASICON are, in fact, quite different materials (Irvine and West, 1989). They are solid solutions based on stoichiometric phases such as y-Li2ZnGe04 or y-Li3(P, As, V)04, but containing... 

To examine this problem more closely it was necessary to develop (1) a model for the nine-component oxide system U02-Zr02-Si02-Ca0-Mg0-Al203-SrO-BaO-La203 to account for the MCCI (Chevalier 1992, Ball et al. 1993) and (2) develop a database for the gas-phase reactions in the oxide subsystem U02-Zr02-Si02-Ca0-Mg0-Al203. The final oxide database included four solution phases and 70 condensed stoichiometric phases. 

Intercalation of Cjq with lithium has been achieved by solid-state electrochemical doping [125]. In this technique, metallic lithium was used as the negative electrode and a polyethylene oxide lithium perchlorate (P(E0)8liCl04) polymer film served as electrolyte. The formation of stoichiometric phases Li Cgg (n = 0.5, 2, 3, 4, and 12) has been observed. 

Above 1127°C, a single oxygen-rich non-stoichiometric phase of UO2 is found with formula U02+, ranging from UO2 to U02.25- Unlike FeO, where a metal-deficient oxide was achieved through cation vacancies, in this example the metal-deficiency arises from interstitial anions. 

Titanium and oxygen form non-stoichiometric phases which exist over a range of composition centered about the stoichiometric 1 1 value, from TiOo.es to TiOi.25. We shall look at what happens in the upper range from TiOi 00 to TiOi.25. 

There are a number of solid phases of the types MScCl and ScCl where the formal oxidation state of scandium is less than three. They are usually made by direct combination at elevated temperatures of MCI, ScCl3 and metallic scandium. Their structures often show evidence of Sc—Sc bonds. Thus CsScCl3 is made by action of Sc on Cs3Sc2Cly at 700 °C. The shiny blue product has the hexagonal perovskite CsNiCl3 structure. This is similar to the Cs3Sc2Cl9 structure but with all Sc positions filled. Non-stoichiometric phases exist between the two end structures.128 When scandium is heated with ScCl3 at 940-960 °C in a sealed Ta... 

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