As non-stoichiometric compounds

Many of the binary compounds of the lanthanides, such as oxides, nitrides, and carbides, can exist as non stoichiometric compounds. These form crystals where some of the anions ate missing from the sites the anions normally occupy. 

The compounds of indefinite structure and composition are known as non-stoichiometric compounds. They are formed because of the ability of the metal to exist in more than one oxidation states. 

Some interstitial compounds are variously referred to as non-stoichiometric compounds or variable composition compounds of the berthollide type. In this context Earley [2005, 91-2] has argued that the distinction between non-stoichiometric... 

Fumarates. lron(Il) fumarate [141 -01 -5], Fe(C4H20, is prepared by mixing hot aqueous solutions of sodium fumarate and iron(Il) sulfate followed by filtration of the resulting slurry. It has limited solubiUty in water but is more soluble in acid solution. The compound is red-orange to red-brown and finds uses as a hematinic. A non stoichiometric compound [7705-12-6] and iron(Ill) fumarate [52118-11-3], Fe2(C4H20 3, are also available. 

The obvious question then arises as to whether the effective double layer exists before current or potential application. Both XPS and STM have shown that this is indeed the case due to thermal diffusion during electrode deposition at elevated temperatures. It is important to remember that most solid electrolytes, including YSZ and (3"-Al2C)3, are non-stoichiometric compounds. The non-stoichiometry, 8, is usually small (< 10 4)85 and temperature dependent, but nevertheless sufficiently large to provide enough ions to form an effective double-layer on both electrodes without any significant change in the solid electrolyte non-stoichiometry. This open-circuit effective double layer must, however, be relatively sparse in most circumstances. The effective double layer on the catalyst-electrode becomes dense only upon anodic potential application in the case of anionic conductors and cathodic potential application in the case of cationic conductors. 

Figure 7.1 The Gibbs energies of formation of stoichiometric and non-stoichiometric compounds in the system Ti305-Ti02 [4]. Composition given as mole fraction O.

Hydrides of variable composition are not only formed with pure metals as solvents. A large number of the binary metal hydrides are non-stoichiometric compounds. Non-stoichiometric compounds are in general common for d,f and some p block metals in combination with soft anions such as sulfur, selenium and hydrogen, and also for somewhat harder anions like oxygen. Hard anions such as the halides, sulfates and nitrides form few non-stoichiometric compounds. Two factors are important the crystal structures must allow changes in composition, and the transition metal must have accessible oxidation states. These factors are partly related. FeO,... 

The use of energy minimization can be extended to solid solutions and highly non-stoichiometric compounds. In principle the method is simple we take a suitable thermodynamic average over the results of minimizations of different possible arrangements of the atoms. The overall procedure for a solution A0.5B0.5 is then as follows ... 

Non-stoichiometric compounds are of potential use to industry because their electronic, optical, magnetic, and mechanical properties can be modified by changing the proportions of the atomic constituents. This is widely exploited and researched by the electronics and other industries. Currently, the best known example of non-stoichiometry is probably that of oxygen vacancies in the high temperature superconductors such as YBCO (1-2-3) (YBa2Cu307 J. The structure of these is discussed in detail in Chapter 10. 

It is found to be characteristic of most non-stoichiometric compounds that t/ieir unit cell size varies smoothly with composition but the symmetry is unchanged. This is known as Vegard s Law. 

Non-stoichiometric compounds are found for the higher oxides of tungsten, molybdenum, and titanium—WOs-, MoOs-, and Ti02- f, respectively. The reaction of these systems to the presence of point defects is entirely different from what has been discussed previously. In fact, the point defects are eliminated by a process known as crystallographic shear (CS). 

Conversely, the presence of some metal ions of lower oxidation state in the metal ion sublattice requires vacant anion sites to balance the charge. In some cases, the charge imbalance is caused by ions of some other element or, rarely, by multiple valence of the anions. In any event, the empirical formula of a recognizable solid transition metal compound may be variable over a certain range, with nonintegral atomic proportions. Such non-stoichiometric compounds may be regarded as providing extreme examples of impurity defects. 

As mentioned above, the non-stoichiometric compounds originate from the existence of point defects in crystals. Let us consider a crystal consisting of mono-atoms. In ideal crystals of elements, atoms occupy the lattice points regularly. In real crystals, on the other hand, various kinds of point defects can exist in thermodynamic equilibrium. First, we shall consider vacancies , which are empty regular lattice points. Consider a crystal composed of one element which has N atoms sited on regular lattice points and vacancies,... 

It is noted that by increasing <) to some extent the interaction energy between vacancies plays an important role in non-stoichiometric compounds, as mentioned below. 

There are many non-stoichiometric compounds in the 3d transition metal chalcogenides. Among them, the NiS phase with the NiAs-type structure has been investigated in detail from both chemical and physical viewpoints. Let us adopt this compound as a typical example of a non-stoichiometric compound. 

CASE STUDIES OF NON-STOICHIOMETRIC COMPOUNDS 83 concentration 8 in a mole fraction of Cu2- O is written as... 

In this chapter non-stoichiometric compounds derived from point defects have been reviewed mainly from a thermodynamical point of view, and many examples have been presented for the purpose of understanding the nature of non-stoichiometry. As mentioned above it is not necessary to take the interaction between defects for vacancies) into consideration if the... 

In the above sections, we have described the four types of non-stoichiometric compounds derived from extended defects based on the difference of structural characteristics. The concept of adaptive structure, which was proposed by the late Professor J. S. Anderson in 1973, is a more general concept which explains some of the examples mentioned in the preceding sections. The compounds which have the adaptive structure are defined by Anderson as ... 

As mentioned above, there exist two types of interstitial site for H three equivalent sites with eight coordination (I J and six equivalent sites with four coordination (12) in a unit cell. Accordingly, it is likely that there are two non-stoichiometric compounds around the compositions CaNijHj and CaNijHg. However, this is not observed for the CaNi5-H2 system. In this system, three phases appear at c. x = 1, 5, 6 (or 7). This result suggests that... 

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