Stoichiometric ionic compounds

Fig. 3.4 Energies of a working ion in a stoichiometric ionic compound the dots indicate an occupied site.

It should be noted that the discussion so far has assumed that all compounds are stoichiometric, i.e. that all the atomic or molecular proportions are integral. It has become increasingly clear that many compounds are to some degree non-stoichiometric. These rules fail for non-stoichiometric compounds, for which further formalisms need to be developed. Electroneutrality must, of course, be maintained overall in such compounds, in one way or another. For example, in an ionic compound where there is apparently a deficit of negative ions, the consequent formal excess of cations may be neutralised by the presence of an appropriate number of cations of the form rather than of the prevalent form Various... 

The ionic compound [Ta(Se2)2]2[TaBr6] was prepared in high yield from the elements by stoichiometric reaction at 450 °C.377 The positively charged polymeric component (formally [TavTaIV]) is constructed from chains of Ta(//2-f72-Se2)2Ta units. The ionic lattice is completed... 

Sodium tetraphenylborate, (C5H5>4B Na, is an important example of an organic precipitating reagent that forms salt-like precipitates. In cold mineral acid solutions, it is a near-specific precipitating agent for potassium and ammonium ions. The precipitates have stoichiometric composition and contain one mole of potassium or ammonium ion for each mole of tetraphenylborate ion these ionic compounds are easily filtered and can be brought to constant mass at 105°C to 120°C. Only mercury(Il), rubidium, and cesium interfere and must be removed by prior treatment. 

Most of the cases we encounter in the real world involve ionic lattices containing charged cations and charged anions. Up to now, we have considered simple hypothetical compounds such as MX and delineated various kinds of non-ionlzed intrinsic defects that would be present. Section 2-6 was concerned with various types of defects intrinsic to the stoichiometric MXs compound and the non-stoichiometric compound. 

In the vicinity of the stoichiometric composition (REGION II), the concentration of defects depends upon whether Ks > Kion. or vice-versa. The former usually holds for large band-gap ionic compounds. Then, 2.7.1.- g becomes ... 

Differentiate between the formula equation, the complete ionic equation, and the net ionic equation. For each reaction in Question 6, write all three balanced equations. What is an acid-base reaction Strong bases are soluble ionic compounds that contain the hydroxide ion. List the strong bases. When a strong base reacts with an acid, what is aiways produced Expiain the terms titration, stoichiometric point, neutralization, and standardization. 

Coordination compounds of platinum(II) halides with tertiary alkylphosphines have long been known.Treatment of a potassium tetrachloroplatinate(II) solution with the stoichiometric amount of triethyl- or tri-n-propylphos-phine yields a precipitate of [Pt(R3P)4][PtCl4], which is transformed to the monomeric isomers, cis- and trans-[PtCl2(R3P)2], on several weeks standing, or more rapidly on heating. The stability of the ionic compound decreases... 

Solvent extraction of ionic compounds from the aqueous to organic phase is well known to analytical and industrial chemists. For the extraction equilibrium of Q" X salt, Qaq + Xaq (Q X )oj.g, the stoichiometric extraction constant Eqx is defined by Schill and Modin [31,32] as... 

This aspect of the theory of defect structures of non-stoichiometric compounds is usually covered in the main text of books on high-temperature oxidation. The subject of doping is interesting for its own sake, and it is vitally important for the study of the physical chemistry and electrochemistry of ionic compounds. In the case of an introduction to high-temperature oxidation our opinion is that, since the control of oxidation rates by controlling the ionic and electronic transport properties of oxides by impurity solution is not generally used as a technique for the development of oxidation-resistant alloys, this subject should be dealt with in an appendix. This allows it to be covered adequately without over-emphasizing its importance. 

As the -ide" ending suggests, the electrons in electrides take on the role of anions.[ This implies that electrides are crystalline ionic compounds with one electron per cation. They could be viewed as salts that contain stoichiometric F-centers. with electrons as the only anions. But because one cannot suppress the quantum nature of electrons, these "anions" are special. The line between localized weakly interacting electrons and delocalized, or metallic electrons is blurred, and both extremes are encountered in electrides. Because most of the electron density in electrides is present in otherwise empty, well-defined pseudo-one-dimensional cavities and channels, they could also be viewed as forming lowdimensional correlated electron gasses. 

When metals react with gases, the main corrosion products are ionic compounds that can be stoichiometric or nonstoichiometric. Generally, only defect ions (ion condnctors) arise in stoichiometric componnds snch as silver chloride (AgCl) and NaCl. Four border cases of imperfections are possible When cation vacancies in the lattice and cations at interstitial lattice sites are found in an undisturbed anion lattice, the cations are mobile. Alternatively, the anions are mobile. In compounds with anion and cation vacancies, both can migrate, as they can when an equal number of cations and anions are present at interstitial lattice sites. 

Some are well-defined ionic compounds (containing Se ), whiie others are non-stoichiometric interstitial compounds (e.g. Pd4Se, PdSe2). 

The Schottky defect, which is unique to ionic compounds, consists of a stoichiometric pair of cation and anion vacancies. For an MO compound with full ionization, its formation equation is expressed as... 

A stoichiometric amount of R4NCI will usually lead to the quantitative conversion of the acceptor chloride into the respective chloro-complex. On the other hand owing to their small DNsbCh they behave as poor solvents for most ionic compounds and also for most transition metal halides. Alkali or alkaline earth salts of the chloro-complexes cannot be formed from their solutions and chloro-complexes of most transition metal ions are also inaccessible in such media. Their use is therefore restricted to the formation of chloro-complexes of certain representative elements with large cations, such as [R4N]+. 

It is known that ionic compounds have appreciable electrical conductivity, which is inseparable from diffusion, due to atomic defects, such as Schottky and Frenkel defects to an extent, and ionic migration and diffusion. For instance, Schottky defects are combinations of cation and anion vacancies necessary to maintain ionic electrical neutrality and stoichiometric ionic structure. In general, ions (cations and anions) diffuse into adjacent sites. 

For ionic compounds that have a 1 1 stoichiometric ratio of cation to anion (like NaCl, CsCl, or MgO), the relative sizes of the ions determine whether the compound will have one of three possible unit cells. We use a label to define each type of unit cell. The following table summarizes this experimentally determined generality, in which the ratio in column 1 determines columns 2 and 3 ... 

Similar diagrams and analyses may be made for many other combinations of non-stoichiometric and stoichiometric defect situations of pure (undoped) oxides and other ionic compounds. 

Low temperature (in relation to the melting point of the material) creep of metals is usually controlled by dislocation movements, because their structures contain sufficient active slip systems and have small Peierls stresses (the force needed to bring about dislocation movement) [4-8]. Deformation can also be controlled by dislocation climb, a process requiring vacancy diffusion. At high temperatures, deformation in metals is usually controlled by diffusion creep mechanisms that do not involve dislocation movement. In ceramics, however, diffusion creep may be the dominant mechanism under most processing conditions due to the small number of slip planes, the high Peierls stresses, and to the need to move stoichiometric amounts of the different atomic species present in the material (both anions and cations for an ionic compound). 

We can better understand precipitation reactions by revisiting a concept fiom Chapter 14—the reaction quotient (0. The reaction quotient for the reaction by which an ionic compound dissolves is the product of the concentrations of the ionic components raised to their stoichiometric coefficients. 

The ratio of cations to anions is not altered by the formation of either a Frenkel or a Schottky defect. If no other defects are present, the material is said to be stoichiometric, stoichiometry Stoichiometry may be defined as a state for ionic compounds wherein there is the exact... 

---