Stoichiometric solid compound

Graphite reacts with alkali metals, for example potassium, to form compounds which are non-stoichiometric but which all have limiting compositions (for example K C) in these, the alkaU metal atoms are intercalated between the layers of carbon atoms. In the preparation of fluorine by electrolysis of a molten fluoride with graphite electrodes the solid compound (CF) polycarbon fluoride is formed, with fluorine on each carbon atom, causing puckering of the rings. 

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. 

Palladium hydride is not a stoichiometric chemical compound but simply a metal in which hydrogen is dissolved and stored in solid state, in space between Pd atoms of crystal lattice of the host metal. Relatively high solubility and mobility of H in the FCC (face-centered-cubic) Pd lattice made the Pd H system one of the most transparent, and hence most studied from microstructural, thermodynamic, and kinetic points of view. Over the century that followed many metal-hydrogen systems were investigated while those studies were driven mostly by scientific curiosity. Researchers were interested in the interaction of hydrogen molecule with metal surfaces adsorption and diffusion into metals. Many reports on absorption of in Ni, Fe, Ni, Co, Cu, Pd, Pt, Rh, Pd-Pt, Pd-Rh, Mo-Fe, Ag-Cu, Au-Cu, Cu-Ni, Cu-Pt, Cu-Sn, and lack of absorption in Ag, Au, Cd, Pb, Sn, Zn came from Sieverts et al. [30-33]. 

Carbon fluoride [also known as carbon monofluoride, polycarbon monofluoride, graphite fluoride, or (CFx)n] is a solid, layered, non-stoichiometric fluorocarbon of empirical formula CFX, where 0 < x < 1.25, obtained by the action of elemental fluorine on carbon. Fluorine combines with carbon and yields three solid compounds CFX, C2FX, and C4FX as well as varying amounts of volatile fluorocarbons as byproducts. With appropriate selection of fluorination conditions nearly 100% conversion of carbon to carbon tetrafluoride can occur. 

A relatively simple example of a group contribution technique is the method for estimating liquid and solid heat capacities (159). This method is a modification of Kopp s rule (160,161) which was originally proposed in 1864. Kopp s rule states that, at room temperature, the heat capacity of a solid compound is approximately equal to a stoichiometric summation of the heat capacities of its atoms (elements). The Hurst-Harrison modified equation is as follows ... 

The chemical reaction for mineral dissolution in Eq. 3.1 (the forward reaction) represents the stoichiometric decomposition of a binary solid compound into aqueous ionic species. It is an overall reaction based on a chemical formula for the solid phase and the hypothesis that free ionic species in aqueous solution will be created in proportion to their stoichiometry in the solid for at least some time... 

At that date, palladium hydride was regarded as a special case. Lacher s approach was subsequently developed by the author (1946) (I) and by Rees (1954) (34) into attempts to frame a general theory of the nature and existence of solid compounds. The one model starts with the idea of the crystal of a binary compound, of perfect stoichiometric composition, but with intrinsic lattice disorder —e.g., of Frenkel type. As the stoichiometry adjusts itself to higher or lower partial pressures of one or other component, by incorporating cation vacancies or interstitial cations, the relevant feature is the interaction of point defects located on adjacent sites. These interactions contribute to the partition function of the crystal and set a maximum attainable concentration of each type of defect. Conjugate with the maximum concentration of, for example, cation vacancies, Nh 9 and fixed by the intrinsic lattice disorder, is a minimum concentration of interstitials, N. The difference, Nh — Ni, measures the nonstoichiometry at the nonmetal-rich phase limit. The metal-rich limit is similarly determined by the maximum attainable concentration of interstitials. With the maximum concentrations of defects, so defined, may be compared the intrinsic disorder in the stoichiometric crystals, and from the several energies concerned there can be specified the conditions under which the stoichiometric crystal lies outside the stability limits. 

Mn(OH)2 is a well-characterized, stoichiometric, crystalline compound with the hexagonal Mg(OH)2 structure. Although preparations of it require careful exclusion of 02, the pure, isolated solid has good stability in the air it occurs naturally (pyrochroite) and finds use as an industrial catalyst. It is weakly amphoteric, and can be recrystallized from hot, concentrated alkali solutions. This, of course, implies the formation of hydroxo and/or oxomanganates(II), and, indeed, solid, yellow Na2[Mn(OH)4] has been desscribed, as have Ba2Mn(OH)6 and Sr2Mn(OH)6. 

By the direct fluorination with elemental fluorine, it is still difficult to prepare pure compounds C6oF.v and C70F of specific fluorine content because the reaction between fluorine gas and solid state C6o and C70 proceeds under heterogeneous conditions and, therefore, lacks control. Attempts to prepare stoichiometrically pure compounds have become successful using solid phase reactions between fullerenes and metal fluorides. These are described in the next section. It has been unsuccessful so far, but efforts are being made to produce fluorinated fullerenes C6oF v of lower fluorine content (x < 18) [24],... 

A stoichiometric amount of propynyllithium is then used to perform the hexaalkynylation of the organo-ditin hexachlorides at low temperature (Scheme 3.7.18). The reaction leads to solid compounds, which can be easily purified by recrystallization. 

Clathrate hydrates (known also as gas hydrates) belong to a large class of crystalline, non-stoichiometric, inclusion-compound materials that are stable within a certain range of pressure and temperature. The host solid framework structure is made up of water molecules, connected through hydrogen bonds that form cavities (cages) . The cavities can be stabilized by the inclusion of small molecules such as CH4, CaHg, CO2, N2, Ar, etc. Over 100 different molecules are known to form hydrates. 

The titanium-oxygen system is particularly complex among the binary metal-oxygen systems. It exhibits many stoichiometric and widely non-stoichiometric solid phases as well as two gaseous compounds, TiO(g) and Ti02(g). The complexity explains the large number of investigations carried out... 

Praseodymium(iv). Only a few solid compounds are known, the commonest being the black non-stoichiometric oxide formed on heating Pr111 salts or oxide in air. The oxide system which is often formulated as Pr6On is actually very complicated,49 with five stable phases each containing Pr3 + and Pr4+ between Pr203 and the true dioxide Pr02. 

Here a. (T) is the activity of component i in a stoichiometric liquid at the liquidus temperature, T, AS (IC) is the entropy of fusion of compound IC at the melting temperature, T, and AC (IC) is the difference in heat capacity between themstoichiom6tric liquid and the solid compound. This sequence is the same as that proposed for binary III-V systems by Vieland (5.). 

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