Binary compounds defined

Based on voltammetry and solid-state characterization results. Pettier and Maurin [175] defined the electrochemical conditions leading to the formation of smooth crystalline deposits of CuInSe2 and also of the related binary compounds... 

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

The symbols suggested by Jensen, based on Niggli s proposals, describe the local coordination by means of coordination number ratios. For instance, a formula AEm/n will indicate a binary compound where m is the coordination number (defined as the nearest-neighbour number (NNN)) of atoms E around A and n is the similarly defined coordination number of A around E. 

Figure 5.41. Schemes of ternary compound formation in ternary alloys. For a few metal pairs (Al-Cu, Al-Fe, etc.) the third elements are indicated (defined by their position in the Periodic Table) with which true ternary phases are formed that is, phases are formed which are homogeneous in internal regions of the composition triangle not connected with the corners or edges. Compare these data with those shown for the formation of binary compounds in the figures relevant to the involved metals.

Elemental solids and binary compounds display numerous well-defined structural trends. We saw in chapter 1 that we would like theory to be able to explain at least the following ... 

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. 

Indium and thallium form a number of binary compounds with alkali metals in which the group 13 elements form well-defined anionic clusters (Zintl ions, see Section 1-9). Examples are K8Inu (Fig. 6-3) which has considerably fewer (2n-4) electrons than the minimum described by Wade s rules (2n + 2), KgIn10Zn and K10In10M (M = Ni, Pd, Pt).6 Closo-In16 and nido-Inu clusters have also been found. Thallium, too, forms Zintl clusters Na2H contains Tlf tetrahedra, while K8T1ii is similar to In, and KT1 contains Tl octahedra.7... 

The II State. All the elements Ti to Cu inclusive form well-defined binary compounds in the divalent state, such as oxides and halides, which are essentially ionic. Except for Ti, they form well-defined aqua ions [M(H20)6]2+ the potentials are summarized in Table 17-1. 

The description of bonding within these compounds has been treated by several different approaches that come to the same conclusion. The (8 — N) rule that is generahzed by Mooser and Pearson uses a covalent model with collective counting of electrons. The generahzed 8 — N rule can be easily defined for simple binary compounds of the general formula, as 8x electrons are required in order... 

How well can we tell the difference between an ionic bond and a polar covalent bond The only honest answer to this question is that there are probably no totally ionic bonds between discrete pairs of atoms. The evidence for this statement comes from calculations of the percent ionic character for the bonds of various binary compounds in the gas phase. These calculations are based on comparisons of the measured dipole moments for molecules of the type X—Y with the calculated dipole moments for the completely ionic case, X+Y. We performed a calculation of this type for HF in Section 13.3. The percent ionic character of a bond can be defined as... 

AI2O3, SigN, P/>5, Mn O, OsFjj. Some elements are known the atoms of which usually do not combine with more than one atom of any other element. Hydrogen is an element of this sort its binary compounds have formulas such as HCl, H S, H3N. Such an element is said to have the valence one —to be univalent. The valence of another element may be defined as the number of atoms of a univalent element with which one atom of the element combines. Thus one atom of oxygen combines with two atoms of hydrogen to form water, HjO ... 

Silver(n) Compounds, d9.12 The Ag2+ ion is well defined and numerous complexes are known but only one binary compound, AgF2 the oxide, AgO, is actually Ag Agin02 and is accordingly discussed under Ag111. 

Oxides can be defined as binary compounds formed between various elements and oxygen, while phosphates can be defined as salts based formally on phosphorus (V) oxoacids and in particular salts of phosphoric (V) acid, H3PO4. Both oxides and phosphates are among the most important classes of inorganic compounds. For example, silicon dioxide (Si02) is the main compound in the Earth s crust, and apatite, a complex mineral form of calcium phosphate, Ca5(P04)3 (OH, F, Cl) is the main compound on the enamel of teeth [1-3],... 

As presented in Table 1, these salts are, usually, binary compounds. Sometimes, a third component is involved it can be either a neutral solvent (solv.), or an ionic species (e.g.tri-iodide). When the stoichiometry and the counter-ion charge are defined, the average charge density is known (p = p / n for singly charged species). Usually, we observe a radical-ion (p < 1) but, in a few particular cases, a diamagnetic di-ion (D++ or A" ) has been detected (1 < p < 2). 

RIs of elemental solids are listed in Table SI 1.2 and those of binary compounds in Tables S11.3-S11.7. For substances with the same structure, RI increases with the covalence and metallicity of bonding, as follows from Eq. 11.3 as on transition from ionic to covalent and then to metallic substances the absorption frequency (Vi) decreases and approaches v, the denominator of Eq. 11.3 approaches 0, hence n 00. Later we shall make use of this circumstance to define atomic polarizabilities... 

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