Binary compounds, structures

Table 8. Properties and Crystal Structure Data for Important Actinide Binary Compounds...

Transition elements, for which variable valency is energetically feasible, frequently show non-stoichiometric behaviour (variable composition) in their oxides, sulfides and related binary compounds. For small deviations from stoichiometry a thermodynamic approach is instructive, but for larger deviations structural considerations supervene, and the possibility of thermodynamically unstable but kinetically isolable phases must be considered. These ideas will be expanded in the following paragraphs but more detailed treatment must be sought elsewhere. " ... 

Other binary compounds include MAs3 (M = Rh, Ir), which has the skutterudite (CoAs3) structure [33] containing As4 rectangular units and octahedrally coordinated M. The corresponding antimonides are similar. M2P (M = Rh, Ir) has the anti-fluorite structure while MP3 has the CoAs3 structure. In another compound of this stoichiometry, IrSi3, 9-coordination exists for iridium. 

C.19 Aluminum oxide, alumina, exists in a variety of crystal structures, some of which are beautiful and rare. Write the formula for aluminum oxide, which is a binary compound of aluminum and oxygen. The mass of a rectangular slab of aluminum oxide of dimensions 2.5 cm X 3.0 cm X 4.0 cm is 102 g. What is the density of aluminum oxide ... 

Other Binary Compounds.—Scandium nitride and zirconium and titanium carbide do not conform with the theoretical radii. It is possible that these crystals do not consist essentially of Sc+3, N 3, Ti+4, Zr+4 and C-4 ions, especially since zirconium and titanium nitride, ZrN and TiN, also form crystals with the sodium chloride structure but possibly also the discrepancy can be attributed to deformation of the anions, which have very high mole refraction values. 

It is also shown that theoretically a binary compound should have the sphalerite or wurzite structure instead of the sodium chloride structure if the radius ratio is less than 0.33. The oxide, sulfide, selenide and telluride of beryllium conform to this requirement, and are to be considered as ionic crystals. It is found, however, that such tetrahedral crystals are particularly apt to show deformation, and it is suggested that this is a tendency of the anion to share an electron pair with each cation. 

In Fig. 3.1, several ideal structures are also plotted with the + mark. All of these structures have no adjustable parameter and most of them lose some of the symmetry elements when they are distorted. As shown in the figure, most of the ideal structures have some deviation from the fitting curve. It may be related to the fact that some of these ideal structures are deformed in real binary compounds. 

Appendix, ICSD Codes, D and N Parameters of the Structures Used 43 Table A3 Structures of binary compounds. 

Carbon forms a huge number of binary compounds with hydrogen. Three major categories of these compounds are alkanes, alkenes, and alkynes. An alkane has only single bonds between carbon atoms. The four simplest alkanes, which are shown in Figure 3-7. are methane, ethane, propane, and butane. An alkene, on the other hand, contains one or more double bonds between carbons, and an alkyne has one or more triple bonds between carbon atoms. Figure shows the structures of ethylene, the simplest alkene, and acetylene, the simplest alkyne. 

Solid solutions are very common among structurally related compounds. Just as metallic elements of similar structure and atomic properties form alloys, certain chemical compounds can be combined to produce derivative solid solutions, which may permit realization of properties not found in either of the precursors. The combinations of binary compounds with common anion or common cation element, such as the isovalent alloys of IV-VI, III-V, II-VI, or I-VII members, are of considerable scientific and technological interest as their solid-state properties (e.g., electric and optical such as type of conductivity, current carrier density, band gap) modulate regularly over a wide range through variations in composition. A general descriptive scheme for such alloys is as follows [41]. 

The binary compounds of palladium and platinum show a higher diversity of structures than found for the Rh-Ir and Ru-Os systems. Aside from several other reported binary phases, four general families can be distinguished ... 

Of particular interest is the fundamental science and technology of the solid solutions between II-VI binary compounds. These isovalent alloys may be classified according to the scheme introduced previously (see Sect. 1.2.3) - a convenient matrix diagram comprising their observed structures can be found in a publication of Wei and Zunger [102]. 

By substituting alternately the carbon atoms in cubic diamond by zinc and sulfur atoms, one obtains the structure of zinc blende (sphalerite). By the corresponding substitution in hexagonal diamond, the wurtzite structure results. As long as atoms of one element are allowed to be bonded only to atoms of the other element, binary compounds can only have a 1 1 composition. For the four bonds per atom an average of four electrons per atom are needed this condition is fulfilled if the total number of valence electrons is four times the number of atoms. Possible element combinations and examples are given in Table 12.1. 

It should not be inferred that the crystal structures described so far apply to only binary compounds. Either the cation or anion may be a polyatomic species. For example, many ammonium compounds have crystal structures that are identical to those of the corresponding rubidium or potassium compounds because the radius NH4+ ion (148 pm) is similar to that of K+ (133 pm) or Rb+ (148 pm). Both NO j and CO, have ionic radii (189 and 185 pm, respectively) that are very close to that of Cl- (181 pm), so many nitrates and carbonates have structures identical to the corresponding chloride compounds. Keep in mind that the structures shown so far are general types that are not necessarily restricted to binary compounds or the compounds from which they are named. 

Although the products in these equations are written as if they were simple ionic binary compounds, this is not always the case. For example, some nonmetals form clusters containing several atoms arranged in polyhedral structures. One such species is the P73- cluster, which has six phosphorus atoms at the vertices of a trigonal prism with the seventh occupying a position above a triangular face... 

Because nitrogen compounds of Se and Te are much less important than those of sulfur, this section will be devoted to the sulfur compounds. The binary compounds containing sulfur and nitrogen have several unusual structures and properties that make them an interesting series. Probably the most studied compound of this type is S4N4, tetrasulfur tetranitride, which is prepared by the following reactions ... 

LEDs and semiconductor, 22 174-175 in organic semiconductors, 22 201, 202 silicon, 22 485, 488 silicon carbide, 22 530 Band gap transition type, for binary compound semiconductors, 22 145, 146-147t Band structure... 

Villars, P., Mathis, K. and Hulliger, F. (1989) Environment classification and structural stability maps. In The Structures of Binary Compounds, eds. de Boer, F. and Pettifor, D. (North-Holland, Amsterdam), Vol. 2, p. 1. 

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