Binary compounds simple structures

Related topics Electronegativity and bond type (Bl) Binary compounds simple structures (D3)... 

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... 

It has already been noticed (see 3.9.4) that according to the mentioned concepts several ternary compounds may be considered as the result of a sort of structural interaction between binary compounds. As a consequence some regular trend could also be predicted for their occurrence in their phase diagrams and in the description (and perhaps modelling) of their thermodynamic properties. A few details about this type of structural relationships will be considered in the following and, in this introduction, examples of blocks of simple structural types and of their combination in more complex types will be described. 

Among the actinide compounds the interest is concentrating on binary compounds of simple structure (e.g. 1 1 compounds with elements of the groups V and VI of the periodic table) for which the theoretical treatment is rather advanced, and on intermetal-lic (e.g. Laves-) phases. 

V. Crystal Structures and Thermodynamic Properties of Simple Actinide Binary Compounds... 

In this chapter, we have examined relevant structural and thermodynamic properties of metals and simple binary compounds, and which information they may convey on the... 

As the valency of the metal increases, the bonding in these simple binary compounds becomes more covalent and the highly symmetrical structures characteristic of the simple ionic compounds occur far less frequently, with molecular and layer structures being common. Many thousands of inorganic crystal structures exist, ffere we describe just a few of those that are commonly encountered and those that occur in later chapters. 

A special technique which has yielded highly unstable and simple dinitrogen complexes trapped in low-temperature solids (N2 or Ar) is the metal atom cocondensation technique in low-temperature matrices. By the cocondensation of nickel atoms and N2 at 4.2-10 K the binary compounds Ni(N2) (n = 1-4) result, whose structure has been inferred from IR and... 

Here we consider the factors which determine whether a given compound prefers an ionic structure or a covalent one. We may imagine that for any binary compound - e.g. a halide or an oxide - either an ionic or a covalent structure can be envisaged, and these alternatives are in thermochemical competition. Bear in mind that there may be appreciable covalency in ionic substances, and that there may be some ionic contribution to the bonding in covalent substances. Since there is no simple means - short of a rigorous MO treatment - of calculating covalent bond energies, and since quantitative calculations based upon the ionic model are subject to some uncertainties, the question of whether an ionic or a covalent structure is the more favourable thermodynamically cannot be answered in absolute terms. We can, however, rationalise the situation to some extent. 

Structure of Simple Binary Ionic Compounds.—To be electrically neutral, every ionic compound must contain some positive and some negative ions. The simplest ones are those binary compounds that contain one positive and one negative ion. Obviously the positive ion must have lost the same number of electrons that the negative one has gained. Thus monovalent metals form such compounds with monovalent bases, divalent with divalent, and so on. In other words, this group includes compounds of Li+, Na+, K+, Rb+, Cs+, Cu+, Ag+, and Au+ with F-, C1-, Br, I- of Be++, Mg++, Ca++, Sr++, Ba++, Zn++, Cd++, Hg++, with 0—, S—, Se—, Te of B+++, A1+++, Ga+++, In+++, T1+++, with... 

Structural data are available (Table 30) for a range of binary, ternary and quaternary sulfides of manganese, almost invariably Mn", and these set the scene for the structures to be expected in the compounds with the more discrete polyhedra.319 Indeed, the structural pattern is established in the simple binary compound MnS. Whereas, the stable modification of this (a-MnS) is green and has the cubic rock salt structure with [MnS6] octahedra, the well-known flesh-coloured precipitates of the qualitative analysis system are metastable / - and y-modifications, which have [MnS4] tetrahedra with respectively the zinc blende or diamond (cubic) and wurtzite (hexagonal) structures. And so, in the rest of the known solids, there are almost equal numbers of four-coordinate tetrahedra and six-coordinate octahedra with no other polyhedra having been detected. 

If the substance is a binary compound AB, and if its unit cell is simple cubic P with one formula (one atom of A and one of B) per cubic cell, the relative positions of the two atoms are fixed by symmetry. This is tme of the salt cesium chloride, CsCl, the structure of which is shown in Fig. 4. One of the ions, Cs say, may without loss of generality be placed at the origin. The other ion, Cr, must be at the center of the unit cell if it is in any other position, the stracture will lack the threefold rotational axes of synunetry that are always present along all four body diagonals of the unit cell in the cubic system. 

Binary compounds (compounds consisting of two elements) may have very simple crystal structures and can be described in several different ways. Two simple structures are shown in Figure 7-7. As described in Section 7-1-1, there are two tetrahedral holes... 

The number of binary compounds alone is evidently considerable, and there is an indefinitely large number of compounds built of atoms of three or more elements. It seems logical to concentrate our attention first on the simplest compounds such as binary halides, chalconides, etc., for it would appear unlikely that we could understand the structures of more complex compounds unless the structures of the simpler ones are known and understood. However, it should be noted that simplicity of chemical formula may be deceptive, for the structures of many compounds with simple chemical formulae present considerable problems in bonding, and indeed the structures of some elements are incomprehensibly complex (for example, B and red P). On the other hand, there are compounds with complex formulae which have structures based on an essentially simple pattern, as are the numerous structures described in Chapter 3 which are based on the diamond net, one of the simplest 3-dimensional frameworks. We shall make a point of looking for the simple underlying structural themes in the belief that Nature prefers simplicity to complexity and also because structures are most easily understood if reduced to their simplest terms. 

Crystal structures may be described in terms of the coordination polyhedra MX of the atoms or in terms of their duals, that is, the polyhedra enclosed by planes drawn perpendicular to the lines M-X joining each atom to each of its neighbours at the mid-points of these lines. Each atom in the structure is then represented as a polyhedron (polyhedral domain), and the whole structure as a space-filling assembly of polyhedra of one or more kinds. We can visualize these domains as the shapes the atoms (ions) would assume if the structure were uniformly compressed. For example, h.c.p. and c.c.p. spheres would become the polyhedra shown in Fig. 4.29. These polyhedra are the duals of the coordination polyhedra illustrated in Fig. 4.5. These domains provide an alternative way of representing relatively simple c.p. structures (particularly of binary compounds) because the vertices of the domain are the positions of the interstices. The (8) vertices at which three edges meet are the tetrahedral interstices, and those (6) at which four edges meet are the octahedral interstices. Table 4.9 shows the octahedral positions occupied in some simple structures c.p. structures in which tetrahedral or tetrahedral and octahedral sites are occupied may be represented in a similar way. (For examples see JSSC 1970 1 279.)... 

Binary compounds are ones with two elements present. Simple crystal structures may be classed as ones in which each atom (or ion) is surrounded in a regular way by atoms (or ions) of the other kind. Even with this limited scope many structures are possible. Figure 1 shows a selection of simple ones that exemplify some important principles. Although many are found with ionic compounds, some of these structures are shown by compounds with covalent bonding, and a discussion of the bonding factors involved in favoring one structure rather than another is deferred to Topic D4. 

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