Layer lattice compounds

Structure of nickel arsenide showing (a) 3 unit cells, (b) a single unit cell NiaAsa and its relation to (c) the unit cell of the layer lattice compound Cdia (see text). 

Many layer-lattice compounds can intercalate additional metal atoms of the same element as comprised in the original structure (e.g. niobium in niobium diselenide), but molybdenum disulphide will not do so. The behaviour may be determined by the availability of electrons suitably oriented to form bonds with the additional metal atoms, although it seems unlikely that this single factor applies to all intercalation effects. 

Haltner, A.J., Sliding Behaviour of Some Layer Lattice Compounds in Ultrahigh Vacuum, ASLE Trans., 9, 136, (1966). 

Various other soft materials without the layer—lattice stmcture are used as soHd lubricants (58), eg, basic white lead or lead carbonate [598-63-0] used in thread compounds, lime [1305-78-8] as a carrier in wire drawing, talc [14807-96-6] and bentonite [1302-78-9] as fillers for grease for cable pulling, and zinc oxide [1314-13-2] in high load capacity greases. Graphite fluoride is effective as a thin-film lubricant up to 400°C and is especially useful with a suitable binder such as polyimide varnish (59). Boric acid has been shown to have promise as a self-replenishing soHd composite (60). 

The crystal structure of many compounds is dominated by the effect of H bonds, and numerous examples will emerge in ensuing chapters. Ice (p. 624) is perhaps the classic example, but the layer lattice structure of B(OH)3 (p. 203) and the striking difference between the a- and 6-forms of oxalic and other dicarboxylic acids is notable (Fig. 3.9). The more subtle distortions that lead to ferroelectric phenomena in KH2PO4 and other crystals have already been noted (p. 57). Hydrogen bonds between fluorine atoms result in the formation of infinite zigzag chains in crystalline hydrogen fluoride... 

A detailed discussion of individual halides is given under the chemistry of each particular element. This section deals with more general aspects of the halides as a class of compound and will consider, in turn, general preparative routes, structure and bonding. For reasons outlined on p. 805, fluorides tend to differ from the other halides either in their method of synthesis, their structure or their bond-type. For example, the fluoride ion is the smallest and least polarizable of all anions and fluorides frequently adopt 3D ionic structures typical of oxides. By contrast, chlorides, bromides and iodides are larger and more polarizable and frequently adopt mutually similar layer-lattices or chain structures (cf. sulfides). Numerous examples of this dichotomy can be found in other chapters and in several general references.Because of this it is convenient to discuss fluorides as a group first, and then the other halides. 

Heterocyclic sulfides, 23 645 Heteroepitaxial layers, for compound semiconductors, 22 145 Heteroepitaxy, on lattice mismatched substrates, 22 160 Heterofullerenes, 12 231—232 chemistry of, 12 252—253 Heterogeneous azeotropic distillation, 8 819-845... 

Square-planar complexes of platinum(II) and palladium(II) have been known for a long time the comparatively simple unit cells of compounds such as K2PdCl4, K2PtCl4, and Pd(NH3)4Cl2H20 led to early elucidation of the structures (257) and they all contain square-planar ions. The simple halides PdCl2 and Pt,Cl2 (71) consist of chains in which the metal is bonded from the corners of a square. Nickel chloride, on the other hand, has a layer lattice in which the nickel is octahedrally coordinated, and in the halide complexes the coordination is tetrahedral, as described in Section IV,B. 

Investigations in this field have mainly used calcium silicides. In the Ca/Si system there are two silicides with Si-Si bonds in polymeric systems. In calcium monosilicide the silicon atoms are arranged in form of chains, while the calcium disilicide CaSi2 has a layer lattice, consisting of silicon- and calcium layers. It is possible to form polymeric compounds (SiX) and (SiX2) from both silicides by replacing calcium. 

It is possible to insert additional atoms or molecules into the inter-lamellar gap of many layer-lattice materials, including molybdenum disulphide, creating what are called intercalation compounds. The intercalated substances may be alkali or alkalyne-earth metals (sodium, potassium, rubidium, caesium, calcium, strontium), salts or organic bases such as ethylene diamine or pyridine . 

Salt hydrates hold water molecules as (i) co-ordinated water, (ii) anion water, (iii) lattice water, or (iv) zeohte water. The term water of constitution is a misnomer often applied to compounds wrongly formulated as FegOg.HgO and MgO.HgO. These are the true hydroxides FeO(OH) and Mg(OH)2 and most of the class have a layer lattice consisting of sheets of OH and Onions with cations between them. 

The sulphides M2S3 are all made by direct combination of the elements. But GaS is also known and has an unusual layer lattice containing Ga2 ions (Hahn and Frank, 1955). The nitride GaN,unreactive to water and acids, is made by heating gallium in ammonia at 1000° the corresponding indium compound is best made by heating (NH4)3lnFg. Both have the wurtzite lattice. 

The structure of PbS is of particular interest as the compound crystallises with the typically ionic NaCl lattice in marked contrast to the layer lattice of PbO. It may well be the least ionic compound to do so. 

There are compounds of terpositive molybdenum they are strong reducing agents like those of Cr b Corresponding simple compounds of tungsten arc unknown, although complexes of exist. Both Mo and W are quadri-positive in chlorides, oxides and sulphides MO.3 and WOg have a distorted rutile structure, and MoSg a layer lattice. 

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