Non-molecular solids

Besides the molecular solids discussed above, there are numerous and very important solid substances which are formed by extended arrays rather than by molecular units. Indeed, many of the elements are found under normal conditions as non-molecular solids built up as infinite atomic arrays. 

Three limiting types of non-molecular solids may be distinguished  

Metallic solids. Infinite arrays of closely packed atoms held together by delocalized electrons. Each atom in a metal has a high coordination number, often eight or twelve. Physical properties of metals are quite different from those of other solid substances. They are good conductors of heat and electricity, have high reflectivity, and special mechanic properties such as strength and ductility. 

Ionic solids. They consist of infinite arrays of ions in which each ion is surrounded by a certain number of ions of the opposite sign coordination numbers are often high, six or eight. In general ionic crystals are hard solids with high melting temperatures and behave as electric insulators (vide infra). 

There are also other types of solids, for instance glasses and polymers, that cannot be classified into the groups defined above. These substances may be considered to be intermediate cases between the limiting situations described above. Specially in the cases of both non-metallic and ionic solids the instances for which such classifications match totally are very few. In most cases the behavior of actual solids may be better described by assuming an hypothetical mixture of these limiting cases. 

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After an extensive review of MMCT transitions involving ions in solids, it seems wise to start this paragraph with some molecular species, because many of these have been investigated in much more detail than their counterparts in non-molecular solids. It is suitable to make a distinction between outer-sphere charge-transfer (OSCT) and inner-sphere charge-transfer (ISCT) transitions [1], In the former the metal ions do not have ligands in common, in the latter they are connected by a common ligand. Studies are usually performed on metal-ion pairs in solution. 

The photochemical process will in general not occur in non-molecular solids due to the restriction in the nuclear coordinates. However, the nonradiative return to the ground state does, using MMCT states as an intermediary [35]. This can be nicely illustrated on a molecular solid, viz. the lanthanide-decatung-states which contain complex ions [Ln(III)Wio036] [40, 121]. 

G. M. Clark, The Structures of Non-molecular Solids. Applied Science Publishers, 1972. 

G. M. Clark, The Structures of Non-molecular Solids. Applied Science Publishers, 1972. D. M. Adams, Inorganic Solids. John Wiley Sons. Ltd, 1974. 

Electronegativity thus permits a quantitative interpretation of the bonding in SiO or any other compound for which appropriate data are known, The same principles allow a superior alternative to the ionic" model of non molecular solids, and offer high hope of eventually elucidating the thermochemistry of mineral substances in general. 

T Mixed metal oxides T Inorganic, non metallic, non molecular solids Hybrid ceramic materials... 

The method of X-ray diffraction is widely used for the determination of the structures of molecular solids (i.e. sohds composed of discrete molecules) and of non-molecular solids (e.g. ionic materials). As the technique has been developed, its range of applications has expanded to include poljrmers, proteins and other macromolecules. The reason that X-rays are chosen for these experiments is that the wavelength (a 10 m) is of the same order of magnitude as the internuclear distances in molecules or non-molecular solids. As a consequence of this, diffraction is observed when X-rays interact with an array of atoms in a solid (see below). 

The electric forces linking atoms in molecules or non-molecular solid phases. Three basic types are usually distinguished ionic or electric bonding, covalent bonding and metallic bonding... 

Until 1995, micrometer scale whiskers of the lll-IV semiconductors have been grown only by metal-catalyzed or metal-organic CVD processes at relatively high temperatures, e.g., SiSOO C. Lack of useful crystallization mechanisms for highly covalent non-molecular solids has so far prevented their growth at lower process temperatures. A recent breakthrough [18]... 

The wavelength of X-rays ( 10 °m, i.e. 100pm) is of the same order of magnitude as the intemuclear distances in molecules or non-molecular solids. As a consequence of this, diffraction is observed when X-rays interact with the electrons in an array of atoms in the crystalline solid. 

Jensen proposes using quotation marks or underhning for non-molecular solids [50]. Thus, NaCl or NaCl would show that solid sodium chloride does not actually have separate molecules with that formula, unlike, say, P4. This resembles somewhat the suggestion just above for handling structurally misleading conventional formulae, but we have not yet adopted this promising idea. 

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