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Friday, April 27, 2012

electronic lattice bound materials

Classic Definitions: Of the millions of different chemical systems discovered since chemistry began, many are solids at room temperature. From the early days these solids have been classified in the four families, molecular, ionic, covalent and metallic solids, based on the nature of the forces which bind the atoms. Ideal solids consist of an arrangement of atoms repeated indefinitely in space. The term extended solid refers to the case where the bonding between the atoms is reasonably uniform in strength throughout the structure (e.g. diamond, metallic Cu). An ionic material is an extended solid in which substantial charge transfer has occurred (e.g. NaCl). A molecular (or atomic) solid consists of small discrete molecules (or atoms) weakly bound together (e.g. solid CH4 or Xe, white P). The latter category should be characterised by low melting points (or low sublimation temperatures). Molecular solids are composed of groups of covalently bound atoms, i.e., molecules, held by weak charge-polarization (van der Waals) forces. In ionic solids, electrostatic attraction is the primary force binding cations and anions. Bonding in covalent solids is similar to that within molecules but extends over the whole crystallite. Metallic solids also exhibit extended bonding but, in addition, possess weakly bound, highly delocalized electrons easily moved by applied fields. Of course, this classification is somewhat artificial and many solids exhibit complex bonding in which more than one type of bonding is displayed. Molecular clusters in the solid state are naturally described nowadays with molecular-orbital models. The forces binding the fundamental units (atoms, ions or molecules) together may be essentially non-directional or directional. Non-directional forces result in structures based on close packing, while directional forces lead to more open structures, often based on linked polyhedra or groups of polyhedra. The two categories are now considered separately.

Darpa Definition: Extended solids are polymorphs/phases of simple molecules that are currently formed under ultrahigh pressure conditions where strong intermolecular bonding and tight crystal packing can be induced, leading to dramatic changes in physical, mechanical, and functional properties.

Darpa talks about extented high pressure phases. By this they apparently mean phases that can be induced by high pressure although high pressure is not needed to maintain them. This is not a very good use of the word, extended, which has been used to refer to the case where atomic or molecular bonding is reasonably uniform.. I assume they actually mean materials bound by an electronic lattice. The strength and QED properties of the electronic lattice often confers exceptional properties on the materials.

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