Diamond-like Structures

By subjecting boron nitride (a white powder) to high pressure and temperature small crystals of a substance harder than diamond, known as borazon, are obtained. This pressure-temperature treatment changes the structure from the original graphite-like layer structure (p. 163) to a diamond-like structure this hard form can withstand temperatures up to 2000 K. 

The work on carbon nitride solids is strongly related to research on diamondlike carbon (DLC) materials [5, 6]. DLC materials are thin film amorphous metastable carbon-based solids, pure or alloyed with hydrogen, which have properties similar to that of crystalline diamond (high hardness, low friction coefficient, high resistance to wear and chemical attack). This resemblance to diamond is due to the DLC structure, which is characterized by a high fraction of highly cross-linked sp -hybridized carbon atoms. To obtain this diamond-like structure... 

A large number of binary AB compounds formed by elements of groups IIIA and VA or IIA and VIA (the so-called III-V and II-VI compounds) also fcrystallize in diamond-like structures. Among the I-VII compounds, copper (I) halides and Agl crystallize in this structure. Unlike in diamond, the bonds in such binary compounds are not entirely covalent because of the difference in electronegativity between the constituent atoms. This can be understood in terms of the fractional ionic character or ionicity of bonds in these crystals. 

Ternary materials that incorporate carbon into a boron-nitrogen framework are expected to exhibit extreme hardness as a result of their diamond-like structures. Alkyl-, vinyl- and alkynyl-substituted borazines are potential... 

Boron nitride (BN) can normally be prepared from the reaction of boric acid and urea or melamine. For example, the pyrolysis of MB can yield hexagonal BN. It is commonly referred to as white graphite because of its platy hexagonal structure similar to graphite. Under high pressure and at 1600°C, the hexagonal BN is converted to cubic BN, which has a diamond-like structure. 

Diamondoid A material with a superior strength-to-weight ratio (100 to 250 times as strong as titanium) but much lighter. These diamond-like structures contain dense, three-dimensional networks of covalent bonds, formed mostly from first- and second-row atoms with a valence of three or more. 

FIG. 3.39. Projections of the diamond-like structures of (a) the POiCOH) anion in KH2PO4,... 

Apart from the irradiation with high-energy electrons, the conversion of carbon onions into diamond also succeeds by bombardment with ions like Ne. The latter are 36000 times heavier than the rather light-weight electrons. Consequently, they require far less velocity and thus smaller accelerator voltages to bear the same effect Diamond-like structures can further be generated by thermal treatment in air at 500 °C or by irradiation with a C02-laser. 

This termed is frequently deployed in the literature to describe ill-defined and unwelcome substances deposited during hydrocarbon reactions on metals, leading to loss of catalytic activity. The work on hydrocarbon adsorption which we have reviewed has shown that, in almost every case, adsorbed species when heated decompose first to Ci species (e.g. =CH2 or H), and that these then lose their hydrogen atoms to form carbon atoms, which on Ir( 100) have been shown by LEED (and DFT) to reside in fourfold sites in a (2 x 2) overlayer. Further heating may lead to their polymerisation to graphite or to a diamond-like structure. Carbon... 

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