Structural chemistry, hard materials

Most well-known hard materials have surprisingly simple crystal structures. Here we give an overview of the crystal chemistry of hard materials. Different aspects of the various crystal structures are discussed, such as close-packed structures with filled octahedral voids like titanium carbide, the tetrahedral arrangements in SiC or the three-dimensional inorganic networks in a- and P-Si3N4. Also we briefly mention the synthesis and some applications of various hard materials. 

Putz, M. V. (2011c). Quantum paiabohc effects of electronegativity and chemical hardness on carbon n-systems. In Putz, M. V. (Ed.), Carbon Bonding and Structures Advances in Physics and Chemistry, Carbon Materials Chemistry and Physics series Vol. 5, Springer Verlag, London, Chapter 1, pp. 1-32. 

Carbides are binary compounds in which carbon is the more electronegative partner. We shall come across silicon carbide, SiC, again later when we look at the chemistry of silicon this is a very hard material, manufactured in large amounts as an abrasive known as carborundum. Its hardness can be accounted for by the fact that its crystal structure is the same as that of diamond, with alternate carbon atoms replaced by silicon (Figure 10.13). A major industrial use of SiC is in steel refining where its addition to the molten metal removes metal oxide impurities with production of CO and a silicate slag, which is skimmed off. 

The work described in this paper is an illustration of the potential to be derived from the availability of supercomputers for research in chemistry. The domain of application is the area of new materials which are expected to play a critical role in the future development of molecular electronic and optical devices for information storage and communication. Theoretical simulations of the type presented here lead to detailed understanding of the electronic structure and properties of these systems, information which at times is hard to extract from experimental data or from more approximate theoretical methods. It is clear that the methods of quantum chemistry have reached a point where they constitute tools of semi-quantitative accuracy and have predictive value. Further developments for quantitative accuracy are needed. They involve the application of methods describing electron correlation effects to large molecular systems. The need for supercomputer power to achieve this goal is even more acute. 

Despite the vast number of reported syntheses, crystal structures, properties and applications of monomeric Pcs, it is still difficult to clarify clearly the synthetic mechanisms of Pcs in various conditions, to predict fully the supramolecular structures of Pcs in the solid state (except for a very few types of simple Pcs), and to elucidate completely the correlation between molecular structure and properties. The large-scale preparation and separation of some novel Pcs with interestingly properties is still very hard. On the basis of the great potential applications of Pcs in high-tech fields, exploitation of multi-functional Pc materials needs to be strengthened in the future. There is still plenty of room for further investigation of Pc chemistry. 

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