Crystal structure carbonitrides

An extensive compilation of the crystal structures of transition metal carbides is found in Pearson s Handbook. Ward has discussed the structures of carbides extensively. Epicier and de Novion have suimnarized the results of investigations on ordering in transition metal carbides. Lengauer recently reviewed the knowledge on transition metal carbides and carbonitrides. For a comparison of close-packed transition metal carbides with close-packed transition nitrides, see Nitrides Transition Metal Solid-state Chem istry. 

Because of the evident structural similarities between transition metal carbides and transition metal nitrides the carbon atoms in group 4 and 5 carbides can be replaced completely by nitrogen without changing the structure of the binary phases. So far only one distinct ternary phase Cr2 (C,N)2 has been reported. Intersolubility between the binary nitrides and carbides in the group 6 carbonitride systems Cr-C-N and Mo-C-N is not complete because of the differences in the crystal structures of the carbide and nitride phases. 

Amineboranes were thermally decomposed at 1273 K in Ar to boron carbonitrides [262]. These materials were reported to show a turbostratic structure where atoms in a carbon-like structure are partially substituted by boron and nitrogen, respectively. At temperatures higher than 2073 K (Ar-atmosphere) the single phase material of compositions BC4N and BC2N, respectively, started to crystallize and a-BN, B4C and elemental C (graphite) were observed. 

The boundary between hardmetals and cermets is not strict because many of these compacts resemble microstructure features of both type of materials [106] faceted WC crystals together with round-shaped titanium carbonitride-based hard particles. Generally, these titaniiun carbonitride hardmetals are comparable with respect to properties and microstructure to WC-based hardmetals. The powders of these materials are liquid phase sintered with Ni or Ni-Co binder metal alloys. The core-and-rim structure of the hard phase usually exhibit a molybdenum- and carbon-rich (Ti,Mo)C rim and a titanium- and nitrogen-rich Ti(C,N) but can also be inverted (compare Fig. 26). The metallurgy of the phase reactions is (because of the complexity of the multicomponent system) not yet fully understood [69]. 

Because fonnation of cubic boron carbonitride is of great fundamental interest with respect to superhard materials much additional effort is needed to succeed in the preferential synthesis of the cubic B-C-N phase. As follows from the above results the most promising way would be synthesis under nonequilibrium conditions such as flash-heating at static pressures or shock-wave compression [140]. Successful synthesis of cubic BC2.5N solid solution in 18% yield by shock-compression of hexagonal BC2.5N has been reported [143]. The material obtained was a single cubic BC2.5N phase with a diamond-like structure and crystals between 5 and 20 nm in size. 

The powder of silicon nitride of the SHS-Az type can contain in the structure from 40% up to 95% (a-nitride phase as anisotropic whiskers by a diameter 1 pm forming wave structure (Figure 8.2)). The particles of the boron nitride powder of the SHS-Az type are ultra fine and have the disc form, and the diameter of disks at 10-15 of time exceeds their thickness having the linear size about 20 nm. The crystal lattice BN has deformations of turbostrate kind with a degree of three-dimensional order 0.40-0.50. The powder of titanium carbonitride with composition TiCggNog of the SHS-Az type differs from similar powders of traditional technologies of synthesis of more branched structure, typical for formation of... 

The data available on the structure of the electronic states in more complicated systems are extremely limited. The MO LCAO calculations for the clusters [MoN Xi Mi2] (n = 0, 1,..., 5 M = Ti, V X = C, O) were performed by Ivanovsky (1988). Such clusters serve as models for Ti and V carbonitrides and oxynitrides, when the solid solution contains vacancies (Chapter 4). On the emergence of vacancies the occupied part of the metal subband broadens while the 2s- and 2p-nonmetal subbands (whose atoms are removed from the crystal) become narrower. The localised bonds between the metal and second nonmetal component (X) change in this case also. For example, the Ti-C bond population in the [TiN5CTii2] cluster is 16% less, than its counterpart in the [TiN4DCTii2] cluster, corresponding to nonstoichiometric carbonitride. 

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