Transition-Metal Silicides

The general understanding of the electronic structure and the bonding properties of transition-metal silicides is in terms of low-lying Si(3.s) and metal-d silicon-p hybridization. There are two dominant contributions to the bonding in transition-metal compounds, the decrease of the d band width and the covalent hybridization of atomic states. The former is caused by the increase in the distance between the transition-metal atoms due to the insertion of the silicon atoms, which decreases the d band broadening contribution to the stability of the lattice. 

Among metal borides of the formula MjM B or (Mj, M/r)2B, the competing structural units are (a) the antiprism and (b) the trigonal metal prism. In many cases the CUAI2 structure with BMg-antiprismatic B coordination is adopted in close resemblance to transition-metal silicides, but no boron-carbon substitution is ob-served - " . 

Silicon-containing ceramics include the oxide materials, silica and the silicates the binary compounds of silicon with non-metals, principally silicon carbide and silicon nitride silicon oxynitride and the sialons main group and transition metal silicides, and, finally, elemental silicon itself. There is a vigorous research activity throughout the world on the preparation of all of these classes of solid silicon compounds by the newer preparative techniques. In this report, we will focus on silicon carbide and silicon nitride. 

In this article the intention is to place the main emphasis on studies of molecular silicon-transition-metal compounds, in particular those from the author s laboratory and those that have been reported since the comprehensive reviews noted above. As, however, it is now known that these molecular compounds can act as convenient precursors for certain transition-metal silicides, aspects of the properties of silicides will also be outlined, and relationships between the two classes of compounds examined. Compounds with bonds between transition metals... 

Slaugh has claimed that a catalyst consisting of MoSi2 has resistance to sulphur.130 This material gave 20% conversion of CO + H2 mixture (1 3) at 500-600 °C and the conversion was >2% after 90 min of H2S treatment. Other transition metal silicides (e.g., ZrSi, WSi2) were tested for methanation but had no activity. 

As the lattice spacings of chemical compounds are usually of the order of 0.5 nm or greater, it follows that any compound layer, 5 nm thick, can contain at most 10 crystal-lattice units. Therefore, the results of an analysis of the nucleation process, obtained by F.M. d Heurle31 for transition-metal silicides, appear to be quite realistic. 

F.M. d Heurle evaluated a specific thickness of the layers (an analogue of the critical radius of nuclei in a homogeneous system for more detail, see Ref. 31) for compounds of the Ni-Si binary system. For Ni2Si, its value was found to be 0.15 nm, i.e. the nucleus does not contain even one lattice unit. Although higher values were obtained for other nickel silicides, they never exceeded 1 nm. Therefore, the nucleation process can hardly play any significant role in the formation of most transition-metal silicides, except in some special cases. This conclusion is likely to be valid for any other chemical compound layer. It should be noted, however, that there is also a different viewpoint.38 132... 

Consider the thermodynamic data on the formation of a few transition metal silicides presented in Table 3.1. The values of AfG° were taken from the work by V.N. Yeremenko et al. When comparing the free energy changes calculated per mole of the compound, the silicides of the Me5Si3 type should be regarded as most stable. However, in none of the systems under consideration the compound of such a composition was found to form first. 

Improvements in the performance of integrated circuits and the trend towards VLSI-technology require the replacement of polycrystalline silicon by materials with a lower resistivity for use as gate electrodes. Transition metal silicides appear to be valuable possibilities for these applications. Timgsten-silicon compounds could be suitable precursors for the precipitation of tungsten-silicide thin films. Moreover tungsten-silicon compounds are nearly unknown and of scientific interest. 

Transition-, Inner-Transition, Group-IB and -MB Metal Silicides 5.10.3.2.1. Transition-Metal Silicides... 

Sarkisyan, A. R., Dolukhanyan, S. K., and Borovinskaya, I. P., Self-propagating high-temperature synthesis of transition metal silicides. Sov. Powd. Metall., 17,424 (1978). 

Keywords compensation effect, isokinetic effect, Rochow synthesis, transition metal silicide phases, selective energy transfer model... 

Summary Both in the Rochow synthesis of methylchlorosilanes and in the reaction of transition metal silicides with HCl, catalytic reactions of silicon, bound as metal silicide, with gaseous reactants are involved. With both reactions, the kinetic parameters ko and Ea exhibit consequent compensation effects, with the isokinetic temperature positioned within the range of reaction temperatures investigated. In this paper, we ply the model of selective energy transfer fiorn the catalyst to adsorbed species to the kinetic data. With Rochow synthesis Si-CHs rocking frequencies, and with hydrochlorination of silicides Si—H vibration frequencies could correspond to the isokinetic temperatures observed. An interpretation in terms of accessibility of the reactive silicon atom to reactant molecules is given. 

Fig. 2. Arrhenius plot of the reaction between transition metal silicide phases and hydrogen chloride. (Reprinted with permission from [7]. Copyri t 2002 American Chemical Society.)...

Fig. 3. Pre-exponential fector - apparent activation energy compensation plot of the reaction between transition metal silicides and hydrogen chloride under isothermal conditions (confidence interval 95%) [8]. Silicide phases are labeled as follows 1 FeSi2, 2 NiSij, 3 FeSi, 4 NiSi, 5 CuaSi, 6 Ni2Si, 7 NisSi, 8 CusSi + 1.5 at% Zn.

Rochow synthesis Hydrochlorination of transition metal silicides... 

---