Transition metal silicide phases

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

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.

Summary Transition metal silicides, containing small amounts of chlorine, act as catalysts in the hydrodechlorination of silicon tetrachloride into trichlorosilane. Silicides, with comparable properties to the catalytically active phases, can be prepared by the reaction of silicon-rich silicides with the respective metal chloride. For the first time a thermodynamic model of chlorine-containing nickel silicides is given. Chlorine is considered to be dissolved in the silicides and is modeled as a lattice gas. Consequences regarding the bonding state of chlorine in the silicide phases are reported and discussed in relationship with the interactions within the silicide lattice. 

Eq.2. Formation of transition metal silicides in the starting phase of the catalytic hydrodechlorination. 

Synthesis in liquidAl Al as a reactive solvent Several intermetallic alu-minides have been prepared from liquid aluminium very often the separation of the compounds may be achieved through the dissolution of Al which dissolves readily in several non-oxidizing acids (for instance HC1). For a review on the reactions carried out in liquid aluminium and on several compounds prepared, see Kanatzidis et al. (2005) binary compounds are listed (Re-Al, Co-Al, Ir-Al) as well as ternary phases (lanthanide and actinide-transition metal aluminides). Examples of quaternary compounds (alumino-silicides, alumino-germanides of lanthanides and transition metals) have also been described. As an example, a few preparative details of specific compounds are reported in the following. 

Line compounds. These are phases where sublattice occupation is restricted by particular combinations of atomic size, electronegativity, etc., and there is a well-defined stoichiometry with respect to the components. Many examples occur in transition metal borides and silicides, III-V compounds and a number of carbides. Although such phases are considered to be stoichiometric in the relevant binary systems, they can have partial or complete solubility of other components with preferential substitution for one of the binary elements. This can be demonstrated for the case of a compound such as the orthorhombic Cr2B-type boride which exists in a number or refractory metal-boride phase diagrams. Mixing then occurs by substitution on the metal sublattice. 

This is a very fundamental hypothesis (very well verified indeed within the accuracy limits +0,02 A) of EXAFS and allows the analysis of an unknown system, say an interface between a transition metal and silicon, by using the amplitudes and phase shifts from a model compound of known crystallography, say a silicide. 

Note that the silicide layer may grow not only between silicon and a transition metal, but also between a silicon-containing phase and a transition metal or an intermetallic compound. Such layers are known to occur in the process of brazing the transition metals by their own melts with Si3N4-base ceramics239 and also during the interaction of transition metals with silicon carbide.238 240 245... 

The formation of silicides in reaction couples, for example, of the MesAl-SiC type, where Me is a transition metal, is more complicated. In this case, in addition to the Me2Si layer, the MeAl layer (or some other aluminide layer) also grows, i.e. the Me3Al-MeAl-(Me2Si+C)-SiC system is formed. The mechanism of its occurrence is probably as follows. The Me3Al phase is decomposed at the Me3Al-MeAl interface by the reaction... 

The transition metals are used as metallization layers in Si device technology Upon heating, the thin (a few thousand A thick) transition-metal layers react uniformly with the Si substrate to form a silicide. From a typical transition-metal-Si binary phase diagram (see Fig. 1), the lowest T at which a liquid appears is greater than 900°C, which is above the process T used in integrated circuit fabrication. In Si device processing, silicide formation is therefore usually a solid-phase interaction. 

The rule for predicting first-phase silicide formation (see 5.10.3.2.1) works for germanide formation during solid-state interactions between thin films ( 1000 A) of the transition metals Co, Hf, Mn, Ni, Pd and Rh and a thick single-crystal Ge substrate , with the exception of Pd. 

The partial substitution of the transition element M in MjSi by a second transition metal M leads to ternary silicides of approximate composition MM Si corresponding to (M,M )2Si. Such ternaries are primarily the Si-containing E phases and V phases (Jeitschko etal., 1969 Jeitschko, 1970) and the ternary Si-containing Laves phases (Bardos etal., 1961), which were discussed in Sec. 8, as well as many other phases, which all differ by composition and crystal structure (Nowotny, 1972 a). This is exemplified by the Fe-Nb-Si system with the ternary silicides E, V, Xj, Xj, Xj and the Laves phase Nb(Fe,Si)2 with up to 25 at.% Si (Raghavan, 1987), or the Co-Nb-Si system with the ternary silicides E, T, v, Ti, v i, and the ternary Laves phase Nb(Co,Si)2 with Si contents between about 10 and 20 at.% (Argent, 1984). Finally, it is noted that other phases - in particular a phases and A13 Mn-base phases - dissolve large amounts of Si by which these phases are stabilized (Gupta et al., 1960 Bardos et al., 1966). 

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