The stability and elastic constants of X2Ni3Si (X=Ti, Mo and W): The novel ternary metal silicides
Introduction
Recently, the binary transition metal carbides or silicides compounds with hexagonal crystal class (space group P63/mmc) are extensively investigated both by theoretical calculations and experimental methods. Because many of them exhibit outstanding mechanical properties and chemical stability, for instance they are very hard compounds and have high melting point, low density, high thermal conductivity, etc. One of the most important carbides is the MAX phase or more precisely Mn+1AXn (M refers to the early transition metal, A usually denotes Al and X is carbon or nitrogen) [1], [2], [3], [4], [5]. More recently, Wang et al. [6], [7], [8], [9], [10] reported the abrasive performances of several newly discovered ternary metal silicides, for example Mo2Ni3Si, Ti2Ni3Si, W2Ni3Si and Mo2Co3Si. The abrasive resistance and chemical stability of these metal silicides were determined and the results indicated that they could be used as the reinforced phases in anti-oxidation coatings and wear resistance materials. The compounds we will investigate in this paper are represented by M2A3X, and in our case M, A and X are Ti/Mo/W, Ni/Co and Si, respectively. In this paper, the chemical stability, electronic structures and mechanical properties for the single phase of X2Ni3Si are studied. In Table 1 the optimized cell structures are listed, such as cell constants and atomic positions. Fig. 1 shows the representative crystal structure of the novel ternary metal silicides.
Section snippets
Methods and details
The total energy calculations were performed with density functional theory (DFT) [11], [12], [13], [14]. The CASTEP code was used for the whole study which uses the plane wave expansion technology in reciprocal space [15], [16], [17]. The Ultra-soft pseudo-potentials of Vanderbilt type were employed to represent the electrostatic interactions between valence electrons and ionic cores [10]. For Mo and W, the semicore orbitals were also included as valence electrons for electronic minimization
Cohesive energy and formation enthalpy
The stability of the ternary metal silicides is discussed firstly. In Table 1 the optimized cell parameters are listed. Since the GGA approach was used for geometry optimizations, one may expect that the obtained cell constants could be overestimated. On the other hand, the lack of experimental results hampered us to confirm this conclusion. The calculated cohesive energy of the silicide is negative, which indicates that they are stable. However, the bulk stability of the compound is determined
Conclusion
In summary, the crystal structures of several novel ternary metal silicides were discussed. By performing the first principles calculations based on DFT, the chemical bonding characteristics and full set of elastic constants were calculated. We conclude that all of the studied compounds, Mo2Co3Si, Mo2Ni3Si, W2Ni3Si and Ti2Ni3Si are thermodynamically stable structures. The calculated bulk modulus and shear modulus clearly indicate that these compounds are harder than general MAX phases, for
Acknowledgments
Project supported by the National Natural Science Foundation of China (no. 2008CB617609) and Science Foundation of Kunming University of Science and Technology.
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