The phase stability of MoSi 2 -NbSi 2 pseudobinary alloys was examined by Monte Carlo simulation and the cluster expansion technique based on first-principles calculations. We found that formation energies of all possible atomic arrangements exhibited a positive sign, indicating that no stable intermediate phase exists between MoSi 2 with C11 b and NbSi 2 with C40 structures. The C40 phase has significantly greater solubility as well as higher temperature dependence of solubility than C11 b ,

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... ich agrees with previous experimental reports. Lattice vibration is found to significantly affect the solubility of both C11 b and C40 phases, where its impact naturally increases at higher temperatures. From the analysis of Warren-Cowley short-range-order parameters, the C11 b single phase can be interpreted as a nearly disordered state, while the C40 phase exhibits explicit deviation from the disordered state: C40 prefers Mo-Mo and Nb-Nb like-atom pairs for first-nearest-neighbor coordination, especially around equiatomic composition.