| Summary: | Based on first-principles simulations, we have systematically studied physical behavior of impurity carbon in molybdenum. A single carbon atom is preferable to occupy the octahedral interstitial position (oip) rather than the tetrahedral interstitial position (tip) in molybdenum. Two carbon atoms are inclined to bind each other at two neighbor oips along the 〈210〉 direction. A mono-vacancy captures up to four carbon atoms to form CnV (n = 1–4) clusters. The C2V cluster is the most stable cluster owing to its most favourable capturing energy of −1.71 eV. The vacancy concentration in the form of CnV clusters dramatically raises owing to powerfully exothermic reactions of CnV clusters, in according with the experimental results. The diffusion activation energy of carbon/vacancy are 1.22/1.17 eV, agreeing with the experimental value of 1.20/1.35 eV. The CnV formations are ascribed to vacancy capturing mechanism. The CnV clusters are nearly immobile at lower temperature regime. When the temperature achieves a critical point ~1700 K, carbon atoms and vacancy are separating from CnV cluster to produce the isolated interstitial carbon atoms and an individual vacancy, respectively. All CnV clusters do not exist anymore when the temperature exceeds ~1700 K. These CnV clusters do not nearly have the effect on interstitial carbon diffusion.
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