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First-principles calculations are performed on MgB 2 , carbon doped MgB 2 , and MgB 2 C 2 . The trend of calculated lattice parameters of MgB 2 with increasing carbon content agrees with bulk experiments but not with thin films produced by hybrid physical-chemical vapor deposition. In this work, the authors propose a model to explain this behavior based on the coefficients of thermal expansion of MgB 2 and MgB 2 C 2 as predicted from first principles and of graphite from literature. It is concluded that the effect of carbon on the lattice parameters of MgB 2 thin films is extrinsic and due to differences of the coefficients of thermal expansion of different phases.

This paper is concerned with the prediction of oxygen diffusivities in fcc nickel from first-principles calculations and large-scale atomic simulations. Considering only the interstitial octahedral to tetrahedral to octahedral minimum energy pathway for oxygen diffusion in fcc lattice, greatly underestimates the migration barrier and overestimates the diffusivities by several orders of magnitude. The results indicate that vacancies in the Ni-lattice significantly impact the migration barrier of oxygen in nickel. Incorporation of the effect of vacancies results in predicted diffusivities consistent with available experimental data. First-principles calculations show that at high temperatures the vacancy concentration is comparable to the oxygen solubility, and there is a strong binding energy and a redistribution of charge density between the oxygen atom and vacancy. Consequently, there is a strong attraction between the oxygen and vacancy in the Ni lattice, which impacts diffusion. V C 2014 AIP Publishing LLC. [http://dx.