Effects of carbon in MgB₂ thin films: Intrinsic or extrinsic

First-principles calculations are performed on MgB₂, carbon doped MgB₂, and MgB₂C₂. The trend of calculated lattice parameters of MgB₂ 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 MgB₂ and MgB₂C₂ as predicted from first principles and of graphite from literature. It is concluded that the effect of carbon on the lattice parameters of MgB₂ thin films is extrinsic and due to differences of the coefficients of thermal expansion of different phases.