Structural and thermodynamic properties of compounds in the Mg–B–C system from first-principles calculations

To understand the effects of carbon doping on the performance of MgB2 superconductors, the structure and thermodynamic properties of the key compounds in the Mg–B–C system need to be investigated. MgB2C2 is the only known ternary compound in this system, but no experimental data can be found in the literature. Using the first-principles phonon, we have calculated the elastic constant, enthalpy, entropy, heat capacity of the MgB2C2 compound along with pure Mg and the MgB2, MgC2, and Mg2C3 compounds that also appear in the Mg–B–C ternary system. The calculated lattice parameters and enthalpies of formation at 298 K show good agreement with available experimental data. For Mg and MgB2, the calculated thermodynamic properties agree with the experiment very well. For MgC2 and Mg2C3, the calculated enthalpies of formation are positive, indicating they are unlikely to form in equilibrium with respect to the pure elements.