First-principles study of mechanical stability and thermal properties of MNNi3 (M=Zn, Mg, Al) under pressure

The mechanical stability, elastic, and thermodynamic properties of the anti-perovskite superconductors MNNi3 (M=Zn, Mg, Al) are investigated by means of the first-principles calculations. The calculated structural parameters and elastic properties of MNNi3 are in good agreement with the experimental and the other theoretical results. From the elastic constants under high pressure, we predict that ZnNNi3, MgNNi3, and AlNNi3 are not stable at the pressures above 61.2 GPa, 113.3 GPa, and 122.4 GPa, respectively. By employing the Debye model, the thermodynamic properties, such as the heat capacity and the thermal expansion coefficient, under pressures and at finite temperatures are also obtained successfully.