VARIABLE-CELL-SHAPE-BASED STRUCTURAL OPTIMIZATION APPLIED TO CALCIUM NITRIDES

Symmetry corrections are added to a structural optimization at constan t pressure in order to preserve the symmetry during the numerical rela xation of a structure. The enthalpy is minimized as a function of the electronic structure, the metric g(ij), and the atomic positions. For the antiperovskite structures AsNCa3, PNCa3, and BiNCa3 the structural parameters and the band structure are calculated. The results are in good agreement with the available experimental data. For AsNCa3 the pr essure dependence of the structural parameters and of the band-gap ene rgy is also discussed. A possible transition from the orthorhombic pha se to an eightfold-coordinated cubic phase (SC15) is found at 59 GPa.