  The thermodynamic stabilities of various phases of the nitrides of the
platinum metal elements are systematically studied using density functional
theory. It is shown that for the nitrides of Rh, Pd, Ir and Pt two new crystal
structures, in which the metal ions occupy simple tetragonal lattice sites,
have lower formation enthalpies at ambient conditions than any previously
proposed structures. The region of stability with respect to those structures
extends to 17 GPa for PtN2. Calculations show that the PtN2 simple tetragonal
structures at this pressure are thermodynamically stable also with respect to
phase separation. The fact that the local density and generalized gradient
approximations predict different values of the absolute formation enthalpies as
well different relative stabilities between simple tetragonal and the pyrite or
marcasite structures are further discussed.
