Interstitial dinitrogen makes PtN2 an insulating hard solid

Recent synthesis of platinum nitride has provoked considerable interest on account of the compound's anomalously high bulk modulus, which is more than 30% higher than that of the parent metal. Numerous theoretical studies have since offered contradicting hypotheses on the structure and properties of this compound. Here we show, based on first-principles calculations, that the recently synthesized phase of platinum nitride has the pyrite structure. In the PtN2 pyrite structure single-bonded N-2 units occupy the octahedral interstitial sites of the Pt close-packed lattice, giving rise to strong, directional Pt-N bonds and to an insulating character. Excellent agreement with x-ray, Raman, and compressibility measurements is obtained.