Nature of N-N Bonding within High-Pressure Noble-Metal Pernitrides and the Prediction of Lanthanum Pernitride

摘要

The nature of nitrogen-nitrogen bonding and the metal oxidation states within late-noble-metal pernitrides have been determined by a series of density-functional electronic-structure calculations. In contrast to alkaline-earth pernitrides such as BaN_2 which contain quasi-molecular double-bonded N_2~(2-) units, compounds such as PtN_2 incorporate a tetravalent metal and a N_2~(4-) species with a N-N single bond due to four surplus electrons within the antibonding 1πg* molecular orbital. This fact is the source of the huge bulk moduli of PtN_2 and related materials such as OsN_2 and lrN_2. The crystal structure of lanthanum pernitride, LaN_2→ La~(3+) + N_2~(2-) + e~-, yet to be made, has been predicted, and its electronic structure is compared with a likewise hypothetical LaN_2 which consists of both N_2~(2-) and N_2~(4-) pernitride units together with a trivalent lanthanum cation. Finite-temperature DFT calculations predict a very moderate reaction pressure toward LaN_2 starting from LaN and elemental nitrogen of less than 2 GPa at 300 K.