Interstitial binary nitride ReNx phases prepared by pulsed laser deposition: Structure and superconductivity dependence on nitrogen stoichiometry

Pulsed laser deposition under nitrogen radical irradiation was applied to the synthesis of ReNx crystalline phases, and the effect of the varying nitrogen stoichiometry on the crystal structure and superconductivity was investigated. X-ray diffraction revealed that, at low nitrogen content (x similar to 0.06, deposition temperature T-d = 873 K), ReNx maintained the hexagonal structure of the Re metal (having a hexagonal close-packing (hcp) structure), where the nitride ions occupy interstitial sites among Re atoms. It was found that the crystal structure is cubic when x = 0.17 (T-d = 603 K) and 0.22 (T-d = room temperature). X-ray photoelectron spectroscopy detected N 1s(1/2) signals close to those for Si3N4 and BN, indicating the strong covalent character of Re-N bonds. First principle band calculations indicated that the density of states at Fermi energy of ReN consists of mainly Re 5d bands. The superconducting transition temperature T-c increased steeply from 1.6 K (Re metal) to 4.8 K (ReN0.22). (C) 2009 Published by Elsevier B.V.