Journal of Solid State Chemistry 179 (2006) 1761–1766 Crystal structure and high-pressure properties of g-Mo 2 N determined by neutron powder diffraction and X-ray diffraction Craig L. Bull a,b,Ã , Tetsuya Kawashima c,d , Paul F. McMillan c,e,Ã , Denis Machon e , Olga Shebanova e , Dominik Daisenberger c , Emmanuel Soignard f , E. Takayama-Muromachi d , Laurent C. Chapon b a SUPA, School of Physics and Centre for Science at Extreme Conditions, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, UK b ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK c Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, WBX 4BS, London, UK d Superconducting Materials Centre (Namiki-site), National Institute for Materials Science (NIMS) 1-1 Namiki, Tsukuba Ibaraki, 305-0044 Japan e Department of Chemistry and Materials Chemistry Centre, Christopher Ingold Laboratory, University College London, 20 Gordon Street, WC1H 0AJ, London, UK f Department of Geological Sciences, Arizona State University, US Received 31 August 2005; received in revised form 31 January 2006; accepted 15 March 2006 Abstract We prepared samples of cubic g-MoN x (x0.5) by high-pressure–high-temperature synthesis. N atom site occupancies within the defect rock salt structure were determined from time-of-flight neutron diffraction and powder X-ray diffraction data by Rietveld analysis. The results show that N atoms occupy only octahedral sites within the structure. The semi-metallic compound is a superconductor, with T c ¼ 5:2 K determined by SQUID magnetometry. The compressibility of the material was determined by synchrotron X-ray diffraction measurements at high pressure in the diamond anvil cell. The vibrational density of states was studied by Raman scattering spectroscopy. r 2006 Elsevier Inc. All rights reserved. Keywords: High pressure; Raman spectroscopy 1. Introduction Refractory transition metal nitrides possess technologi- cally important properties including high hardness and superconductivity [1–5]. Within the Mo–N system, known phases include the stoichiometric hexagonal compound d-MoN and the non-stoichiometric cubic phase g-MoN x , with x0.5 (g-Mo 2 N) [1,4]. d-MoN is a high-hardness material with very low compressibility [6]. It is also a superconductor, with T c values ranging between 12 and 15 K, depending upon the preparation conditions and degree of structural order [7–9]. The cubic B1-structured phase prepared by conventional methods has a stoichio- metry near MoN 0.5 (i.e., Mo 2 N). It is also superconducting, with T c ¼ 425K [5]. The corresponding stoichiometric cubic nitride NbN has T c ¼ 17 K [1,5]. It was predicted that if stoichiometric cubic ‘‘g-MoN’’ could be synthesized, it could achieve T c values as high as 29 K [10]. However, ab initio theoretical predictions and recent high-P,T synthesis experiments have indicated that a fully nitrided cubic phase is unlikely to be achieved in the Mo–N system [11,12]. Within B1-structured nitrides, one fcc lattice is defined by the transition metal atoms, and the N atoms are presumed to occupy interstitial octahedral sites to form the second interpenetrating fcc lattice. Vacancies on the anion sublattice result in non-stoichiometric nitrides such as MoN 0.5 , VN x and TiN x . However, although the N atoms are usually modelled as occupying only octahedral ARTICLE IN PRESS www.elsevier.com/locate/jssc 0022-4596/$ - see front matter r 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jssc.2006.03.011 Ã Corresponding authors. Davy Faraday Laboratory, School of Physics and Centre for Science at Extreme Conditions, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, UK. Fax: +44 1235 445720. E-mail addresses: c.bull@rl.ac.uk (C.L. Bull), p.f.mcmillan@ucl.ac.uk (P.F. McMillan).