Journal of Solid State Chemistry ] (]]]]) ]]]–]]] Determination of the crystal structure of d-MoN by neutron diffraction Craig L. Bull, a,b Paul F. McMillan, a,b, Emmanuel Soignard, b and Kurt Leinenweber c a Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London WIX 4BS, UK b Department of Chemistry, UCL, Christopher Ingold Laboratories, 20 Gordon Street, London, UK c Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, USA Received 23 September 2003; received in revised form 18 November 2003; accepted 25 November 2003 Abstract We have determined the crystal structure of ordered hexagonal d-MoN by use of powder X-ray diffraction and time-of-flight neutron diffraction. A disordered variety of the compound was first prepared by high-temperature ammonolysis of MoCl 5 . This material has hexagonal symmetry with the space group P6 3 mc with a ¼ 2:87ð2Þ and c ¼ 2:81ð1Þ A ˚ . Upon high pressure annealing, the N-atoms become ordered and the unit cell doubles in size: a ¼ 5:73659ð10Þ and c ¼ 5:61884ð17Þ A ˚ . The superconducting transition temperature increases from 4 K in the disordered compound to 12.1 K in the ordered phase. r 2003 Elsevier Inc. All rights reserved. Keywords: Hardness; MoN; Neutron diffraction; High pressure; Structure determination 1. Introduction Various physical properties, including high hardness, high melting points, and high T c superconductivity, make the transition metal nitrides of industrial impor- tance as well as of academic interest. The stoichiometric hexagonal phase of molybdenum nitride (d-MoN) is a high-hardness material with a low compressibility (bulk modulus K 0 ¼ 345 GPa) [1], and it is found to be superconducting. Transition temperatures ranging from 4 to 12 K have been recorded, depending upon the preparation and annealing conditions [2]. It has been determined that the superconducting T c depends upon the ordering of N-atoms within the hexagonal metal sublattice. However, the crystal structure of this material has not been determined reliably to date, and in particular, the nitrogen positions have not been located with certainty. Here we have used a combination of powder X-ray diffraction methods and time-of-flight (TOF) neutron diffraction studies to determine the crystal structure and to locate the N-atom positions. Various methods have been used to synthesize molybdenum nitride materials, including direct nitrida- tion of molybdenum powder in an ammonia atmosphere at 1000 K [3], high pressure N 2 nitridation of Mo wire [4], and reaction between MoO 2 and NH 4 Cl at 20 kbar and 1800 K [5]. Often, a mixture of MoN x phases are obtained, primarily cubic Mo 2 N and hexagonal d-MoN. The Mo–N phase diagram was determined by Hagg [3]. In the present work, we used the method of Lengauer [6] to first prepare a N-atom disordered sample of d-MoN by ammonolysis of MoCl 5 at 943 K. The chloride is liquid at this temperature, and the nitride product forms as a fine grained powder. The synthesis conditions were optimized to yield nearly pure stoichiometric d-MoN. This material was then treated at high pressure and high temperature in a multi-anvil apparatus to yield fully ordered d-MoN, as described by Bezinge [2]. The products were examined using powder X-ray diffraction and TOF neutron diffraction. The superconducting T c ’s were determined by SQUID magnetometry. Previous structural studies of d-MoN materials have been carried out using X-ray diffraction. Troitskaya [7] originally proposed the hexagonal space group P % 3m1; while Scho¨nberg [8] suggested the hexagonal space ARTICLE IN PRESS  Corresponding author. Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London WIX 4BS, UK. Fax: +2076293569. E-mail addresses: craig@ri.ac.uk (C.L. Bull), p.f.mcmillan@ucl.ac. uk (P.F. McMillan). 0022-4596/$ - see front matter r 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jssc.2003.11.033