High-Pressure Annealing of a Prestructured Nanocrystalline Precursor to Obtain Tetragonal and Orthorhombic Polymorphs of
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High-Pressure Annealing of a Prestructured Nanocrystalline Precursor to Obtain Tetragonal and Orthorhombic Polymorphs of Hf3N4 Ashkan Salamat,1,2 Pierre Bouvier,3 Benjamin M. Gray,4 Andrew L. Hector,4 Simon A. J. Kimber2 and Paul F. McMillan5 1 Lyman Laboratory of Physics, Harvard University, Cambridge, MA 02138, U.S.A. 2 European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France. 3 Laboratoire des Materiaux et du Genie Physique, CNRS, Université Grenoble-Alpes, 3 Parvis Louis Neel, 38016 Grenoble, France. 4 Chemistry, University of Southampton, Southampton SO17 1BJ, U.K. 5 Department of Chemistry, Christopher Ingold Laboratory, University College London, London WC1H 0AJ, U.K. ABSTRACT Transition metal nitrides containing metal ions in high oxidation states are a significant goal for the discovery of new families of semiconducting materials. Most metal nitride compounds prepared at high temperature and high pressure from the elements have metallic bonding. However amorphous or nanocrystalline compounds can be prepared via metal-organic chemistry routes giving rise to precursors with a high nitrogen:metal ratio. Using X-ray diffraction in parallel with high pressure laser heating in the diamond anvil cell this work highlights the possibility of retaining the composition and structure of a metastable nanocrystalline precursor under high pressure-temperature conditions. Specifically, a nanocrystalline Hf3N4 with a tetragonal defect-fluorite structure can be crystallized under highP,T conditions. Increasing the pressure and temperature of crystallization leads to the formation of a fully recoverable orthorhombic (defect cottunite-structured) polymorph. This approach identifies a novel class of pathways to the synthesis of new crystalline nitrogen-rich transition metal nitrides. INTRODUCTION Solid state compounds with transition metals in high oxidation states are often insulators or wide band gap semiconductors, e.g. TiO2 and ZrO2. The corresponding nitrides do not typically achieve maximum oxidation states and are dominated by metallic bonding. This is due to the highly exothermic formation enthalpy of the lower metal nitrides allied with the thermodynamic stability of gaseous nitrogen. Higher nitrides such as Ti3N4 should be stable1 and could be accessed if a suitable synthesis route is found to achieve them. Lower band gaps are expected relative to the oxides, giving rise to semiconducting and photocatalytic properties, as well as highly colored compounds that can be used as pigments e.g. Ta3N5.2 Recent high pressure and temperature elemental combination reactions in laser heated diamond anvil cells (LH-DAC) have led to new phases such as PtN2, OsN2 and Hf3N4.1 However, most of these reactions at high pressure have only led to surface conversion or mixed phases and consequently low yields. Alternatively, synthesis routes involving metal-organic chemistry such as ammonolysis of metal amide complexes can lead to amorphous or nanocrystalline solids with a very high N:metal ratio.3 Here we have shown t
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