Nitrides DOI: 10.1002/anie.200603851 High-Pressure Synthesis of Crystalline Carbon Nitride Imide, C 2 N 2 (NH)** Elisabeta Horvath-Bordon, Ralf Riedel,* PaulF. McMillan, Peter Kroll, Gerhard Miehe, PeterA. vanAken, Andreas Zerr, Peter Hoppe, Olga Shebanova, Ian McLaren, Stefan Lauterbach, Edwin Kroke, and Reinhard Boehler The main-group-element nitrides Si 3 N 4 and Ge 3 N 4 crystallize in at least four polymorphs, including the technologically important a and b forms, a new spinel-type g phase synthe- sized at high pressure and temperature, and a high-pressure d phase. [1,2] Sn 3 N 4 also forms a cubic spinel-type phase. [1,2] There is still no reliable evidence for analogous dense phases of C 3 N 4 . High-density C 3 N 4 polymorphs were predicted to have very high bulk modulus and hardness values comparable with or exceeding that of diamond. [3–11] Many experimental studies have attempted to produce dense crystalline C x N y phases, by using various techniques, including high-pressure, high-temperature (HP-HT) synthesis. Solids with N:C ratios of 1.3–1.5 have been reported. [1,2] However, these materials are amorphous or nanocrystalline, and their structures and chemical compositions are not well characterized. Further- more, the compounds prepared under HP-HT conditions are not generally recovered to ambient conditions. Herein, we report the first synthesis of a well-crystallized compound with an N:C ratio of 3:2, in which all of the carbon atoms are tetrahedrally coordinated. Crystals of the com- pound are formed from the single-source precursor 1- cyanoguanidine (dicyandiamide (DCDA), C 2 N 4 H 4 ) under HP-HT conditions in a laser-heated diamond-anvil cell (Scheme 1). Single crystals of the new dense carbon nitride phase were recovered to ambient conditions for structural and chemical analysis. The precursor was embedded in an NaCl pressure medium along with ruby chips for pressure determination. Heating was carried out with a CO 2 laser (l = 10.6 mm), and Scheme 1. Synthesis of C 2 N 2 (NH). [*] Dr. E. Horvath-Bordon, Prof.Dr. R. Riedel, Dr. A. Zerr [+] Disperse Feststoffe, Material- und Geowissenschaften Technische Universität Darmstadt Petersenstrasse 23, 64287 Darmstadt (Germany) Fax: (+ 49) 6151-166-346 E-mail: riedel@materials.tu-darmstadt.de Prof. Dr. P. F. McMillan, Dr. O. Shebanova Christopher Ingold Laboratories Department of Chemistry, University College London 20 Gordon Street, London WC1H 0AJ (UK) and Royal Institution of Great Britain Davy Faraday Research, Laboratory 21 Albemarle Street, London W1S 4BS (UK) Priv.-Doz. Dr. P. Kroll Institut für Anorganische Chemie, RWTH Aachen Professor-Pirlet-Strasse 1, 52056 Aachen (Germany) Dr. G. Miehe, Dr. I. McLaren [++] Strukturforschung, Material- und Geowissenschaften Technische Universität Darmstadt Petersenstrasse 23, 64287 Darmstadt (Germany) Priv.-Doz. Dr. P. A. van Aken, [+++] Dr. S. Lauterbach Institut für Angewandte Geowissenschaften Material- und Geowissenschaften Technische Universität Darmstadt Schnittspahnstrasse 9, 64287 Darmstadt (Germany) Dr. P. Hoppe Abteilung Partikelchemie, Max-Planck-Institut für Chemie 55020 Mainz (Germany) Prof. Dr. E. Kroke Institut für Anorganische Chemie Technische Universität Bergakademie Freiberg 09596 Freiberg (Germany) Dr. R. Boehler Hochdruck-Mineralphysik, Max-Planck-Institut für Chemie 55020 Mainz (Germany) [ + ] Present address: Laboratoire des PropriØtØs MØcaniques et Thermodynamiques des MatØriaux, CNRS Institut GalilØe, UniversitØ Paris 13 99 avenue J. B. Clement, 93430 Villetaneuse (France) [ ++ ] Present address: Department of Physics and Astronomy University of Glasgow, Glasgow G128QQ (Scotland) [ +++ ] Present address: Max-Planck-Institut für Metallforschung Heisenbergstrasse 3, 70569 Stuttgart (Germany) [**] The work was financially supported by the Deutsche Forschungs- gemeinschaft (Germany), partially in the form of a Heisenberg Fellowship (P.K.). The P.F.M. group in London is supported by the Engineering and Physical Science Research Council (UK) under Portfolio Grant EP/D504872. P.F.M. is also a Wolfson–Royal Society Research Merit Award Fellow. We thank Elmar Gröner (MPI für Chemie) for his support with the nanoSIMS, and Denis Machon (UniversitØ Claude Bernard, Lyon) and Dominik Daisenberger (University College London) for help with the diamond-anvil-cell and synchrotron experiments. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Communications 1476 # 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2007, 46, 1476 –1480