A new hard allotropic form of carbon: Dream or reality? I. Konyashin a, * , V. Khvostov b , V. Babaev b , M. Guseva b , J. Mayer c , A. Sirenko d a Max Planck Institute for Metals Research, Heisenbergstr. 5, D-70569 Stuttgart, Germany b Department of Physics, Moscow State University, Vorobjovy Gory 119899 Moscow, Russia c Aachen University, Central Facility for Electron Microscopy, Ahornstr. 55, D-52074 Aachen, Germany d Department of Physics, New Jersey Institute of Technology, Newark, NJ 07102, USA Received 11 November 2004; accepted 13 April 2005 Abstract An unusual phase of carbon with the fcc crystal structure, the lattice constant of which is very close to that of diamond, was reported before in a number of publications. In all the published works, the existence of fcc carbon could not be unambiguously established, as only insignificant amounts of fcc carbon in mixture with other carbon modifications were obtained so far. The present work provides for the first time clear evidence for the existence of the carbon modification with the fcc crystal structure. Thin films consisting exceptionally of fcc carbon were obtained by various methods, namely by hot-filament CVD, plasma-assisted CVD and treatment of the diamond surface in hydrogen plasmas. Results of electron diffraction, Raman spectroscopy, AES, XPS, EELS and HRTEM of the films consisting of fcc carbon clearly indicate its unique crystal and electron structure. The micro-hardness of the films of fcc carbon was found to be equal to HV 0.01 = 250 which is quite different from the hardness of all the known carbon modifications. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon; Face-centred cubic crystal lattice; Electron diffraction; Raman spectroscopy; Crystal structure; Micro-hardness 1. Introduction In spite of the fact that carbon is presumably the best ever investigated chemical element, it reserves many sur- prises and riddles for scientists. Among others, one crys- tal form of carbon, namely carbon with the face-centred cubic (fcc) crystal structure, has been a mystery for a long time. The question of the existence of fcc carbon appears to be extremely important, as it would represent the fourth basic form of carbon with delocalised and unhybridised electrons in addition to diamond with sp 3 -, graphite with sp 2 - and carbyne with sp 1 -hybridisa- tion of electron orbitals. Indeed, each carbon atom in the face-centred cubic crystal lattice has a coordination number of 12, which cannot be obtained on the basis of hybridisation of the carbon four valence electrons. Thus, when assuming that the nature of electron bonds in fcc carbon is close to that in metals, it is expected to have a high electrical conductivity and hardness similar to that of fcc metals, i.e. roughly between HV = 100 and HV = 300. There are some publications predicting the existence of a carbon allotrope with the fcc crystal structure [1– 4], however, fcc carbon is thought would be less stable than diamond. The first experimental evidence for the existence of fcc carbon was obtained over 40 years ago. An unusual crystal modification of carbon present in synthetic diamond, which was at that time a new and exciting 0263-4368/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2005.04.015 * Corresponding author. Present address: Boart Longyear Technical Development Centre, Staedeweg 18-24, D-36151 Burghaun, Germany. Tel.: +49 6652 82412; fax: +49 6652 82390. E-mail address: ikonyashin@boartlongyear-eu.com (I. Konyashin). International Journal of Refractory Metals & Hard Materials 24 (2006) 17–23 www.elsevier.com/locate/ijrmhm