Carbon Vol. 36, No. 5-6, pp. 809-815,199s 0 1998 Elsevier Science Ltd Printed in Great Britain. All rights reserved OOOS-6223/98 $19.00 + 0.00 PII: sooo&6223( 98)oooo9-8 SOLID CUBANE: A BRIEF REVIEW T. YILDIRIM,~*~* P. M. GEHRING,~ D. A. NEUMANN,~ P. E. EATONC and ‘T. EMRICK’ “University of Maryland, College Park, MD 20742, USA ‘National Institute of Standards and Technology, NIST Center for Neutron Research, Gaithersburg, MD 20899, USA ‘Department of Chemistry, University of Chicago, Chicago, IL 60637, USA (Received 30 October 1997; accepted in revised form 9 December 1997) Abstract-It is relatively easy to predict the structural properties of a solid made of spherical molecules. However this is not the case for a solid composed of cubic molecules such as solid cubane. The cubic structure of the cubane molecule gives rise to many unusual solid state properties. Here we present a brief review of our X-ray and neutron scattering studies along with our model calculations of the structure and lattice dynamics of solid cubane. The low-temperature phase (below the transition temperature of 394 K) is rhombohedral with or=72.69” and 0=5.20 A. Surprisingly, the crystal structure of cubane in the orientationally-disordered phase is not cubic, but instead remains rhombohedral with a = 103.3”, far from the fee value of 60”. The jump in lattice constant at this transition is 0.05 A, which corresponds to a 5.4% volume expansion, among the largest ever observed. Neutron inelastic scattering measurements indicate that the librons lie higher in energy than do the phonons, but that they soften rapidly with increasing temperature. Remarkably, model calculations predict the correct low and high- temperature crystal structures, and reveal cubane to be a text-book example of a system with anisotropic large-amplitude collective motions in the disordered phase. 0 1998 Elsevier Science Ltd. All rights reserved. Key Words-A. Fullerene, C. neutron scattering, C. x-ray diffraction, C. crystal structure, C. lattice dynamics. 1. INTRODUCTION Carbon is a very interesting and important element for new high-tech materials. Almost 90% of all known compounds contain carbon. Despite the many different forms of carbon, and the considerable rich- ness of their physical properties, there are only two main types of bonding between carbon atoms: sp2 (as in graphite) and sp3 (as in diamond). The opti- mized C-C-C bond angles for the sp’ and sp3 hybridizations are 120 and 109.5”, respectively. Deviations from these bond angles cause strain, thereby decreasing the stability of the structure. In spite of this fact, for more than four decades scientists have been imagining how to synthesize novel carbon- based materials with cage-like structures, such as tetrahedrane (C,HJ, cubane (CsH,), dodecahedrane (CzoHzo) and buckminsterfullerene (C,), each of which are shown in Fig. 1. Of the four polyhedra shown in Fig. 1, CeOdiffers from the others in two respects. First, the icosahedral structure of C, is stable without any hydrogen passivating the carbon atoms. By contrast dodecahe- drane, which also has icosahedral symmetry, is not stable without hydrogen passivating carbons. Second, tetrahedrane, cubane and dodecahedrane do not exist naturally on the earth, whereas C6e has been found in old soot and an ancient carbon-rich rock known as shungite. Indeed, tetrahedrane, cubane and dodecahedrane hold a special fascination for many scientists because they are atomic scale realizations *Corresponding author. of the so-called “platonic solids”, i.e. three-dimen- sional forms with flat sides described by a regular polygon (a triangle, a square, a pentagon, etc.). C&, though equally fascinating, is not a platonic solid because its sides are a mix of 12 pentagons and 20 hexagons. (A) Tetrahedrane (Td) (B) Cubane (0 ,,) (C) Dodecahedrane (I,,) (D) Buckminstetfulbwene (I,,) Fig. 1. Four carbon-based polyhedral molecules. Carbon atoms are represented by black spheres and hydrogen atoms by gray spheres. 809