PHASES IN THE A1C60 SYSTEM (A=K, Rb, Cs). G. Faigel, G. Bortel, L. Gránásy, G. Oszlányi, S. Pekker, T. Pusztai, M .Tegze, Research Institute for Solid State Physics, H-1525 Budapest, POB. 49., Hungary; L.Forró, IGA, Ecole Polytechnique Federale de Lausanne, 1015 Lausanne, Switzerland; P.W. Stephens, G. Bendele, Department of Physics, SUNY, Stony Brook, NY 11794, USA.
The crystalline structure of the various phases of A1C60 type salts (A=K, Rb, Cs) were determined from x-ray powder diffraction.
In the short time since the discovery of C60, an amazing variety of compounds have been found. In most of them the intermolecular separation is mainly determined by van der Waals interactions. Recently some exceptions have been found, in which C60 molecules are linked by covalent bonds. Among them the orthorhombic phase of A1C60 was the first for which x-ray diffraction clearly proved the existence of linear polymer chains. In all cases the basic mechanism of polymerization is [2+2] cycloaddition which involves hexagon-hexagon bonds of neighbouring C60 molecules.
Polymer formation in A1C60 can be prevented by fast cooling from high temperature. In this case covalent (C60)2 dimers are formed through a single C-C bond which differs from the bonding configuration of the polymer.
The sequence of phase transitions between the various phases was successfully modelled by a paramater free Monte Carlo type calculation giving a microscopic picture of these processes.
An inhomogeneous structural phase specific to K1C60 appears at 400 K. In this "intermediate state" the K ions are distributed in such a way that small K3C60 and K free C60 regions are formed while the C60 sublattice remains continuous. The structure has important conseqences on the physical properties.