Temperature Dependent Structural Studies of K- and RB- Doped C 60
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TEMPERATURE DEPENDENT STRUCTURAL STUDIES OF K- AND RB- DOPED C6 o OTTO ZHOU, QING ZHU, GAVIN B.M. VAUGHAN, JOHN E. FISCHER, PAUL A. HEINEY, NICOLE COUSTEL, JOHN McCAULEY Jr., AMOS B. SMITH II, and DAVID E. COX' Laboratory for Research on The Structure of Matter, University of Pennsylvania, Philadelphia, PA 19104.
*Physics Department, Brookhaven National Laboratory, Upton, NY 11973. ABSTRACT
The temperature dependent structural evolutions of RbCo0 (x = 3, 5, 6) and K 4C6 0 were studied using both in-house and synchrotron x-ray powder diffraction and thermal analysis techniques over a temperature range of 10K - 673K. The superconducting face centered-cubic (fcc) Rb3 C60 and the body centered-tetragonal (bct) M4 C60 (M = K, Rb) phases are found to be line compounds in this temperature range, while the body centered-cubic (bcc) phase forms a solid solution in which the solubility of vacant M sites increases with temperature. The orientation of the C60 molecules in the K 4Ceo phase was analyzed. A crystalline fcc Rb1 Co phase is stable only above room temperature. INTRODUCTION The reports of superconductivity at 19K[1] and 29K[2] in potassium- and rubidium-doped Ceo have stimulated a considerable amount of research directed toward determining the composition and the phase equilibria of alkali metal-doped C60. Among the different alkali metal- doped compounds, K- and Rbdoped systems exhibit very similar structural behavior, three stoichiometric phases having been determined. These are a fcc superconducting M 3 C 60 phase[3,4], a bct M 4 C6o phase [5], and a saturation-doped bcc M6Ceo phase [6]. At room temperature, the orientation of the C6o molecules in the doped phases varies with the dopant concentration, changing from complete disorder in solid C6o[7,8] to a state with the fullerene molecules jumping between two symmetry equivalent orientations (merohedral-disorder) in MsCeo[3], and finally to a fully ordered state in the saturation-doped M6Ce0 phase[6] . Orientational disorder is intrinsic in the bct M4Ce0 phase due to the incompatibility of the C60 point group and the bct lattice symmetry[5], and can be described as either completely disordered or merohedral disordered, analogous to the MaCeo phase. Fleming and coworkers have refined this M 4C60 (M =K, Rb and Cs) structure with the former model[5]. A provisional binary phase diagram of MgC6o (M = K and Rb) has been proposed, based mainly on the 300K experimental data and free energy considerations[9]. Na-doped C60 retains an fcc lattice at concentrations up to 6 Na atoms per fullerene molecule(10]. A stoichiometric fcc Na 2C6o phase with Na ions occupying only the tetrahedral sites has also been observed[10]. The superconducting Cs3Ceo compound has recently been reported to have a bcc structure[1 1], different from all other known alkali metal- doped C60 superconductors. Most of the structural studies of alkali metal-doped C60 reported so far have been carried out at ambient conditions. How the structures of these compounds evolve with temperature and pressure is
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