CRYSTAL STRUCTURE AND PHASE TRANSITION OF FULLERENE C76 H. Nakao, Y. Fujii ISSP, Univ. of Tokyo, T. Watanuki, K. Ishii, T. Ino, H. Suematsu, Dept. of Phys., Univ. of Tokyo, H. Kawada, Y. Murakami, KEK-PF, K. Kikuchi, Y. Achiba, Y. Maniwa, Tokyo Met. Univ.

High-resolution synchrotron x-ray powder diffraction experiments of a fullerene C76 crystal has elucidated its structure and successive phase transitions. Two types of crystal lattices, fcc and hcp, coexist with different stacking sequences as conventionally denoted by ABAB(hcp) and ABCABC(fcc). Lattice constants at room temperature are obtained as a=15.42Å (fcc) and a=10.93Å, c=17.72Å (hcp). A precise measurement of them reveals the following phase transition sequence with respect to molecular rotation:

[fcc] Phase I (T<140K) orientationally disordered glass state.

Phase II (140K<T) free rotation state.

[hcp] Phase I (T<140K) c/a = 1.60, orientationally ordered.

Phase II (140<T<420K) c/a continuously varied.

Phase III(420K<T) c/a=1.63, free rotation state.

Calculations of intensity by an ellipsoid-shaped molecular model suggest that in Phase I of hcp the long rotational axis of molecules aligns in the stacking direction. In Phase II a molecule starts rotation about its long axis whose direction fluctuates. By applying pressure we have also studied phase boundary between I and II of the hcp structure. The present experimental results qualitatively agree to the recent NMR studies¹. We also discuss the lattice stability conditions by combining our previous works on C82².

¹Y. Maniwa et al., to be published.

²H. Kawada et al., Phys. Rev. B51,8723 (1995).