X-RAY STUDY OF 2-PHASE COEXISTENCE IN A C-60 SINGLE-CRYSTAL THROUGH THE CUBIC-CUBIC PHASE-TRANSITION AT 260 K

The first-order cubic-to-cubic phase transition in C-60 at similar to 260 K is studied by both synchrotron and in-house x-ray diffraction th rough three thermal cyclings. A range in which the high-temperature (r otationally disordered, fee) phase with lattice parameter of 14.14 Ang strom and the low-temperature (orientationally ordered, sc) phase with lattice parameter of 14.08 Angstrom coexist was observed in all cycli ngs except the first cooling and it is discussed in the context of the experimental data on Bragg intensities, widths, mosaic spreads, and p eak positions (lattice parameters). This transition is also characteri zed by a measurable hysteresis in the order parameter. Elastic interac tions, associated with the volume change, and a varying particle (mosa ic block) size distribution appear to account for our observations. In explanation of the behavior of the fundamental (hkl-unmixed) and orde ring (hkl-mixed parity) intensities below the phase transition, it is suggested that the major C-60 orientation angle moves slightly towards a higher angle from 98 degrees on lowering the temperature.