Lattice dynamics of fullerenes probed by organometallic intercalates

The 14.4 keV nuclear gamma-ray resonance in Fe-57 has been used as a probe of the hyperfine interactions and lattice dynamics of the bisferrocene (Fc) intercalates of C-60 and C-70. Over the temperature range 90 less than or equal to T less than or equal to 350 K the isomer shift is a smooth function of T and is very similar for neat Fc and the two intercalates. In contrast, the T-dependence of the quadrupole hyperfine interaction undergoes a marked change in slope at similar to 300 K in C(60)Fc(2), but not in C(70)Fc(2). A similar discontinuity at similar to 300 K is observed in the temperature dependence of the recoil-free fraction in C(60)Fc(2), but not in C(70)Fc(2). These observations can be related to the known phase change and onset of fullerene rotation in the former (as contrasted to the latter) and are supported by DSC measurements in the interval 240 less than or equal to T less than or equal to 380 K for both intercalates.