Thermal conductivity of C60 at pressures up to 1 GPa and temperatures in the 50-300 K range.

Abstract

The thermal conductivity \ensuremath{\lambda} of ${\mathrm{C}}_{60}$ shows anomalies near 260 K and 90 K which are associated with the well-established phase transition and glass transition, respectively. Both transition temperatures increase with pressure, at the rates 120 K ${\mathrm{GPa}}^{\mathrm{\ensuremath{-}}1}$ and 62 K ${\mathrm{GPa}}^{\mathrm{\ensuremath{-}}1}$, respectively. With increasing temperature, \ensuremath{\lambda} of the simple cubic (sc) phase increased below 170 K (glasslike behavior) but decreased above. The glasslike behavior of \ensuremath{\lambda} is probably due to a substantial amount of lattice defects. Possible reasons for the change of sign of d\ensuremath{\lambda}/dT near 170 K are discussed. In the face centered cubic (fcc) phase (T\ensuremath{\gtrsim}260 K at atmospheric pressure) \ensuremath{\lambda} was almost independent of temperature, a behavior which is far from that of an ordered crystal (\ensuremath{\lambda}\ensuremath{\propto}${\mathit{T}}^{\mathrm{\ensuremath{-}}1}$ for T\ensuremath{\gtrsim}Debye temperature). This result can be attributed to the molecular orientational disorder of the fcc phase. The relaxation behavior associated with the glassy state and its unusually strong dependence on thermal history are discussed briefly, and data which support a previously reported relaxation model are presented. At room temperature, the density dependencies of \ensuremath{\lambda}, (\ensuremath{\partial} ln\ensuremath{\lambda}/\ensuremath{\partial} ln\ensuremath{\rho}${)}_{\mathit{T}}$, were 5.5 and 9.5 for the fcc and sc phases, which are values typical for an orientationally disordered phase and a normal crystal phase, respectively. \textcopyright{} 1996 The American Physical Society.