Spectroscopic characterization of phase transformations in KC 60

Abstract

We have performed inelastic neutron scattering experiments on fullerene samples of KC 60 stoichiometry in the temperature range from 30 to 500 K. The KC 60 polymer phase transforms with a strongly temperature dependent rate to intermediate (int)KC 60. This transition is direct, i.e. in contrast to RbC 60 there are no indications for equilibrated phase separated polymer-monomer states. The dynamic response of int-KC 60 can be expressed as a linear superposition of C 60 and K 3C 60 confirming the picture of an inhomogeneous C 60 K 3C 60 phase separated system. The spectral changes of the internal pressures caused by the phase separation turn out to be small. Apart from a lowering of the orientational ordering temperature the vibrational spectrum of the mixed phase shows no temperature variations going beyond those of the constituent parent phases. The transition towards the homogeneous rotator phase at about 440 K is evidenced by the disappearance of the K 3C 60 libron peak in the high temperature spectra. This transition shows a hysteresis indicating its first order character and is not preceeded by anomalous phonon softening.