In this paper we report the results of x-ray powder diffraction experi
ments on C60-n-pentane. This material is a typical C60 clathrate and s
hows orthorhombic symmetry at room temperature. At low temperature an
orthorhombic to monoclinic phase transition can be observed which invo
lves the distortion of the base plane from rectangular. A microscopic
picture of the phase transition is given by molecular-dynamics calcula
tions.