Structural phase transition of high-stage MoCl5 graphite intercalation compounds

Structural phase transitions of high-stage MoCl5 graphite intercalation compounds (GIC) have been investigated by using specific heat, x-ray (00L) and (HK0) diffraction, electron diffraction, and neutron (HKL) diffraction techniques. Each MoCl5 intercalate layer is uncorrelated with respect to the other MoCl5 intercalate layers but is modulated by the graphite layers. The stage-3 MoCl5-GIC undergoes a structural phase transition at Tc=480 K, where the specific heat shows a type anomaly and the c-axis repeat distance exhibits an abrupt increase as temperature is increased. The MoCl5 intercalate layer consists of the orthorhombic phase and the hexagonal phase. The orthorhombic phase forms a plane body-centered rectangular lattice with a=6.210.02 and b=17.930.02 at room temperature. The rotation angle between the a axis of the rectangular lattice and the graphite 100 direction of real space is =30°, with =8.1°below Tc, and gradually changes to =30°well above Tc. An explanation of the phase transition at Tc is presented in terms of a rectangular superlattice that is commensurate with the graphite lattice.