Structure, slip systems and microhardness of C60 crystals

Comprehensive investigations of structure and micro plasticity of high-purity fullerites C60 are made. The crystal structure, lattice parameters and phase transitions in the temperature range of 30 to 293 K are studied by using an x-ray diffraction technique. The order-disorder phase transition temperature is found to be Tc = 260 K.The samples display a great number of stacking disorder areas that are responsible for a fcc-sc transition smearing at Tc = ± 3 K. The temperature dependence of the lattice parameter a(T) exhibits three anomalies: one at Tc where a jump in the lattice parameter occurs (Δa/a = 3,3 centre dot 10-3) and two at T0 approx= 155 K and Tg approx= 95 K which are related to the onset and the completion of the process of molecule orientation freezing. It is shown that the formation of the orientational glass is followed by a considerable increase in the width of x-ray reflections. The geometry of slip and the temperature dependence of microhardness, HV, are studied within the temperature range of 81 to 293 K. A { 111 } <110> type system is shown to be the only system of slip in the fcc and sc phases. The magnitude of HV depends on the plane of indentation:HV111 > HV100. Below Tc it increases sharply (by about 30%). The temperature interval of the anomaly decreases after the crystal is annealed in vacuum. At T<T0 the dependence HV(T) becomes appreciably steeper. The hardness of C60 normalized to elastic shear modulus is found to be higher than that of typical molecular crystals, the homo logic temperatures being comparable