THE STRUCTURE, SLIP SYSTEMS, AND MICROHARDNESS OF C-60 CRYSTALS

The structure and microplasticity of high-purity fullerite C-60 have b een investigated comprehensively. The crystalline structure, lattice p arameters, and phase transitions have been studied by x-ray diffractom etry in the temperature range 30-293 K. It is found that the temperatu re corresponding to the orientational order-disorder phase transition is T-c = 260 K. A considerable number of regions with stacking faults discovered in the samples leads to blurring of the fcc-->sc phase tran sition in the temperature interval T-c+/-3 K. The a(T) dependences of the lattice parameter display peculiarities at the following character istic temperatures: T-c at which the lattice parameter jump Delta a/a = 3.3 x 10(-3) is observed, and the temperatures T-0 similar or equal to 155 K, and T-g similar or equal to 95 K which are associated with t he beginning and end of molecular orientation freezing. It is shown th at the formation of orientational glass is accompanied by a considerab le increase in the width of x-ray reflections. The slip geometry and t he temperature dependence of microhardness H-V are studied in the temp erature interval 81-293 K. It is shown that a system of the {111}[110] type is the only slip system in the fee and sc phases, The value of H -V depends on the indentation plane: H-V(111)>H-V(100). Below T-c, the microhardness increases abruptly (by approximately 30%). The temperat ure interval of this anomaly decreases after annealing of the crystal in vacuum. At T<T-0, the H-V(T) dependence becomes much stronger. It i s shown that the hardness of C-60 normalized to the elastic shear modu lus is higher than the hardness of typical molecular crystals at compa rable homologic temperatures. (C) 1997 American Institute of Physics.