The structure, slip systems, and microhardness of C-60 crystals

Abstract

The structure and microplasticity of high-purity fullerite C-60 have been investigated comprehensively. The crystalline structure, lattice parameters, and phase transitions have been studied by x-ray diffractometry in the temperature range 30-293 K. It is found that the temperature 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 transition in the temperature interval T-c+/-3 K. The a(T) dependences of the lattice parameter display peculiarities at the following characteristic 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 the beginning and end of molecular orientation freezing. It is shown that the formation of orientational glass is accompanied by a considerable increase in the width of x-ray reflections. The slip geometry and the temperature dependence of microhardness H-V are studied in the temperature 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 temperature 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 is shown that the hardness of C-60 normalized to the elastic shear modulus is higher than the hardness of typical molecular crystals at comparable homologic temperatures.