The structure, slip systems, and microhardness of C60 crystals

Abstract

The structure and microplasticity of high-purity fullerite C60 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 Tc=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 Tc±3 K. The a(T) dependences of the lattice parameter display peculiarities at the following characteristic temperatures: Tc at which the lattice parameter jump Δa/a=3.3×10−3 is observed, and the temperatures T0≃155 K, and Tg≃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 HV 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 fcc and sc phases. The value of HV depends on the indentation plane: HV111>HV100. Below Tc, the microhardness increases abruptly (by approximately 30%). The temperature interval of this anomaly decreases after annealing of the crystal in vacuum. At T<T0, the HV(T) dependence becomes much stronger. It is shown that the hardness of C60 normalized to the elastic shear modulus is higher than the hardness of typical molecular crystals at comparable homologic temperatures.

Micromechanical properties of C70 single crystals in the temperature range 77–350 K

“…Its effectiveness was demonstrated previously in studies of single crystals of C 60 fullerite (Ref. 14) of the same small size as the C 70 crystals. This method is distinguished not only by its simplicity.…”

“…8; at room temperature H V % 170 MPa in C 60 fullerite. 14 In the transition to high-temperature phase severe cracking of the surface, its overturn, and deviation of the indented plane from the normal of indentation were observed. As a result, at T > 350 K indentations became less and less clear, often they were not symmetrical due to the incline of the surface, and severe cracking around the indentation was observed.…”

Dielectric Study of Pressure Induced fcc-sc Phase Transition in Fullerene C60

“…1. This is probably related to the onset of orientation freezing of the rotation of Co molecules visible at ambient pressure at T0 = 150 K. The effect of this freezing was observed in bulk C60 [10][11][12] and C60 thin films [13,14].…”

Low-temperature microhardness of Xe-intercalated fullerite C60

“…Investigations performed by several teams of researchers (see [8] and the references ibid.) have revealed that macro-and microplastic deformation of fullerite C 60 at room and at moderately low temperatures is determined by the glide of dislocations in the {111}á ñ 110 slip system; the system is kept efficient even after the fcc-sc transition.…”

“…As shown in the previous papers [5][6][7][8], the temperature dependence of microhardness of pure C 60 single crystals displays a step-like anomaly, the position of which is correlated with the temperature T c 260 K of the fcc-sc phase transition. It is known [9,10] that intercalation of an impurity into the C 60 lattice results, as a rule, in a decrease of T c and a broadening of the phase transition temperature range.…”