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

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

CORRELATION OF LOW-TEMPERATURE MICROPLASTICITY ANOMALIES WITH STRUCTURE TRANSFORMATIONS IN C-60 CRYSTALS

Structure, microhardness and slip systems were studied on C-60 crystals of high purity in the temperature range 30-290 K, which includes the fcc --> sc phase transition at T-c = 260 K and the orientational glass region T < 100 K. A step-like change of both the lattice parameter and the microhardness was recorded when passing through T-c, as well as the formation of slip lines near the indentations in both phases. The slip system <110>{111}, operative at room temperature in the FEC phase, is shown to retain its activity below T-c, in the sc phase. In the region where the orientation glass is formed, we have found a kink-like anomaly on the microhardness vs temperature curve; this anomaly correlates with a weak anomaly in the temperature dependence of the lattice parameter. Assumptions are Formulated concerning the chancier ol the dislocation motion in C-60 crystals, which make it possible to explain the microplasticity anomalies observed

Structure and photoluminescence of helium-intercalated fullerite C-60

The intercalation of C-60 single crystals with helium is studied by powder x-ray diffractometry. It is established that the intercalation is a two-stage process: octahedral cavities are filled first and then tetrahedral ones, the chemical pressure being negative during both stages. The low-temperature (5 K) photoluminescence spectra of helium-intercalated fullerite C-60 are studied for the first time. The presence of helium in lattice voids is shown to reduce that part of the luminescent intensity which is due to the emission of covalently bound pairs of C-60 molecules, the so-called "deep traps" with the 0-0 transition energy close to 1.69 eV. The mechanism of the effect of intercalation with helium on the pair formation in fullerite C-60 is discussed

Nuclear Spin Crossover in Dense Molecular Hydrogen

The laws of quantum mechanics are often tested against the behaviour of the lightest element in the periodic table, hydrogen. One of the most striking properties of molecular hydrogen is the coupling between molecular rotational properties and nuclear spin orientations, giving rise to the spin isomers ortho- and para-hydrogen. At high pressure, as intermolecular interactions increase significantly, the free rotation of H2 molecules is increasingly hindered, and consequently a modification of the coupling between molecular rotational properties and the nuclear spin system can be anticipated. To date, high-pressure experimental methods have not been able to observe nuclear spin states at pressures approaching 100 GPa and consequently the effect of high pressure on the nuclear spin statistics could not be directly measured. Here, we present in-situ high-pressure nuclear magnetic resonance data on molecular hydrogen in its hexagonal phase I up to 123 GPa at room temperature. While our measurements confirm the presence of I=1 ortho-hydrogen at low pressures, above 70 GPa, where inter- and intramolecular distances become comparable, we observe a crossover in the nuclear spin statistics from a spin-1 quadrupolar to a spin-1/2 dipolar system, evidencing the loss of spin isomer distinction. These observations represent a unique case of a nuclear spin crossover phenomenon in quantum solids