L.S. Kashevarova

'Low-pressure' orthorhombic phase formed from pressure-treated C60

Abstract X-ray and electron diffraction and Raman spectroscopy have shown that the solid formed by a pressure-temperature treatment of C 60 at 1.5 GPa-723 K has an orthorhombic structure O′. It may be considered that phases O′ and O, the previously known orthorhombic phase formed at 8 GPa-573 K, are the same so that our pressure-temperature treatment provides “the optimal way of obtaining the linear-chain orthorhombic fullerene phase”. However, differences in the respective spectra of these two phases led us to re-examine the structure of the previously described phase O: it is found that a rhombohedral structure fits the experimental X-ray data at least as well as an orthorhombic one does. In any case, phase O′ may be an intermediary for the formation of the tetragonal high-pressure modification.

Retraction: Magnetic carbon.

This corrects the article DOI: 10.1038/35099527

Ferromagnetic carbon with enhanced Curie temperature

Abstract The discovery of a ferromagnetic form of carbon (Nature 413 (2001) 716) gives a new perspective in the investigation of magnetic materials. The existence of a ferromagnetic state with the very high Curie temperature T C ≈500 K for a material with only s- and p-electrons as well as the nature of its underlying interaction are of great fundamental interest. Here we report on the observation of the ferromagnetically ordered state in a material obtained by high-pressure high-temperature treatment of the fullerene C60. It has a saturation magnetization more than four times larger than that reported previously. From our data we estimated the considerably higher value of T C ≈820 K .

Pressure-induced dimerization of fullerene C60: a kinetic study

Abstract The kinetics of pressure-induced dimerization of fullerite C60 at 1.5 GPa in the 373–473 K temperature interval was studied by X-ray diffraction, infrared and Raman spectroscopy. Kinetic curves of the dimerization reaction in the fcc and sc phases of C60 were obtained by monitoring the dimeric (C60)2 IR line at 796 cm−1. The value of the dimerization activation energy was determined to be E a ( dim ) =134±6 kJ mol −1 , assuming the second order irreversible reaction. The peculiarities of the dimerization processes in the fcc and sc phases of C60 fullerite are also discussed.

The crystal structure of the 2D polymerized tetragonal phase of C60

Abstract Single crystals of the 2D polymerized tetragonal phase of C 60 without orientational domains were obtained under high pressure and high temperature. The crystal structure of this phase was resolved using single crystal X-ray diffraction data. The cell parameters are a =9.064(3) and c =15.039(8) A with the space group P4 2 / mmc and Z =2. A structural model of this phase proposed early was confirmed and refined to final R =0.075. The structure is slightly disordered. It consists of a random combination of the P4 2 / mmc layers (84%) and of the Immm layers (16%), along the c -axis.

Spectroscopic properties of individual pressure-polymerized phases of C60

Abstract The Raman and IR spectra of pure polymerized phases of C 60 obtained at high pressure and temperature were investigated. Our data showed that some previous spectra were obtained from a mixture of different polymeric states. Determination of the characteristic bands of dimer, chain and layer polymers permit the use of the IR and Raman spectra in the molecular fractional analysis of different polymerized states of C 60 .

Thermal studies of C60 transformed by temperature and pressure treatments

Raman spectroscopy, X-ray and electron diffractions have been applied to study samples of fullerene C60 after they have undergone pressure and temperature treatments up to 8 GPa and 1073 K. It is confirmed that mixtures of rhombohedral and tetragonal structures are formed in the 2–4 GPa and 673–1073 K pressure-temperature domain. Traces of a hexagonal phase are also observed in the same range. Only the rhombohedral one is obtained in the 4–8 GPa range in the same temperature interval. The material obtained at temperatures between 473 and 673 K in the whole pressure range which has previously been described in terms of a cubic structure can be understood in terms of an orthorhombic one. It may also contain mixtures of different phases including a cubic one and even a cubic superstructure. The widened poorly resolved X-ray profiles indicate the existence of disorder within the materials formed and the increasing complexity of Raman spectra as temperature increases at fixed pressures (from 2 to 8 GPa) suggests that this disorder is related to phase mixtures in almost all samples. A rhombohedral sample formed at 6 GPa-873 K reverted to a mixture of rhombohedral and tetragonal phases at 2.5 GPa-873 K. Thus thermodynamic equilibrium between these two kinds of systems could exist.

Electrical properties of two-dimensional fullerene matrices

The electrical properties of two-dimensionally polymerized C60 fullerenes were studied. Fullerene matrices consisting of randomly oriented domains are compared to the highly-oriented rhombohedral phase. The conductivity of the randomly oriented polymers obeys the Arrhenius law and can be described in a multiple trapping model. The oriented phase of polymeric C60 shows a distinct anisotropy in the electrical properties with a metallic-like in-plane conductivity at high temperatures.

Pressure-induced polycondensation of C60 fullerene

Modifications of carbon which are formed from C60 fullerite at pressures up to 10.0 GPa and temperatures up to 1900 K are studied by x-ray diffraction, Raman spectroscopy, and atomic force microscopy methods. The pressures p and temperatures T at which atomic, molecular, and polymolecular structures form under conditions of quasihydrostatic compression are determined. It is shown that, together with polymerization, another type of chemical interaction of the molecules, called polycondensation, which leads to the formation of polymolecular structures with a shortest intermolecular distance of 0.65 nm, is possible in the system. Three-dimensional polycondensation of C60 fullerene is explained by the special properties of the new carbon states.

Identification of the polymerized orthorhombic phase of C60 fullerene

It is established by x-ray diffraction and Raman scattering that the polymerization of C60 fullerene at 1.5 GPa and 723 K leads to the formation of an orthorhombic phase that is different from the previously identified high-pressure orthorhombic phase. It is determined by a calculation of the optimal packing of linear C60 polymers by the method of atom-atom potentials that the energetically favorable structure of the orthorhombic phase belongs to the space group P n n m and not the previously proposed group I m m m. The computed value of the rotation angle of the polymer chains that corresponds to the minimum packing energy was equal to 61°. The mechanisms leading to the formation of the polymerized phases are discussed on the basis of the results obtained.