The temperature and field dependencies of the magnetoresistivity (MR) were studied and analysed for two series of polycrystalline samples of La0.7Ca0.3-xSrxMnO3 (0 <x <0.3) obtained by the nitrate technology under different annealing conditions. X-ray patterns of the samples demonstrate a broad halo (3 degrees of 2 Theta approximate to7 degrees) related with diffusion scattering at the intergranular media. The temperature dependence of MR for these compounds showed, apart from the MR peak close to the ferromagnetic transition temperature T-c (caused by MR-mechanism inside grains), the significant increase of MR below T-c that is common for systems with weak bonds between grains. The low-temperature component of the MR depends on the average ionic radius of the cation at the A-site.
This paper discusses the effect of hydrostatic pressure up to 0.8 GPa on magnetic properties of manganites La0.7Ca0.3 - xSrxMnO3 (0 x 0.3, x = 0.03) and magnetoresistance data in an applied magnetic field of 5.0 T. Application of pressure enlarges the temperature range of the ferromagnetic phase. Curie temperature, TC, as a function of pressure and the temperature of resistance maximum, Tp, showed an anomaly for x = 0.15. The slope of pressure dependence of TC for x < 0.15 and x > 0.15 is higher than for x = 0.15. Dependence of temperature Tp on x consists of two curves: for x 0.15 and for x 0.15. There is a sharp bend on the Tp curve at x = 0.15. The structural phase transition from orthorhombic phase (x < 0.15) to rhombohedral one (x 0.15) corresponds to that concentration.
The linear coefficients alpha(T) of N-2-C-60 solutions with 9.9% and 100% of the C-60 lattice thermal expansion interstitials filled with N-2 are investigated in the interval 2.2-24 K. The dependence alpha(T) has a hysteresis suggesting co-existence of two types of orientational glasses in these solutions. The features of the glasses are compared. The characteristic times of phase transformations in the solutions and reorientation of C-60 molecules are estimated.
We have examined the c-direction variations of resistance, Rc (or resistivity, varrhoc) over the 4.2–295 K. temperature range at ambient pressure, and over the 160–295 K range for pressures up to 1.5 GPa on first stage PdAl2Cl8 graphite intercalation compounds. All samples examined manifest a rapid, nonlinear decrease in Rc with pressure (maximum ¦dln Rc/dp¦ > 300 % GPa−1 at 0.4 GPa. The overall Rc decrease is around 90% by 1GPa, a value greater than that observed in any other GIC to date. Upon slow pressure release, the resistance increases linearly but to only about 20–25% of its initial (1 bar) value for HOPG-based materials: on the contrary, it returns along its initial R-p trajectory for GICs based on single crystal graphite. We discuss these data with reference to the X-ray diffraction results, to the significance of the residual resistivity and to studies on other similar lamellar solids.
This paper presents some new data on the SbCl5-intercalated graphite family as concerns the relationships between c axis resistivity, ρc(T,p), and intercalate layer structure. Results on the influence of intercalate layer crystallization and the nature of the host graphite are discussed and compared with those on other GIC families. Certain data are examined in the light of available theories and such an analysis raises a number of questions.
Upon the occasion of the 10th ISIC conference, it seems appropriate to give a brief overview of some of the major advances made in our understanding of charge transport in intercalated graphite since the 1st Franco-American Conference on Graphite Intercalation Compounds held in 1977.
We have examined several samples of first- to third-stage PdAl2Cl8-intercalated graphite under hydrostatic pressures up to 1 GPa. In stage-1 highly oriented pyrolytic graphite–(HOPG) and single-crystal-graphite-based materials, the c-axis resistivity decreases sharply above a few kilobars; pressure release induces a reversible return to the initial value only in the case of the latter sample. Raman spectra taken in situ under pressure on a HOPG-based material show similarly irreversible effects. Analysis of the spectra taken on higher-stage samples leads to the conclusion that hydrostatic pressure beyond a few kilobars increases the density of the intercalate within the graphitic galleries, transforming the initial sample to a higher-stage material. Since there is no loss of intercalate, the overall intercalate-to-host charge transfer remains constant so that the Raman frequency is approximately the same for both first- and second-stage products. This is an unusual situation in which there is thus an apparent lack of Raman signature in spite of the stage change.
The Raman spectra of the two-dimensional tetragonal (2D(T)) polymeric phase of C60 have been studied in situ at pressures up to 30 GPa and room temperature. The pressure dependence of the phonon modes shows an irreversible transformation of the material near 20 GPa into a new phase, most probably associated with the covalent bonding between the 2D polymeric sheets. The Raman spectrum of the high-pressure phase is intense and well resolved, and the majority of modes are related to the fullerene molecular cage. The sample recovered at ambient conditions is in a metastable phase and transforms violently under laser irradiation: the transformed material contains mainly dimers and monomers of C60 and small inclusions of the diamond-like carbon phase. The photoluminescence spectra of the 2D(T) polymer of C60 were measured at room temperature and pressure up to 4 GPa. The intensity distribution and the pressure-induced shift of the photoluminescence spectrum drastically differ from those of the C60 monomer. The deformation potential and the Grüneisen parameters of the 2D(T) polymeric phase of C60 have been determined and compared with those of the pristine material.
The structural stability of the tetragonal two-dimensional (2D) polymeric phase of C60 has been studied under pressure up to 24 GPa and room temperature by means of in situ Raman scattering. An irreversible transformation of the material to a new phase was observed at pressure 20 GPa. The phonon spectrum of the high-pressure phase provides a strong indication that the fullerene molecular cage is retained and therefore this phase may be related to a three-dimensional (3D) network of C60 cages. The new phase remains stable upon slow release of pressure to ambient conditions. The recovered material is metastable and transforms in air by detonation under laser irradiation to partially dimerized C60.,
It is shown that C60 single crystals can be polymerized under relatively modest pressure-temperature conditions. The resulting single crystals exhibit long-range order and they are made up of 12 orientation variants. The structure is orthorhombic with a short intermolecular distance along the chains (9.14 Å), characteristic of covalent bonding. We propose a structural mechanism for the polymerization of fullerene-based compounds; it involves a shift of one of the (111) cubic planes together with a shortening of the (111) spacing. The configuration of the polymer chains presents interesting relations with that found in the A1C60 polymer compounds.
High-pressure polymerisation of C^o leads to a variety of new crystalline or amorphous phases which display interesting physical properties. We have prepared onedimensional (ID, C6o chains) and two-dimensional (2D, C60 layers) polymers from C6o single crystals. The resulting multi-domain crystals have been studied using x-ray diffraction and Raman spectroscopy. The relative orientations of the chains in the "low-pressure" ID orthorhombic polymer had been characterized previously [1]. We have now determined the specific stacking of the Ceo layers in the 2D tetragonal and rhombohedral polymers. Using these results we analyze the relations between the different polymers and the intermolecular environments which may play a role in stabilizing the observed polymer structures.
Two-dimensional polymerisation of a C60 single crystal has been obtained under high-pressure high temperature conditions (700 K - 2 GPa). Crystalline order is preserved but the crystal splits into variants (orientational domains). The analysis of X-ray diffraction and Raman spectroscopy data reveals that the polymer crystal is primarily tetragonal with some admixture of rhombohedral phase. Furthermore, Raman spectroscopy gives evidence for additional C60-C60 dimers, which are probably disordered. For the tetragonal phase, it is shown that successive polymer layers are rotated by about the stacking axis, according to the P42/mmc space group symmetry. The structure of the rhombohedral phase is also clarified. The role of the interlayer interactions in stabilising the two-dimensional polymer phases of C60 is discussed.
We present a study by Raman spectroscopy and X-ray diffraction/diffuse scattering of C60 single-crystals treated at high-pressure and high-temperature. This allowed us to obtain structural information on the C60 dimer state which can be considered as an intermediate state in the polymerization process. In the 1-6 GPa pressure range the crystals are primarily formed of dimers with additional minor fractions of monomers, 1D and 2D polymers, as shown by the analysis of the Raman spectra. The dimers are disordered within an average cubic lattice derived from that of the monomer. Single-crystal diffraction patterns reveal a characteristic diffuse scattering intensity distribution which has been simulated by calculating the diffuse scattering produced by dimer and trimer model structures. Satisfactory agreement is obtained for random positional and orientational disorder of the C60-C60 dimers although a small concentration of similarly disordered trimers is likely. In a first approximation the dimer/trimer disorder can be considered as random but various inter-dimer correlations are probably present, as discussed.
X-ray diffraction and Raman spectroscopy have been used to characterize the structures obtained when C-60 single crystals are treated at 2GPa-700K. Two different experimental procedures have been applied: the temperature is raised before the pressure is applied, or the opposite. The "heating-then-pressing" path leads to the tetragonal polymer structure (P4(2)/mmc) together with a minor fraction of rhombohedral structure, which confirms previous results. In contrast, the "pressing-then-heating" path leads to a different state presenting similarities with both the rhombohedral and the disordered dimer structures. The results are discussed in light of the orientational and dynamical aspects of the C60 polymerization.
High-resolution capacitance dilatometry was used to study the thermal expansion of both 'normal' and polymer phases of C60. The expansivity, alpha(T), of 'normal' C60 exhibits an unusual temperature dependence above the orientational disordering transition at 260 K; alpha(T) decreases by about 30% from 260 to 500 K and appears to reach a minimum near 500 K. Polymerized C60 has a much smaller expansivity than normal C60 due to the stronger covalent bonding between molecules. The polymerized state converts back to 'normal' C60 at temperatures between 450 and 500 K. We show that this is an activated process with a well-defined activation energy of 1.92 eV and a volume increase of about 2%.
The thermal stability of C60 dimers and 2D pressure-polymerized C60 is studied using high-resolution capacitance dilatometry. The transformation of both the dimer and the polymer phases back to 'normal' C60 is excellently described by a simple thermally activated process with activation energies of 1.75 +/- 0.1 eV (dimer) and 1.9 +/- 0.2 eV (polymer). These results are compared to previous data for 1D-polymerized C60 and photo-polymerized C60. The thermal expansivity of the 2D-polymer phase is as much as a factor of ten smaller than that of pure C60 and approaches values for diamond.
We report on high-resolution thermal expansion measurements of high-temperature-pressure treated C60 [one-dimensional (1D) and (2D) polymers and “hard fullerite”], as well as of C60 dimers and single crystal monomer C60 between 10 and 500 K. Polymerization drastically reduces the thermal expansivity from the values of monomeric C60 due to the stronger and less anharmonic covalent bonds between molecules. The expansivity of the “hard” material approaches that of diamond. The large and irreversible volume change upon depolymerization between 400 and 500 K makes it possible to study the kinetics of depolymerization, which is described excellently by a simple activated process, with activation energies of 1.9±0.1 eV (1D and 2D polymers) and 1.75±0.05 eV (dimer). Although the activation energies are very similar for the different polymers, the depolymerization rates differ by up to four orders of magnitude at a given temperature, being fastest for the dimers. Preliminary kinetic data of C70 polymers are presented as well.
Accurate measurements of the electrical resistance of Nb are presented as a function of temperature and pressure in the region 0-40°C and 0-1 GPa. From these measurements and published results for the compressibility and the pressure dependence of the superconducting transition temperature Tc we evaluate the pressure dependence of the electron-gas plasma frequency ω(p). The electronic structure of Nb is calculated from a self-consistent linear muffin-tin orbital method and results are obtained for the pressure dependence of the density of states at the Fermi surface, the root mean square over the Fermi surface of the Fermi velocity, the optical mass, and the plasma frequency. The experimental and calculated results for the pressure dependence of ω(p) are both close to dlnω2 / dlnV=-1.9. This agreement suggests that measurements of the electrical resistance as a function of temperature and pressure provide a new test of band-structure calculations. From our measurements of the resistance and calculations of ω(p) and from published results for the compressibility we obtain the pressure dependence of the electron-phonon interaction λ(p). With p given in gigapascals, the result is dlnλ / dp=-0.0047.
The ceramic superconductor YBa2Cu4O8 has been produced by high temperature sintering of a mixture of CuO and YBa2Cu3O7 in a glass capsule under high hydrostatic argon pressure. The resulting highly dense material is investigated by X-ray diffraction, optical and electron microscopy, resistance measurements and hardness measurements, and shown to be a homogeneous High transition temperature superconductor.
The polymerization of fullerenes has been an interesting topic for almost three decades. A rich polymeric phase diagram of C60 has been drawn under a variety of pressure-temperature conditions. However, only linear or perpendicular linkages of C60 are found in the ordered phases. Here we used a unique bilayer structural solvate, C60∙1,1,2-trichloroethane (C60∙1TCAN), to generate a novel quasi-3D C60 polymer under high pressure and/or high temperature. Using Raman, IR spectroscopy and X-ray diffraction, we observe that the solvent molecules play a crucial role in confining the [2+2] cycloaddition bonds of C60s forming in the upper and lower layers alternately. The relatively long distance between the two bilayers restricts the covalent linkage extended in a single individual bilayer. Our studies not only enrich the phase diagram of polymeric C60, but also facilitate targeted design and synthesis of unique C60 polymers.
We have studied the structural, thermophysical, and spectroscopic properties of polymeric C60 obtained by high pressure treatment at pressures and temperatures near 1 GPa and 600 K. We present here a brief overview of our results for the structural and thermophysical properties and a more detailed report on recent results obtained by Raman spectroscopy on both thin films, polycrystalline, and single crystal material. The results presented include a comparison between Raman results for photopolymerized and pressure polymerized thin films and a preliminary estimate of the binding energy of polymeric C60.
The properties of C60 have been studied after treatment at high temperature and high pressure (1.1 GPa and 565 K for 2 h). The treated material is insoluble in organic solvents. We present results obtained in NMR and Raman studies and measured data for the specific heat and the thermal expansion. Our results show clearly that there are no covalent bonds and no molecular rotation, but suggest that the molecules are slightly deformed and held together by weak pi-type bonds.
Bulk C60 has been treated at 1.1 GPa and 550–585 K, producing a dense insoluble material which on heating to above 600 K reverts to normal C60. Raman and IR studies on modified material show a large number of new lines, and the Raman pentagon pinch mode shifts from 1469 to 1458 cm−1 as on photopolymerization. MAS NMR shows one broadened line at the original C60 shift 144 ppm and a small peak at about 77 ppm due to the bridging carbons. None of the new resonances observed for C60 polymerized by other methods were observed. The results verify previously suggested polymeric structures where the fullerence cages are connected by four-membered rings.
The thermal dissociation of C60 polymers has been studied using Raman scattering. The measured dissociation rate depends on the intensity of the 1064 nm NIR excitation, showing that i) the band gap is smaller than 1.17 eV and ii) a radiation-enhanced thermal breakdown path exists in addition to the "normal'' thermal breakdown mechanism. Quantum chemical calculations show that the energy barrier Ea for thermal breakdown is about 40% lower in the first (optically) excited state than in the ground state. This agrees well with the ratio between our radiation-enhanced value $E_{a} = (1.1 \pm 0.02)\,eV$ and values near 1.9 eV measured by purely thermal methods.
Polycrystalline fullerite C60 intercalated with Xe atoms at 575 K and a pressure of 200 MPa was studied by powder x-ray diffraction. The integrated intensities of a few brighter reflections have been utilized to evaluate the occupancy of the octahedral interstitial sites in C60 crystals, which turned out to be (34±4)%, and in good agreement with another independent estimate. It is found that reflections of the (h00) type become observable in Xe-doped C60. The presence of xenon in the octahedral sites affects both the orientational phase transition as well as the glassification process, decreasing both characteristic temperatures as well as smearing the phase transition over a greater temperature range. Considerable hysteretic phenomena have been observed close to the phase transition and the glassification temperature. The signs of the two hysteresis loops are opposite. There is reliable evidence that at the lowest temperatures studied the thermal expansion of the doped crystal is negative under cool-down.
Accurate measurements of the electrical resistance as a function of temperature and pressure are reported for Sn, Zr, dhcp La, and V. These measurements cover a temperature region around room temperature and pressures up to 1.3 GPa. From these data, including also our previous measurements for Al and published results for Pb, the pressure dependence of d ρ / dT (the resistivity-temperature derivative) is obtained. This quantity is found to be a significant factor in the pressure dependence of the electron-phonon interaction parameter λ. For the nontransition metals the relative pressure dependence of d ρ / dT is much larger than the compressibility. Therefore the pressure dependence of the superconducting Tc is quantitatively well accounted for by the resistance data for these metals. For the transition metals the pressure dependence of d ρ / dT is relatively smaller and Tc(p) calculated from the resistance data is, at the best, only qualitatively correct. These differences are discussed. Estimates for the pressure dependence of the plasma frequency are obtained.
On the basis of measurements of the resistivity and calculations of the density of states and the plasma frequency it is concluded that palladium may become superconducting at high pressures.
Excitation spectra of polymerized fullerides such as RbC60, Na4C60 and pressurized C60 are studied by inelastic neutron scattering and Raman spectroscopy in view of differences in the interfullerene bonding. Changes in the dynamics are followed by temperature dependent measurements. A detailed analysis is performed by model calculations.
Thermal conductivity of solids and liquids under pressure is covered in this review. Experimental techniques are critically considered and compared, and an introduction to theory is provided. Results are presented and discussed for ionic crystals, normal molecular crystals, plastic crystal phases, clathrate hydrates, polymers and glass-formers, liquids, covalent and semiconducting crystals, rocks and metals. Special attention is given to isochoric conditions, change of crystal structure and molecular orientational disorder. Available reliable measurements at pressures up to a few GPa indicate the need for theoretical development, especially in connection with molecular crystals and ferromagnetic metals.
We have studied tetragonal C60 and C60-based polymers doped with Lithium and Sodium. We show that the intercalated phases Li4C60 and Na4C60 both form two-dimensional polymers. X-ray diffraction diagrams for Li4C60 and tetragonal C60 can be accurately indexed assuming tetragonal structures, but for Na4C60 a monoclinic quasi-tetragonal structure is found. We conclude that in Li4C60 the covalent bonds are formed by (2 + 2) cycloadditions, in the same way as in the tetragonal polymer produced by treating pure C60 at high temperature and high pressure, while single C-C bonds connect the fullerene molecules in Na4C60.
We have examined intercalated compounds of C60 containing four light alkali metal atoms per molecule. The single-metal compounds Na4C60 and Li4C60 form two-dimensional polymers with intermolecular links consisting of one and two C–C bonds, respectively. We have here studied the compounds NanLi(4-n)C60, with n between 0 and 4, to find out what parameters define the polymeric structure, in particular the type of intermolecular bonding. The materials have been studied by X-ray diffraction and Raman spectroscopy. Although the results are compatible with a charge transfer model with different charge transfers for Na and Li ions, other models cannot be ruled out because disorder and mixed phases complicate the analysis.
We have synthesized the series of nominally isoelectronic intercalated fullerene compounds LixNa(4 − x)C60 and investigated these by X-ray diffraction and Raman spectroscopy. All compounds are two-dimensional polymers, with the Li4C60 structure dominating for x > 1 and the Na4C60 structure for x less-than-or-equals, slant 1. We find almost no shift of the Ag Raman modes with x, indicating that the charge transfer is also practically independent of composition. We conclude that the Li4C60 structure is the lowest energy structure for all these compounds, and that the Na-rich compounds choose the Na4C60 structure because of geometrical factors connected with effective ion radii.
The thermal conductivity and the specific heat capacity per unit volume have been measured for two low viscosity grades of Dow Corning 200 fluid (polydimethyl siloxane) in the range 110 to 350 K and under pressures up to 2.0 GPa (20 kbar). Both the quantities studied are found to increase with increasing pressure in the liquid phase. From the measured data the phase diagrams are obtained. The 5 mm2/s (5 cSt) grade fluid does not crystallize, but undergoes a glass transition at 1.0 GPa at room temperature. The 1 mm2/s (1 cSt) grade has a more complicated phase diagram with a partly crystalline phase at low temperatures and pressures and two glass transitions at high temperatures and pressures.
Amorphous materials inherit short- and medium-range order from the corresponding crystal and thus preserve some of its properties while still exhibiting novel properties1,2. Due to its important applications in technology, amorphous carbon with sp2 or mixed sp2–sp3 hybridization has been explored and prepared3,4, but synthesis of bulk amorphous carbon with sp3 concentration close to 100% remains a challenge. Such materials inherit the short-/medium-range order of diamond and should also inherit its superior properties5. Here, we successfully synthesized millimetre-sized samples—with volumes 103–104 times as large as produced in earlier studies—of transparent, nearly pure sp3 amorphous carbon by heating fullerenes at pressures close to the cage collapse boundary. The material synthesized consists of many randomly oriented clusters with diamond-like short-/medium-range order and possesses the highest hardness (101.9 ± 2.3 GPa), elastic modulus (1,182 ± 40 GPa) and thermal conductivity (26.0 ± 1.3 W m−1 K−1) observed in any known amorphous material. It also exhibits optical bandgaps tunable from 1.85 eV to 2.79 eV. These discoveries contribute to our knowledge about advanced amorphous materials and the synthesis of bulk amorphous materials by high-pressure and high-temperature techniques and may enable new applications for amorphous solids.
As an advanced amorphous material, sp3 amorphous carbon exhibits exceptional mechanical, thermal and optical properties, but it cannot be synthesized by using traditional processes such as fast cooling liquid carbon and an efficient strategy to tune its structure and properties is thus lacking. Here we show that the structures and physical properties of sp3 amorphous carbon can be modified by changing the concentration of carbon pentagons and hexagons in the fullerene precursor from the topological transition point of view. A highly transparent, nearly pure sp3−hybridized bulk amorphous carbon, which inherits more hexagonal-diamond structural feature, was synthesized from C70 at high pressure and high temperature. This amorphous carbon shows more hexagonal-diamond-like clusters, stronger short/medium-range structural order, and significantly enhanced thermal conductivity (36.3 ± 2.2 W m−1 K−1) and higher hardness (109.8 ± 5.6 GPa) compared to that synthesized from C60. Our work thus provides a valid strategy to modify the microstructure of amorphous solids for desirable properties.
We have investigated the electronic structure of one and two-dimensional C60 polymers with regard to both their low-lying excitations and C 1s excitation spectra by means of electron energy-loss spectroscopy in transmission. We compare the results with those for pristine C60. In general, the spectra for the polymers resemble those for pristine C60 but show a broadening due to both the lowering of the symmetry and the increased intermolecular overlap. This is also reflected by a reduction of the optical gap in comparison with pristine C60.
The vibrational spectrum of polycrystalline C60 polymerized at 1.1 GPa and 585 K was studied by inelastic neutron scattering. We find drastic changes in the spectrum compared to the vibrational spectrum of pristine C60: the appearance of a new, broad vibrational band at low energies, and splitting and significant changes in the peak positions of various modes. The thermal conductivity lambda of polymerized C60 was measured in the temperature range 150-320 K and was found to increase with a rise in temperature, which reflects strong phonon scattering. A high degree of structural disorder in the crystalline lattice of the polymeric phase is most probably responsible for the glass-like beahviour of lambda(T).
The properties of bulk C60 have been studied after treatment at 1.1 GPa and 550-585 K. The treated material is insoluble in both toluene and 1,2-dichlorobenzene. Raman spectroscopy on modified aterial shows a large number of new lines, and the Raman pentagon pinch mode (Ag2) shifts from 1469 to 1458 /cm as on photopolymerization. MAS NMR shows one broadened line at the original C60 shift 144 ppm and a small peak at about 77 ppm due to the bridging carbons. The results verify previous suggested polymeric structures where the fullerene cages are connected by four-membered rings.
For the first time polymerization of both powder and single crystals of C70 fullerene was established after their subjection to high pressure (1.1 - 2 GPa) at elevated temperature (500 - 580 K). High-resolution capacitance dilatometry, FTIR/Raman spectroscopy and thermal conductivity were employed to characterise the polymeric phase of C70. The results demonstrate drastic changes in the physical properties of C70 on polymerization. We report on a reverse transformation to the monomeric state on heating the polymer to 500 K at ambient pressure. The activation energy of depolymerization was determined to be 1.8(1) eV. We discuss our results in terms of existing structural models for polymerization of C70 and compare the physical properties of C70 and C60 polymers.
The kinetics of C60 polymerization were studied in the temperature range 450-500 K at pressures below 1 GPa by measurements of the time dependence of the thermal conductivity. The vibrational spectrum of polymerized C60 was studied by inelastic neutron scattering. We find drastic changes in the spectrum compared with the vibrational spectrum of pristine C60: hardening of intermolecular modes, appearance ofa new broad vibrational band at low energies, splitting and significant changes in the peak positions of intramolecular modes. The thermal conductivity, lambda, of polymerized C60 was measured in the temperature range 150-320 K and was found to increase with rising temperature, that reflects strong phonon scattering. The presence of polymeric chains of different length and a high degree of structural disorder in the crystalline lattice of the polymeric phase are the most probable factors responsible for the glass-lika behaviour of lambda(T).
We have measured the thermal conductivity ε and the heat capacity per unit volume varrhocp of highly pure C70 in the temperature interval 100–450 K under pressures up to 1 GPa. Anomalies indicating freezing of uniaxial molecular rotation were observed in λ and varrhocp upon both cooling and increasing pressure. The phase boundary for this transition has an approximate slope dT/dp = 70 K Gpa−1. The temperature and pressure dependence of λ indicate a substantial amount of structural defects in the sample and strong metastability effects.
Measurements of the thermal conductivity lambda and the specific heat capacity per unit volume rhocv of polycrystalline C70 were carried out in the temperature range 100 - 450 K under pressures up to 1 GPa. Anomalies have been observed in lambda and rhocv near 300 K on cooling (increasing pressure) indicating a phase transition from a rotationally disordered phase into an ordered phase. The phase boundary for this transition was found to have a slope dT/dp = 70 K/GPa. The temperature dependence of lambda indicates a substantial amount of structural defects in the sample and strong hysteresis effects. We propose a phase diagram describing the probable evolution of orientational structure in T-p space.
Polymeric forms of C60 are now well known, but numerous attempts to obtain C70 in a polymeric state have yielded only dimers. Polymeric C70 has now been synthesized by treatment of hexagonally packed C70 single crystals under moderate hydrostatic pressure (2 gigapascals) at elevated temperature (300°C), which confirms predictions from our modeling of polymeric structures of C70. Single-crystal x-ray diffraction shows that the molecules are bridged into polymeric zigzag chains that extend along the c axis of the parent structure. Solid-state nuclear magnetic resonance and Raman data provide evidence for covalent chemical bonding between the C70 cages.
The high-Tc superconductor with nominal composition BiSrCaCu2Ox has been studied at high pressure, i.e. up to 50 GPa. A tetragonal structure was compatible with the measurements at all pressures, and no phase change was observed. The bulk modulus, B0 = 62.5 GPa, obtained has a somewhat smaller value than the one estimated earlier.
A general overview is given over fullerenes under pressure, with an emphasis on polymeric phases produced by treatment of C60 and C70 under high pressure. The pressure-temperature phase diagrams of these materials are briefly reviewed. The discussion centers on recent advances regarding the details of the crystal structures and the electron structures and electronic transport properties, in particular for the one- and two-dimensional phases of C60. Several polymerized phases of C70 are also discussed.
Results from recent high-pressure experiments in the field of fullerenes are briefly reviewed. In particular, new results on one-, two- and three-dimensional polymerized C60 and C70 are discussed. Results discussed include the first synthesis of a well defined, one-dimensional polymer based on C70, transformations from two-dimensional (2D) to three-dimensional phases in C60, and doping of 2D C60 polymers.
Carbon is an element with extremely versatile bonding properties and theoretical calculations have suggested the possible existence of several hundred structural allotropes. Many, or even most, of these are predicted to be formed under conditions of high pressure and temperature. On the other hand, experimental high pressure studies have identified surprisingly few structural allotropes. In this paper, physical properties and structural transformations observed in high pressure experiments, at and above room temperature, are reviewed for a large number of solid carbon allotropes. The materials discussed include bulk carbon such as graphite, diamond, glass-like and amorphous carbon, two-dimensional graphene, and molecular carbon in the form of one-dimensional carbon nanotubes and zero-dimensional fullerenes. Results from recent studies on twisted graphene, graphdiyne, graphyne, carbon dots and other interesting all-carbon allotropes are also briefly described. Observed similarities and differences between the high pressure behavior and evolution of carbon materials are discussed. In spite of the enormous volume of experimental work carried out on these materials, few new structural allotropes have been identified and most carbon materials studied convert into diamond at sufficiently high temperature and pressure. Further theoretical work thus seems to be needed to elucidate possible transformation processes and transition paths for the many undiscovered allotropes proposed from calculations. In particular, it is recommended that, for every new allotrope predicted by theory, suitable precursors and transformation conditions should also be investigated. Efficient creation of new structural allotropes or functional materials based on pure carbon by high pressure methods should ideally start from designed, preassembled precursor structures or composites for which transition paths can be theoretically predicted.