Shock compressibility and shock-induced phase transitions of C60 fullerite

Shock compressibility of C₆₀ fullerite and sound velocities in shock-compressed fullerite were experimentally studied at the pressures range up to 50 GPa. In our experiments, we used polycrystalline C₆₀ specimens with a density of 1.64 g/cc. The Hugoniot of C₆₀ fullerite had a set of peculiarities, which may be attributed to a series of polymorphic phase transitions. The jump of sound velocity in shocked C₆₀ at pressure 9 GPa indicates the formation of a rather hard carbon phase. It is possible to assume that it is a polymerized C₆₀ phase. In the region of pressures 9–25 GPa, destruction of this phase and formation of a graphite-like carbon occurs. With further growth of shock pressure, phase transition of the graphite-like carbon to a diamond-like phase is observed with a transition onset pressure 25 GPa. If shock pressures are higher than 33 GPa, Hugoniot of C₆₀ fullerite is determined by the thermodynamic properties of the diamond-like phase.     

Stepwise shock compression of C70 fullerene

Shock-induced phase transitions of C₇₀ fullerene are experimentally studied up to a pressure of 36 GPa and a temperature of 1200 K. Pressure–temperature histories of C₇₀ specimens are estimated and overlaid on the tentative phase diagram of C₇₀. The crystalline phase of fullerene C₇₀ with a hexagonal close-packed structure remained practically unchanged under stepwise shock compression up to а pressure of 8 GPa. Contrariwise the crystalline phase of fullerene C₇₀ with a rhombohedral structure fully converted to the phase with a cubic structure under similar conditions. Shock-induced transformation of the hexagonal phase into the face-centered cubic phase was observed at pressures in the range of 9–19 GPa. The amount of transformed material increases with the shock intensity. Upon further increase in the shock pressure, the destruction of C₇₀ molecules begins. In the sample recovered from 26 GPa, we observed the traces of C₇₀ with a face centered cubic structure only. The destruction of C₇₀ is accompanied by formation of graphitic carbon.     

Synthesis of ultrahard fullerite with a catalytic 3D polymerization reaction of C60

3D polymerization of C₆₀ realizes under conditions of large plastic deformation at pressure 6–7 GPa and room temperature in the presence of CS₂. The phase of 3D-polymerized C₆₀ is identical to ultrahard fullerite synthesized from pure C₆₀ at 18 GPa pressure: in both cases, the samples plough diamond during the rotation of the sample in a shear diamond anvil cell, bulk module is 585 GPa, and a sequence of phase transitions preceding to ultrahard phase is also the same in both cases (in the presence of CS₂, the phase transitions take place at lower pressures than in pure C₆₀). Raman and transmission microscope studies confirm the structure equivalence of samples of both types. The absence of sulfur in the structure of ultrahard fullerite synthesized in the presence of CS₂ proves the catalysis role of CS₂ in the 3D polymerization of C₆₀.     

Shock-wave synthesis of diamonds from fullerenes C60−C100

Shock-wave synthesis of diamond from C₆₀–C₁₀₀-fullerene powder was first accomplished by using the explosive compaction technique with plane wave loading in the pressure range of 24–40 GPa. The compacts of various initial composition comprised diamond, FCC C₆₀-fullerite, graphite, and amorphous carbon. The largest diamonds of 0.1–1.0 m were obtained under shock loading of pellets consisting of copper powder with 5 wt. % fullerite at 24 and 38 GPa, and pellets consisting of copper powder with 10 wt. % fullerite at 40 GPa. The end product consists of diamond without intermediate diamondlike phases such as n-diamond and hexagonal diamond (lonsdaleite).Central Machine-Building Technology Research Institute, 109088 Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 2, pp. 131–138, March–April, 1995.     

Recover of C3N4 nanoparticles under high-pressure by shock wave loading

Theory predicts that β-C₃N₄ with its dense structure is a superhard material. In order to synthesize this material, light-gas gun loading and shock recovery technology were used to perform experiments. The amorphous nitrogen-enriched g-C3N4, produced by the thermal decomposition of melamine, was used as a precursor. The shock synthesis experiment was completed under the pressure of 50?GPa. A high-density phase with a β-C₃N₄ structure was detected only in the desired product. It is suspected that the elemental composition and synthetic pressure of precursors may be the main factors affecting the phase composition of products. This has significant potential for the synthesis of pure superhard carbonitride compounds.     

Extended polymerization in ABC-stacked C70 fullerite

This letter reports on the extended polymerization occurring in ABC-stacked C₇₀ fullerite, which up to now was thought to occur only in the AB-stacked phase. Frustration effects on ABC close-packing of C₇₀ molecules were believed to prevent extended polymerization based on 2 + 2 cycloaddition bonds, but here we show that an extended polymeric phase is obtained at 10 GPa and 270 °C. Its structure, obtained by Rietveld analysis of the X-ray diffraction data combined with density functional theory methods, consists of zigzag chains running within the dense hexagonal (1 0 1) pyramidal planes.     

Shock Compression of Boron Carbide: A Quantum Mechanical Analysis

The shock Hugoniot of boron carbide, from 0 to 80 GPa, has been obtained using first principles quantum mechanics (density functional theory) and molecular dynamics simulation. The Hugoniot for six different structures which vary by structure or stoichiometry were computed and compared to experimental data. The effect of stoichiometry, and structural variation within a given stoichiometry, are shown to have marked effects on the shock properties with some compositions displaying bilinear behavior in the computed shock velocity‐particle velocity profiles while others show a continuous Hugoniot curve with no evidence of a phase transition over the pressure range considered in this work. Two structures, B₁₂(CBC) and B₁₁C_p(CCB), have predicted phase transition pressures lying within the 40–50 GPa range suggested experimentally. It is shown that the phase transition is driven by deformation of the 3‐atom chain within the boron carbide crystal structure which induces a discontinuous volume change at the critical shock pressure. The effect of defects, in the form of chain vacancies, on the shock response is presented and the ability of shear to significantly lower the phase transition pressure, in accord with experimental observation, is demonstrated.     

Superhard and superelastic films of polymeric C60

The C₆₀ thin film deposited on steel substrate was transformed by high pressure–high temperature treatment to a superhard and superelastic material. The films were studied by Raman spectroscopy in situ at 20 GPa after heating at 300°C and ex situ after the quenching. The hardness and elastic properties of the high-pressure phases have been characterized with nanoindentation. The hardness of the films were determined to be 0.5±0.1 GPa and 61.9±9 GPa for unmodified C₆₀ and HPHT treated films, respectively. The hardness of the pressurized film is higher than for cubic BN but lower than hardness values reported for ultrahard fullerite samples prepared from powders. An interesting observation was that the HPHT treated film showed an extreme elastic response with an elastic recovery of approximately 90%.     

Phase transformations in shock-compressed ytterbium

In order to study the phase transformations of ytterbium under shock compression, the electrical resistance of ytterbium at the initial temperatures of 77 and 290 K and a shock pressure of p ? 20 GPa is measured. The dependence of ytterbium resistance on pressure is nonmonotonic and indicates three successive phase transitions. At p ≈ 2 GPa, ytterbium enters a state with a high electrical resistance of the semiconductor type. The ytterbium bandgap at p ≈ 1.8 GPa is estimated as ≈0.02 eV. At p ≈ 3 GPa, the electrical resistance of ytterbium decreases due to a polymorphic phase transition The electrical resistance grows with further increase in pressure, and at p > 11 GPa, it does not change. The nature of the third transition is determined by calculating the temperature of the sample under shock compression. Analysis of the dependence of sample temperature on shock pressure, together with the phase diagram of ytterbium, suggests that the third transition is caused by ytterbium melting.     

Reversible pressure-induced polymerization of Fe(C5H5)2 doped C70

High pressure Raman, IR and X-ray diffraction (XRD) studies have been carried out on C₇₀(Fe(C₅H₅)₂)₂ (hereafter, “C₇₀(Fc)₂”) sheets. Theoretical calculation is further used to analyze the Electron Localization Function (ELF) and charge transfer in the crystal and thus to understand the transformation of C₇₀(Fc)₂ under pressure. Our results show that even at room temperature dimeric phase and one dimensional (1D) polymer phase of C₇₀ molecules can be formed at about 3 and 8 GPa, respectively. The polymerization is found to be reversible upon decompression and the reversibility is related to the pressure-tuned charge transfer, as well as the overridden steric repulsion of counter ions. According to the layered structure of the intercalated ferrocene molecules formed in the crystal, we suggest that ferrocene acts as not only a spacer to restrict the polymerization of C₇₀ molecules within a layer, but also as charge reservoir to tune the polymerization process. This supplies a possible way for us to design the polymerization of fullerenes at suitable conditions.     

Domain evolution and phase transition dynamics of BaTiO3 ferroelectric single crystal under high pressure with various loading methods: Phase field simulation

The microstructure and macroscopic properties of ferroelectric materials at high pressure are of great interest in both the engineering and scientific arenas. The effect ofthe pressure value, loading time (the time taken for the pressure to increase from atmospheric pressure to the highest pressure) and loading direction on the evolution of domains and the ferroelectric phase transition for a BaTiO₃ single crystal was investigated using a phase field approach. It was found that under symmetrical compression loading the pressure loading time affected the phase transition path and rate but did not affect the phase transition pressure or the ultimate stable phase. For example, at room temperature, even when the loading time increased from 1 ns to 10 μs, the phase transition pressure remained stable at 2.1 GPa, but the phase transition time was prolonged. At −70 °C the orthorhombic–cubic phase transition was induced when the loading pressure was 5 GPa and the loading time was 1 ns, whereas the orthorhombic–tetragonal–cubic phase transition occurred when the loading time increased to 10 μs. In addition, it was found that the application of symmetrical pressure tended to reduce the degree of ferroelectricity, while one-dimensional compression favored the ferroelectric phase.     

Phase transitions in shock-loaded titanium at pressures up to 150 GPa

Phase transformations in VT1-0 titanium were studied. Shock profiles in the pressure range of 10–26 GPa were recorded by polyvinylidene fluoride sensors. Sound velocities in shock-compressed titanium samples were measured by two methods. At a pressure less than 30 GPa, the speed of sound in titanium was determined by the counter unloading method using Manganin gauges, and at a pressure of 30–150 GPa, it was determined by the overtaking unloading method using indicator liquids. At a pressure of 20–40 and 60–90 GPa, the pressure dependences of the speed of sound have breaks, the first of which is apparently associated with the α → ω conversion, and the second with melting. X-ray analysis revealed the presence of the ω phase in the samples in steel capsules recovered after loading at a pressure of 9–23 GPa. The dependence of the yield of the ω phase on the loading pressure has the form of a curve with a maximum at p ≈ 15 GPa.     

Synthesis of a hexagonal modification of C60 using cryoextraction

Samples of the hexagonal close-packed phase (h.c.p.) of fullerite C₆₀ with a low (less then 7%) content of more stable face-centered cubic (f.c.c.) phase were synthesized using cryoextraction with n-hexane. X-Ray diffraction analysis and thermogravimetry were applied for characterization of the samples. The role of n-hexane in the formation of the h.c.p. structure is discussed.     

Hydrogen in fullerites

The peculiar kind of fullerene molecule structure is also reflected in the crystal structure of fullerites. The cubic lattices of metal fullerides and hydrofullerenes behave similarly to the cubic lattices of different metals and alloys. They form interstitial solid solutions when impurity elements are distributed in octahedral and tetrahedral interstitial sites. By replacement of C₆₀ and C₇₀ molecules in lattice sites they make up substitution solid solutions. Forming exo- and endohedral compounds, the fullerene molecules, located in sites of the crystal lattice, can modify greatly the crystal properties with no change of crystalline structure. Some peculiarities of fullerite crystalline structures will be discussed in the present paper.     

Effect of pressure on electric generation of PZT(30/70) and PZT(52/48) ceramics near phase transition pressure

Impact experiment of Pb(Zr_0.3Ti_0.7)O₃ and Pb(Zr_0.52Ti_0.48)O₃ ceramics were conducted by empolying shock reverberation techniques within 3-7 GPa and X-ray diffraction patterns of these materials have been measured at pressure up to 32 GPa with a diamond anvil cell and synchrotron radiation. To refine the crystal structure, Rietveld analysis was performed and bulk moduli were calculated using Birch-Murnaghan equation of state. We found a tetragonal phase transforming to a cubic phase in Pb(Zr_0.3Ti_0.7)O₃ and Pb(Zr_0.52Ti_0.48)O₃ ceramics at ∼7.4 GPa and ∼4 GPa respectively. For dynamic pressure experiment, a metal flyer accelerated by a gas gun facility impacts into PZT ceramics to investigate electric energy. As pressure increased, output voltage of Pb(Zr_0.3Ti_0.7)O₃ and Pb(Zr_0.52Ti_0.48)O₃ ceramics slightly increased below ∼7 GPa and ∼4 GPa. But the voltage increased near ∼7 GPa and ∼4 GPa. From the result, we could confirm that the phase transition influenced the considerable effect on the electrical power generation.     

Structure and stability of carbon nitride under high pressure and high temperature up to 125 GPa and 3000 K

The crystal structure of carbon nitride under high pressure and temperature was investigated up to megabar pressures using graphitic C₃N₄ as a starting material. It transformed to an orthorhombic phase above 30 GPa and 1600 K, which has a similar unit cell parameters (a = 7.6251(19), b = 4.4904(8), and c = 4.0424(8) Å at 1 atm) to those of reported hydrogen-bearing carbon nitride phases such as C₂N₂(NH) and C₂N₂(CH₂). Although the C:N ratio of this orthorhombic phase was carefully determined to be 3:4, FT-IR analysis showed a strong possibility of hydrogen contamination both in the starting and recovered samples. These results suggest that in the studied wide pressure and temperature range, hydrogen-bearing carbon nitride favors the orthorhombic structure with a fundamental composition of C₂N₂X where NH, CH₂, and even potentially vacancies can be flexibly accommodated in the X site.     

Nanoindentation of diamond,graphite and fullerene films

The recently developed method of nanoindentation is applied to various forms of carbon materials with different mechanical properties, namely diamond, graphite and fullerite films. A diamond indenter was used and its actual shape determined by scanning force microscopy with a calibration grid. Nanoindentation performed on different surfaces of synthetic diamond turned out to be completely elastic with no plastic contributions. From the slope of the force–depth curve the Young's modulus as well as the hardness were obtained reflecting a very large hardness of 95 GPa and 117 GPa for the {100} and {111} crystal surfaces, respectively. Investigation of a layered material such as highly oriented pyrolytic graphite again showed elastic deformation for small indentation depths but as the load increased, the induced stress became sufficient to break the layers after which again an elastic deformation occurred. The Young’s modulus was calculated to be 10.5 GPa for indentation in a direction perpendicular to the layers. Plastic deformation of a thin fullerite film during the indentation process takes place in the softer material of a molecular crystalline solid formed by C₆₀ molecules. The hardness values of 0.24 GPa and 0.21 GPa for these films grown by layer epitaxy and island growth on mica and glass, respectively, vary with the morphology of the C₆₀ films. In addition to the experimental work, molecular dynamics simulations of the indentation process have been performed to see how the tip–crystal interaction turns into an elastic deformation of atomic layers, the creation of defects and nanocracks. The simulations are performed for both graphite and diamond but, because of computing power limitations, for indentation depths an order of magnitude smaller than the experiment and over indentation times several orders of magnitude smaller. The simulations capture the main experimental features of the nanoindentation process showing the elastic deformation that takes place in both materials. However, if the speed of indentation is increased, the simulations indicate that permanent displacements of atoms are possible and permanent deformation of the material takes place.     

Shock-induced high-concentration nitrogen doping of titania

High-concentration nitrogen-doped titania is obtained by detonation-driven flyer impacting on mixtures of TiO₂ and different nitrogen precursors. XRD, IR, and XPS spectra are employed to characterize the phase composition, surface absorption, and N-doping concentration of recovered samples. The N-doping concentration is affected by doping nitrogen resources, initial content of doping nitrogen resources, and flyer velocity. A high nitrogen concentration of 13.6 at.% is achieved by shock loading of the mixture of P25 TiO₂ and 10 wt.% dicyandiamide (C₂N₄H₄) at 3.37 km/s. A possible shock doping mechanism is discussed.     

Microstructure of Shocked Preheated Bismuth and Detection of Melting at Pressures of 1.6–2.4 GPa

The structure of bismuth samples after shock-wave loading at pressures of 0.7–2.4 and 22–32 GPa was studied. Before loading, the samples were at room temperature or heated to 230–240°C. Loading by a pressure of 1.5–2 GPa at an initial temperature of 233–240°C led to a structural change in bismuth, indicating melting of the sample in the shock wave. The time of shock-wave loading was ≈0.7 μs.     

Phase transitions in single crystal tubes formed from C60 molecules under high pressure

Pure single crystal tubes formed from C₆₀ molecules, with a face-centered cubic (fcc) structure were fabricated by a liquid–liquid interfacial precipitation method using C₆₀ powder. A bulk transition from fcc to a simple cubic structure and a surface transition from (1 × 1) to (2 × 2) have been observed around 246 K (bulk transition temperature T_B) and 214 K (surface transition temperature T_S), respectively, during the measurement of the temperature dependence of electrical resistance. The initiation of the two transitions under pressure was investigated using a piston cylinder high pressure apparatus and it was found that both T_B and T_S increase with increasing pressure. And the C₆₀ molecules at the surface of the tube exhibit the same behavior of that in the bulk at a pressure of about 2.1 GPa.