Anisotropic Nanoclustered Carbon Phases Prepared from Fullerite C60 Under Non‐hydrostatic High Pressure

Abstract

We show that application of the non‐hydrostatic pressure to the cluster‐based molecular material, like fullerite C₆₀, provides a new opportunity to create elastically and structurally anisotropic carbon materials, including 2D polymerized rhombohedral C₆₀ and graphite‐type (sp ²) disordered atomic‐based phases. The elastic anisotropy, detected by the difference in the ultrasound velocities propagating along and across the loading axis, is directly confirmed by the results of x‐ray diffraction.     

Equilibrium Phase Diagram of Polymerized C60 and Kinetics of Decomposition of the Polymerized Phases

Abstract

Equilibrium phase diagram (T=0–1000 K; p=0–8 GPa) of pressure‐temperature polymerized C₆₀ has been constructed. It included rhombohedral; orthorhombic and tetragonal polymerized phases of C₆₀ along with fcc monomeric phases of C₆₀. The isothermal kinetics of depolymerization (T=373–513 K) of the O‐, R‐, T‐ and D‐phases was studied by means of the IR‐spectroscopy. The isothermal rate of decomposition goes up along the series: (T‐)<(O‐)<(R‐)<(D‐).     

Photoluminescence Study of the Two‐Dimensional Tetragonal and Rhombohedral Polymers of C60 at High Pressure

Abstract

The pressure behavior of the photoluminescence (PL) spectra of the planar polymeric phases of C₆₀ have been studied at pressure up to 4 GPa. The PL spectra and the pressure‐induced shift of the principal bands differ considerably for the pristine C₆₀, two‐dimensional tetragonal (2D‐T) and rhombohedral (2D‐R) polymers of C₆₀. The changes in the PL spectra may be related with the transformation of the electron energy spectrum of polymers.     

B3–B1 phase transition and pressure dependence of elastic properties of ZnS

We have performed the ab initio calculations based on density functional theory to investigate the B3–B1 phase transition and mechanical properties of ZnS. The elastic stiffness coefficients, C₁₁, C₁₂, C₄₄, bulk modulus, Kleinman parameter, Shear modulus, Reuss modulus, Voigt modulus and anisotropy factor are calculated for two polymorphs of ZnS: zincblende (B3) and rocksalt (B1). Our results for the structural parameters and elastic constants at equilibrium phase are in good agreement with the available theoretical and experimental values. Using the enthalpy–pressure data, we have observed the B3 to B1 structural phase transition at 18.5 GPa pressure. In addition to the elastic coefficients under normal conditions, we investigate the pressure dependence of mechanical properties of both phases: up to 65 GPa for B1-phase and 20 GPa for B3-phase.     

Effect of pressure on the structural and elastic properties of ZnS and MgS alloys in the B3 and B1 phases

From the first-principles calculations, we have investigated the elastic stiffness coefficients C₁₁, C₁₂, C₄₄ and the bulk modulus B of the II-VI semiconductors ZnS and MgS under hydrostatic pressure. The calculations are based on the density functional theory within the generalized gradient approximation (GGA) for exchange-correlation interaction. For the structural properties we have shown that ZnS adopt the rocksalt (NaCl or B1) structure over 11.87 GPa pressure, the same character is adopted by MgS over 0.8 GPa. The elastic coefficients have the same behavior for the different structures of alloys; they increase with increasing pressure values. Our results for the structural parameters and equilibrium phase elastic constants are in good agreement with the available theoretical and experimental data.     

Structural and Vibrational Properties of Li‐ and Na‐Doped Fullerene Polymers

Abstract

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.     

The elastic stiffness constants and their hydrostatic pressure derivatives for TGS

The thirteen adiabatic elastic stiffness moduli of triglycine sulphate (TGS) have been determined at room temperature from measurements of the ultrasound wave velocities. Results are used to describe the elastic behaviour of TGS. Accidental pure mode directions have been found at 9.4° and 106.5°. A method has been developed for obtaining the hydrostatic pressure derivatives (∂C_IJ/∂P) of the elastic constants of a monoclinic crystal from the hydrostatic pressure dependences of ultrasound wave velocities. This method has been applied to find the ∂C_IJ/∂P for TGS. There is no evidence for acoustic phonon mode softening under pressure.     

On the Action of Ultraviolet Light on C70 Fullerene

Abstract

C₇₀ was photopolymerized in solution of CCl₄ under nitrogen flow by an high pressure mercury lamp. The resulting photoproduct was still soluble in common solvents and has been studied by electronic and FT-IR spectroscopy. FT-IR spectroscopy reveals a very peculiar band pattern never reported to date for C₇₀ photoproduct. Since this band pattern is very similar to that of C₆₀ photopolymer, it suggests that C₇₀ photoproduct should have a chemical structure very close to that assigned to C₆₀ photopolymer and piezopolymer.

The role played by CCl₄ in C₇₀ photopolymerization seems to be comparable to that already reported for C₆₀, i.e. it acts as a polymerization promoter.     

Yield behavior of porous nuclear fuel (UO2)

Uranium dioxide (UO₂) is one of the most common nuclear fuels. During burn-up, the fuel undergoes substantial microstructural changes including the formation of pressurized pores, thus becoming a porous material. These pores reduce the elastic modulus and alter the yield behavior of the material. In this work, a finite-element-based homogenization technique has been used to map the yield surface of UO₂ with pressurized pores. Two scenarios are considered; in the first, the fuel matrix is a ductile material with a Von-mises type behavior, while in the second, the matrix is quasi brittle, which is simulated using the concrete damaged plasticity (CDP) model available in ABAQUS. For both of the scenarios, it is found that the yield strength decreases with an increase in porosity for a given internal pore pressure. For a given porosity, the yield surface shifts towards the negative hydrostatic axis in the Haigh-Westergard stress space with an increase in pore pressure. When the matrix is quasi brittle, the decrease in tensile hydrostatic strength is less than the increase in compressive hydrostatic strength, whereas in the case of a ductile matrix, the changes in the hydrostatic strengths are same. Furthermore, the shape of the yield surface changes from one deviatoric plane to another in both scenarios. Analytical equations, which are functions of pore pressure and porosity, are developed to describe the yield surface of porous UO₂ while accounting for the changes in shape of the yield surface from one deviatoric plane to another. These yield functions can be used to predict the failure of porous UO₂ fuel.     

Some New Aspects of Chlorination of Fullerenes

Abstract

The effect of the reagent ratio, reaction time and power of the reagent on the product composition in chlorination of [60]fullerene was studied. Chlorofullerenes C₆₀Cl₆, C₆₀Cl₈, C₆₀Cl₁₀, C₆₀Cl₁₂, C₆₀Cl₁₄, and C₆₀Cl₂₆ were synthesized and characterized by chemical analysis, FTIR, ¹³C NMR, and MALDI TOF mass spectrometry. The experimental data supported the coexistence of several isomers of C₆₀Cl _n (n = 8, 10, 12, 14, 26); the mixtures were not separated so far. Semiempirical calculations (AM1, PM3) were used to analyze the addition patterns and resulted in the most favorable structures of C₆₀Cl_8–26. Chlorination of C₇₀ under various conditions invariably yielded C₇₀Cl₁₀.     

Statistical Mechanical Study of Thermodynamic Properties of a Family of Fullerites from C36 to C96 in the Equilibrium with Their Vapors

Abstract

The temperature dependence of saturated vapor pressure of fullerites C₃₆–C₉₆ and their thermodynamic properties along with the sublimation curves up to the spinodal are calculated. The saturated vapor pressures along all the range of temperatures are approximated by logP _sat = A ? (B/T) ? CT. The coefficient A practically does not depend on the number of atoms in the molecule (varying only by 2.2%), B increases noticeably while C decreases from the C₃₆ to the C₉₆, both of them by approximately two. The isothermal bulk modulus B _T and the shear modulus C₄₄ vanish at the spinodal points.     

Synthesis,Spectral Investigations,and Thermal Stability of [60]Fullerene Complexes with TMTSF: 2(C60) · 2(TMTSF) · (C6H6), C60 · TMTSF · 2(CS2), and 2(C60) · 2(TMTSF) · (C6H5Cl)

Abstract

Molecular complexes of [60]fullerene with tetramethyltetraselenafulvalene (TMTSF): C₆₀ · (TMTSF) · 2(CS₂) (1), 2(C₆₀) · (2(TMTSF) · (C₆H₆) (2), and 2(C₆₀) · (2(TMTSF) · (C₆H₅Cl) (3) have been synthesized and their thermal stability and IR spectra vs. light polarization and temperature have been performed.     

He+ Ion Bombardment of C70 Fullerene: An FT‐IR and Raman Study

Abstract

C₇₀ fullerene films deposited on a silicon substrate have been bombarded with He⁺ ions at 30 keV at room temperature in vacuum. The structural changes undergone by C₇₀ have been followed by both FT‐IR and Raman spectroscopy. The results have been compared to the behavior of C₆₀ fullerene and discussed in an astrochemical context. The main conclusion is that C₇₀, contrary to C₆₀, does not form oligomers at low radiation dose but it is directly and gradually degraded to amorphous carbon (carbon black).     

Singlet‐Triplet Transition in the C60 2− Dianion

Abstract

The C₆₀ complexes with decamethylcobaltocene: (Cp*₂Co)₂C₆₀(C₆H₄Cl₂, C₆H₅CN)₂ (1) and [K · (18‐crown‐6)]₂ · C₆₀ · (DMF)₄ (2) have been obtained as single crystals by the diffusion method. The IR‐ and UV‐VIS‐NIR‐spectra justify the formation of the C₆₀ ^2? dianions in these salts. EPR measurements show that the low temperature signals of 1 in the 4–140 K range and 2 in the 4–60 K range have intensity corresponding only to 0.4% and 3.5% from total C₆₀. Because of this, most of the complexes are EPR silent, and, consequently, C₆₀ ^2? has a diamagnetic singlet (S = 0) state in these temperature ranges. The appearance of a broad EPR signal in the spectum of 1 above 140 K and 2 above ～60 K is assigned to a thermal population of a close lying excited triplet (S = 1) state. The singlet–triplet energy gap for C₆₀ ^2? in solid 1 and 2 was estimated to be 730 ± 10 and 300 ± 10 cm^?1.     

Light Scattering Study of Aqueous Poly(Vinyl)pyrrolidone–Fullerene C70 Solutions

Abstract

It was shown by static and dynamic light scattering that poly(vinyl)pyrrolidone (PVP) molecules form large intermolecular complexes (clusters) with C₇₀ in aqueous solutions. The molecular weights and dimensions of PVP–C₇₀ clusters increase both with the increase of fullerene content and the molecular weight of the matrix PVP. However, two different diffusion coefficients were detected by dynamic light scattering. The slow mode was explained as diffusion of large PVP–C₇₀ clusters. The fast mode represents free PVP molecules in solution. Dimensions of clusters revealed in aqueous PVP–C₇₀ solutions are less than that for PVP–C₆₀ by factor of 2.5–3.     

High-pressure structural phase transitions and mechanical properties of calcite rock

In the present work, a state of the art first principles theory is used to examine the structural and mechanical properties of calcium carbonates CaCO₃. Our calculations allowed full structural relaxation, which permits an appropriate evaluation of material properties at ambient conditions as well as under hydrostatic pressure. Compared to experimental measurements the calculated ground state properties show a suitable agreement. By performing a structural phase stability analysis, we were able to predict both first and second order phase transitions that calcium carbonates minerals undertake under hydrostatic pressure. The first one occurs between the calcite and aragonite phases at 3.3 GPa and the second one between the aragonite and post-aragonite phases at ～40 GPa. The previous value agree very well with experimental one (40 GPa) reported by Ono et al. In order to verify the reliability of such phase transitions, we study the mineral high pressure stability by means of mechanical properties behaviour. Both transversal wave velocity and elastic moduli show an unexpected decrease at phase transition pressure range.     

Ti2SiC在高压下结构、弹性和电子性质的第一性原理研究

采用第一性原理方法,研究了Ti_2SiC在高压下的结构、弹性和电子性质。结果表明,随着外压的增大,Ti_2SiC的晶格常数a、c和体积V均减小,且a比c减小幅度更大,表明Ti_2SiC在a轴方向比c轴方向更容易被压缩,体现了该材料的各向异性。计算分析了Ti_2SiC的弹性常数、体模量、剪切模量、杨氏模量、泊松比等弹性性质,这些弹性性质均随着外压的增加而增大,并根据弹性常数证明了Ti_2SiC在0~50GPa范围内均是力学稳定的。此外,还从电子态密度的角度考察了Ti_2SiC的电子性质,认为其具有共价键和金属键的双重性质,并发现在0~50GPa范围内压力对Ti_2SiC的态密度性质影响较小。     

Ab Initio Study of the Electronic Structure,Elastic Properties,Magnetic Feature and Thermodynamic Properties of the Ba<Subscript>2</Subscript>NiMoO<Subscript>6</Subscript> Material

We report first-principles calculations of the elastic properties, electronic structure and magnetic behavior performed over the Ba₂NiMoO₆ double perovskite. Calculations are carried out through the full-potential linear augmented plane-wave method within the framework of the Density Functional Theory (DFT) with exchange and correlation effects in the Generalized Gradient and Local Density Approximations, including spin polarization. The elastic properties calculated are bulk modulus (B), the elastic constants (C₁₁, C₁₂ and C₄₄), the Zener anisotropy factor (A), the isotropic shear modulus (G), the Young modulus (Y) and the Poisson ratio (υ). Structural parameters, total energies and cohesive properties of the perovskite are studied by means of minimization of internal parameters with the Murnaghan equation, where the structural parameters are in good agreement with experimental data. Furthermore, we have explored different antiferromagnetic configurations in order to describe the magnetic ground state of this compound. The pressure and temperature dependence of specific heat, thermal expansion coefficient, Debye temperature and Grüneisen parameter were calculated by DFT from the state equation using the quasi-harmonic model of Debye. A specific heat behavior C_V?≈?C_P was found at temperatures below T = 400 K, with Dulong–Petit limit values, which is higher than those, reported for simple perovskites.     

Pressure derivatives of elastic constants of monoclinic M(NO3)3·9H2O (M=Al,Fe, Cr) and third-order elastic constants of Al(NO3)3·9H2O

The elastic constants and their pressure and temperature derivatives of monoclinic M(NO₃)₃·9H₂O (M=Al, Fe, Cr) have been determined from ultrasonic resonance frequencies of thick plates and from their temperature- and pressure-induced shifts. In addition, the complete third-order elasticity tensor of Al(NO₃)₃·9H₂O has been evaluated from a combination of measurements of shifts of ultrasonic resonance frequencies produced by hydrostatic pressure and uniaxial stresses in various directions. The elasticity tensors of the three isotypic salts and their temperature and pressure derivatives are almost identical. The elasticity tensor exhibits a distinct anisotropy as well as the tensor of thermal expansion. The thermoelastic constants behave quite normally, whereas the pressure derivatives show anomalous features, i.e. some of them are negative similar to the situation in cubic Ba(NO₃)₂ and α-ammonium or α-selenate alums. These effects are accompanied by a very small, even negative, thermal expansion. The third-order elastic constants of Al(NO₃)₃·9H₂O also reflect this anomalous behaviour.     

Raman Modes and Stability at High Pressure of the Two‐Dimensional Rhombohedral Polymer of C60

Abstract

Phonon modes and stability of the planar rhombohedral polymer of C₆₀ have been studied at pressure up to ～30 GPa by means of in situ Raman scattering. At P ～ 15 GPa the phonon frequencies show an irreversible transition to a new phase related with random covalent bonding between the molecules in adjacent polymeric sheets.