Macroscopic Elastic Studies of Orientational Ordering in Single-Crystal Fullerites

Abstract

We give an overview on dynamic mechanic experiments in single-crystal fullerites C₆₀ and C₇₀. Elastic properties and thermal expansion in the vicinity of ordering phase transitions were studied. Landau theory and simple microscopic models are used to describe the experimental results.

     

Kinetic Properties of Orientational Phase Transition in Polymerized States of Fullerites C60

Abstract

In the present work, we have shown that at early stages of dimerization, which occur during synthesis of C₆₀ fullerite T _syn = 10–40°C and P _syn = 1.5–8 GPa, kinetics of the sc→fcc phase transition, may be well described by Avrami law with Avrami's exponent n _Avr = 3 (i.e., in this case we are dealing with martensite‐like transformation). Fullerite's samples produced at higher temperatures (40°C < T _syn < 120°C) exhibit different kinetics with lower Avrami‐exponent. This behavior we attribute to the transformation switching to diffusion‐controlled kinetics.     

Molecular Orientational Ordering in Solid C60

Abstract

Single-crystal X-ray diffraction studies of the orientational ordering in solid C₆₀ are reviewed. The temperature dependence of selected Bragg reflections was carefully examined, allowing to obtain original information on the first-order orientational ordering transition at T_o = 259K, the unusual behavior of both fundamental and superstructure reflections below T_o and the freezing in of the C₆₀ reorientations at T_g ≈ 85K. The diffuse intensity due to orientational disorder of the C₆₀ molecules at room temperature is found to be strongly modulated, both radially and azimuthally. showing that the molecular orientations are indeed correlated. The corresponding intensity distribution has been calculated within a mean-field theory for different microscopic models of inter molecular interactions. It should allow a better understanding of these interactions.     

Molecular Orientational Ordering in Solid C60

Single-crystal X-ray diffraction studies of the orientational ordering in solid C₆₀ are reviewed. The temperature dependence of selected Bragg reflections was carefully examined, allowing to obtain original information on the first-order orientational ordering transition at T_o = 259K, the unusual behavior of both fundamental and superstructure reflections below T_o and the freezing in of the C₆₀ reorientations at T_g ≈ 85K. The diffuse intensity due to orientational disorder of the C₆₀ molecules at room temperature is found to be strongly modulated, both radially and azimuthally. showing that the molecular orientations are indeed correlated. The corresponding intensity distribution has been calculated within a mean-field theory for different microscopic models of inter molecular interactions. It should allow a better understanding of these interactions.     

The Orientational Ordering of Molecular Hydrogen in 2D

The orientational ordering of ortho-hydrogen molecules on a triangular lattice is calculated as a function of ortho concentration using the method of restricted traces. The calculated temperatures for the transitions from the disordered phase to the long range pinwheel ordered phase show a strong concentration dependence with a critical concentration X _c=73% below which the long range ordering is not realized in this model. The results are consistent with recent NMR studies of ortho-para hydrogen mixtures adsorbed as a commensurate × structure on hexagonal boron nitride for which X _c=69±4%. The values of the observed transition temperatures can only be understood if there is a large renormalization of the effective quadrupolar interaction constant in 2D.     

Orientational Ordering of Electric Quadrupoles in FCC Lattices

Rigid rotors with electric quadrupole moment which are localized at FCC lattice sites have been studied by Monte Carlo simulations. It is found that as the temperature lowers the classical rotors are orientationally ordered to form a Pa3 structure. Molecular solids of nitrogen and orthohydrogen were observed to show this orientational ordering at low temperatures, which is understood to be due to the anisotropic interactions between the molecules. Since most of the anisotropy in the intermolecular interactions can be explained by electric quadrupole-quadrupole interaction, we compare our calculation with the experimental results for solid N₂ and ortho-H₂.      

Orientational Ordering in Monolayer Films of Molecular Hydrogen on Boron Nitride

NMR studies of monolayer hydrogen films for both the expected commensurate solid and the incommensurate solid (for full monolayer coverage) on hexagonal boron nitride (BN) are reported for ortho-H ₂ concentrations 0.30X0.74 and temperatures 0.08<T<4.2 K. Phase diagrams for these systems are mapped out in terms of X and T from clear structural changes of the NMR spectra, including the observation of a 2D orientational quadrupolar glass ordering for X<X _crit =0.69, and long range orientational ordering for X>X _crit for commensurate films.     

Orientational Ordering in Ultra-Thin Films of Quantum Rotors on Boron Nitride

We report the results from NMR studies of submonolayer and monolayer films of solid hydrogen (corresponding to the expected commensurate and incommensurate solid, respectively) on a well defined insulating substrate (boron nitride). The orientational ordering of the ortho-H₂ molecules (J=1 quantum rotors) was explored for ortho-H ₂ concentrations 0.3<X<0.74 and temperatures 0.05<T<4.2 K. The phase diagram for submonolayer hydrogen films (75% of full monolayer) has been identified in terms of X and T from clear structural changes of the NMR spectra, including the observation of a new 2D orientational quadrupolar glass ordering for 0.48<X<0.69 and 0.2T0.6 K. Two distinct long range orientationally ordered phases are identified for monolayer hydrogen films: an expected pinwheel lattice structure for high X and an expected herringbone structure for low X.     

Elastic Energy and Elastic Interaction Ordering of Binary Solid Solutions

A theory in the framework of continuum elasticity has been developed to calculate the totalcontribution of "atomic size effect" or "strain energy effect" to free energy of binary solidsolutions. It is found that elastic free energy consists of two parts f elastic self energy (ESE),and elastic interaction energy(EIE). The former is a function of composition alone, the latter isalso a function of atomic configuration. Minimization of total elastic free energy with respect toatomic arrangement resuIts in an ordered arrangement of atoms, which is caIIed elastic interactionordering (EIO), as it originates from elastic interaction among atoms. EIO is a kind of localordering within a "characteristic range", and it is found to be important in determining the Stateof solid solutions and structures of superlattices. The present theory also gives good explanationto the coexistence of ordering and decomposition which can not be understood in conventionaltheories.     

Critical Concentrations for the Orientational Ordering of Molecular Hydrogen in 2D

Recent NMR studies of molecular hydrogen physisorbed on hexagonal boron nitride have been used to determine the phase diagram for the different types of orientational ordering in 2D. Long range pinwheel orientational ordering is only observed for ortho concentrations greater than a critical concentration of 69±3% for the commensurate solid structure. The increase in the critical concentration for long range ordering of quantum quadrupoles from 55% in 3D to 69% in 2D is discussed in terms of theories of the ordering that include treatments of the correlations between the ortho molecules.     

Photoacoustic Monitoring of the Macroscopic Orientational Order in Disperse Red 1 Azo-Dye-Based Dissolutions

The polarized pulsed-laser photoacoustic (PLPA) methodology was implemented to characterize the macroscopic molecular orientational order as a function of the applied electric field in an azo-dye-based dissolution. It is shown in this work that photoacoustic signals are highly sensitive to the material anisotropy when PLPA analyses are performed. In particular, the macroscopic molecular symmetries and structural organization of rod-like azo-dyes (Disperse Red 1: DR1) prepared in heavily loaded toluene dissolutions, have been studied. Experiments evidence unambiguous information concerning the molecular organization detected via simultaneous optical transmission and PLPA measurements. Indeed, in the electrically poled stage, it is shown that the Fourier, correlation, and root-mean-square analyses performed on the PLPA signals, as functions of the incident linear polarization of the laser source (orthogonal states $P_\mathrm{in}$ and $S_\mathrm{in}$ , respectively), reveal the main optical axis, symmetry, and structure of the aligned compounds in the liquid phase. An important advantage of the PLPA method is that it can be implemented with poor or null optical transmission anisotropic materials.     

Interfullerene Interaction and Properties of Fullerites

The correlation is revealed between the mass of a molecule of C_cn fullerene and the properties of fcc-fullerites such as the sublimation energy, the distance between the centers of the nearest molecules, the GЬ neisen parameter, bulk modulus at zero pressure and temperature. The data of correlation dependences are used to determine the parameters of the Mie-Lennard-Jones pair potential for interfullerene interaction in fcc-fullerites. The thus determined parameters of the potential are used to calculate the properties of fullerites and obtain good agreement with known literature data. It is demonstrated that, at nc ≤ 15 to 20, crystals of C_nc fullerites are unstable. At the same time, fullerites with nc ≥ 100 will have anomalously low values of surface energy, and this must bring about the fragmentation of nanoclusters of C_nc molecules this large. The parameters of the triple and critical points of fullerenes are estimated. A study is made of the evolution of the potential parameters and of the properties identified above with the variation of the molecular mass of fullerene.__________Translated from Teplofizika Vysokikh Temperatur, Vol. 43, No. 3, 2005, pp. 385–395.Original Russian Text Copyright © 2005 by M. N. Magomedov.     

New Phases for the Orientational Ordering of Quantum Rotors: H2 on Hexagonal BN

The phase diagram for the orientational ordering of 2D films of ortho–para H ₂ mixtures has been explored for both commensurate and incommensurate monolayer coverages on hexagonal BN. NMR techniques were used to study ortho concentrations 0.30<x<0.74and temperatures 0.05 <T<4.2. Three distinctly different classes of line shapes were observed and we interpret these in terms of the different phases predicted for the orientational ordering in 2D. At high concentrations a Pake doublet line shape was observed which is characteristic of the pinwheel (PW) ordering predicted by Harris and Berlinsky. For intermediate concentrations a distinctive helmet shaped NMR line is observed which is the signature of a quadrupolar glass state. This line shape was seen only for commensurate coverages. At low concentrations (x<0.45) a new very sharply defined Pake doublet shape is seen which corresponds to a very narrow distribution of order parameters. We interpret this latter phase as a herringbone (HB) ordering.     

Shock-Wave Synthesis of Micron-Size Diamond From Fullerites

Fullerite powders in copper powder matrix were subjected to shock loading in the pressure range of 20-40 GPa. Diamond, nonreacted fee C₆₀ and graphite were detected in all analyzed specimens. The dimensions of synthesized diamond crystallites were up to 6 micron.     

Shock-Wave Synthesis of Micron-Size Diamond From Fullerites

Abstract

Fullerite powders in copper powder matrix were subjected to shock loading in the pressure range of 20–40 GPa. Diamond, nonreacted fee C₆₀ and graphite were detected in all analyzed specimens. The dimensions of synthesized diamond crystallites were up to 6 micron.     

Neutron Diffraction Studies of Nuclear Magnetic Ordering in Silver

Nuclear antiferromagnetism in fcc silver metal, already investigated by NMR measurements, has been studied in a single crystal of ¹⁰⁹ Ag by neutron absorption and diffraction techniques. Below the Neel temperature T_N , a (001) Bragg reflection with a resolution limited width demonstrates long range order in a simple type-I AFM structure with the ordering vector k = (2/a)(001) (up-down structure). The entropy at the transition, ln 2 in zero magnetic field, corresponding to a critical polarization P_c = 0.75 and T_N = 700 ± 80 pK. Magnetic field B versus entropy S phase diagrams of the (001) structure have been constructed for two directions of B: [001] and The critical field extrapolated to S = 0 is 100 ± 10 T, and for both field directions S_c is highest around B = 30 T. The transition to the paramagnetic state is presumably of second order. The nuclear magnetization was measured by transmission of unpolarized neutrons, and the dimensionless static volume susceptibility in SI units was found to be 0.36 ± 0.01 in the ordered state independently of B and S. The ac susceptibility at 7.9 Hz showed a kink at the transition only when the sample was not exposed to neutrons.     

Orientational Ordering of ortho–para Mixtures of Crystals of Homonuclear Diatomic Molecules: Theoretical Evidence for Reentrance

We have developed a theoretical formalism to calculate the orientational phase diagram of ortho–para (or odd-J/even-J) mixtures of homonuclear diatomic molecules in the low-pressure solid phases. In particular, our formalism allows for the explicit disorder present in such mixtrues. While the formalism is general, here we apply it to the quantum anisotropic planar rotor model, a two-dimensional model of coupled rotors. Our calculated phase diagram, separating phases of disorder and short-range order is reentrant, when an equilibrium mixture of odd-J/even-J species is considered. A reentrant phase diagram separating states of disorder and long-range order is known to exist in all-J species in both two and three dimensions. The phase diagram we find for the thermal mixture of odd-J/even-J species exhibits reentrance over a wider range of coupling constants than the corresponding all-J species. We also investigate systems where the odd-J fraction is fixed as a function of temperature. We find that even 1% odd-J mixture exhibits a phase diagram different from the pure even-J case, indicating that the even-J molecules play important role in orientational ordering.     

新型弹性合金研究

阐述了铍青铜弹性合金的发展和应用,讨论了它的优点和缺点所在.同时,讨论了新型弹性合金如Cu-Ni-Sn合金的发展状况,阐述了Cu-Ni-Sn合金的制备工艺和组织性能,最后讨论了新型Cu-Ni-Sn合金的研究发展方向.     

Thermal Stability of C60 and C70 Fullerites

High Temperature - The thermal properties of C60 and C70 fullerenes and fullerites have been studied by X-ray structural analysis and UV spectroscopy. It was shown that C70 fullerite is stabler...     

Macroscopic Studies on the Properties of the Anodic Oxide Films on Titanium

The effect of fluoride ions on the formation and dissolution behaviour of anodic oxide films on Ti has been investigated in acidic fluoride media (pH=1) using impedance and galvanostatic techniques. A5 the fluoride ion concentration and temperature increase the rate of oxide film formation decreases while the dissolution process increases. oxide film formed at high tem-perature and formation voltage was found to contain more defect sites in the film than that formed at a lower one. Activation energies are calculated during the oxide film formation and dissolution and found to be 20.76 and 28.72 kJ/mol, respectively. Formation rate and reciprocal capacitance data are reported as a function of polarizing current density. Values are recorded for the electrolytic parameters A and B. Potentiostatic curves are derived from the galvanostatic results.