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.     

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.     

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.     

Macroscopic Elastic Studies of Orientational Ordering in Single-Crystal Fullerites

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.     

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.      

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.

     

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.     

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.     

Orientational ordering in solid D2 from NMR studies. I. The hcp phase

Using a coherent pulsed NMR spectrometer operating at 5.9 MHz, measurements have been made of the free induction decay and of the solid echo in hcp solid D₂. From these data, and using Fourier transform techniques, the NMR line shapes and longitudinal relaxation times have been found for both o-D₂ (angular momentumJ=0 and spinI=2) and p-D₂ (withJ=1 andI=1) separately. The concentration and temperature extended over the ranges 0.05X(J=1)0.56 and 0.04<T<3 K, respectively. For both o-D₂ and p-D₂, the second moment of the NMR line shape rises smoothly as the temperature decreases, while the relaxation times pass through a minimum. No evidence of a thermal hysteresis could be found in the line shape or relaxation times. The orientational ordering increases continuously as the temperature decreases and the results lead to the conclusion that there is no evidence for a well-defined transition into a phase that has the characteristics of a glassy state, at least above 0.05 K. Calculations of the rms order parameter for p-D₂ as a function ofX andT are presented and the results compared with those for solid H₂. Above 0.5 K, the agreement is very good, while below 0.3 K, (H₂)>(D₂). The orientational polarization of theJ=0 molecules in D₂ by the surroundingJ=1 molecules is measured from the ratio of the lineshape second moments and is found to be in order-of-magnitude agreement with the predictions by A. B. Harris in the high-temperature limit. An analysis and correlation of the various measured relaxation times via energy diffusion models is presented. From the spin-lattice relaxation times of theJ=1 molecules, the orientational fluctuation rates are estimated for various concentrations as a function ofT and compared with the results from H₂. Good agreement is found. Some unusual features in the intensity ratio of theI=1 andI=2 signals that is different from the expected one are described. In an Appendix, corrections made to the observed line shapes to compensate for the instrumental limitations of the pulse spectrometer are outlined.     

有序度对Ni2Cr合金氢致脆断的影响

研究了环境气氛、动态渗氢及预渗氢对不同有序度Ni2Cr合金脆化的影响.结果表明,有序度对Ni2Cr合金在室温空气及氢气中的环境氢脆没有明显影响;动态渗氢拉伸时Ni2Cr合金存在严重的氢脆敏感性,无序和高度有序合金比部分有序合金脆化严重;预渗氢时,氢原子通过晶格扩散及晶界扩散进入合金,Ni2Cr合金氢脆敏感性随有序度的增大而减小.     

Orientational ordering in solid D2 from NMR studies. II. The cubic ordered phase

An NMR study in the cubic phase of D₂ which exhibits long-range orientational order is reported. The spin systems in both para D₂ (with angular momentum J=1 and spinI=1) and in ortho D₂ (with J=0 andI=2) were investigated. The integrated intensity ratio of theI=1 andI=2 components in the solid echo, the NMR line shapes obtained by Fourier transforming the solid echoes, and the longitudinal relaxation times for theI=1 andI=2 systems are presented. Samples with D₂ concentrationX between 0.69 and 0.62 were studied over the temperature range 0.12<T<4 K. Measurements were carried out before and after repeated thermal cycling through the ordering transition to study the effect of this cycling on the temperature dependence of the relaxation times. The complex behavior of these relaxation times was found to be similar to that in cubic H₂ at a comparable concentration, and the interpretation of this behavior is discussed. Over most of the temperature range, the relaxation time of theI=2 spins is larger than that of theI=1 spins and is believed to be determined by cross-relaxation with the latter, whose intrinsic spin-lattice relaxation is observed. However, at low enough temperatures, the relaxation time for theI=2 spins is found to be shorter than that for theI=1 spins, and this observation cannot be understood on the basis of predictions. Furthermore, theI=2 relaxation time in this temperature region is found to depend on the position within the NMR line. Another observation not yet understood is that the ratio of the integrated intensitiesS(I=1)/S(I=2) in the solid echo is smaller than the theoretically predicted one by about 20%. The NMR line shapes for theI=1 spins in the ordered phase obtained from pulse measurements are compared with those from continuous-wave methods and to those from H₂.     

Orientational Order Parameter in Molecular Cryocrystals from Diffraction Intensities

A method for the determination of absolute values of orientational order parameter from diffraction intensities in molecular crystals is suggested and implemented for a few molecular cryocrystals. The method is based on an expansion of the scattering amplitude in spherical harmonics of molecular orientations. This method is applied for a few particular orientationally ordered cryocrystals of different crystal symmetries (α-nitrogen, α-oxygen, orientationally ordered J = 1 deuterium, and Ar-CO₂ mixed crystals). Prospectives of the method’s further applications are discussed.PACS numbers: 61.10 −i, 61.66 −f, 65.70 +y     

Thermodynamic Properties of Molecular Orientational Glasses with Indirect Interaction

The general regularities of the behaviour of heat capacity and thermal expansion of solutions of substances with simple linear molecules in solidified inert gases at concentrations of about 1 mol % when the direct quadrupole interaction between impurities does not play an important role are discussed. In such solutions indirect (static and dynamic) interactions between impurity molecules (rotators) are dominant. The static interactions are realized through deformation fields produced by impurities in a lattice. The dynamic ones are performed by the phonon exchange between impurity molecules. Under certain conditions the indirect interactions mentioned lead to stochasticity of an energy spectrum of the subsystem of rotators and the appearance of an orientational glass–like state. Special attention is paid to the behaviour of the systems in which an energy spectrum of rotational motion of impurity molecules is determined by intramolecular degrees of freedom. The experimental results of the studies of thermal properties of N ₂ and O ₂ in solid Ar and Kr are presented.     

Orientational Order in Solid N2O-Kr Solutions

Structure of solid N₂O-Kr mixtures was studied with the transmission electron diffraction technique. The measurements were made at temperatures between 6 K and 44 K. The upper boundary of the temperature interval was dictated by the intensive sublimation of krypton. The mixture compositions varied from 0 to 50 mol.% Kr. A phase separation in the N₂O-Kr system was not observed. The crystallographic structure of the investigated, solutions corresponds to the fcc lattice The effect of krypton impurity atoms on the orientational order of the N₂O molecules in solid solutions was also studied. The kinetic of relaxation processes induced by annealing of the low-temperature films are considered.     

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.     

Ultrasonic Study of the Ordering of ortho–Hydrogen in Quench–Condensed Films

The ultrasonic attenuation and dispersion of quench–condensed H ₂ films was measured for ortho–concentrations ranging from x = 75% to x = 32%. Orientational ordering was observed for all ortho–H ₂ concentrations. The obtained results are compared to recent specific heat experiments by Birmingham et al.     

Molecular Ordering at the Interface Between Liquid Water and Rutile TiO2(110)

The pivotal importance of TiO₂ as a technological material involves most applications in an aqueous environment, but the single‐crystal TiO₂/bulk‐water interfaces are almost completely unexplored, since up to date solid/liquid interfaces are more difficult to access than surfaces in ultrahigh vacuum (UHV). Only a few techniques (as scanning probe microscopy) offer the opportunity to explore these systems under realistic conditions. The rutile TiO₂(110) surface immersed in high‐purity water is studied by in situ scanning tunneling microscopy. The large‐scale surface morphology as obtained after preparation under UHV conditions remains unchanged upon prolonged exposure to bulk water. Moreover, in contrast to UHV, atomically resolved images show a twofold periodicity along the [001] direction, indicative of an ordered structure resulting from the hydration layer. This is consistent with density‐functional theory based molecular dynamics simulations where neighboring interfacial molecules of the first water layer in contact with the bulk liquid form dimers. By contrast, this dimerization is not observed for a single adsorbed water monolayer, i.e., in the absence of bulk water.