Orientational Glass Phase Research Articles

Observation of an Orientational Glass in a Superlattice of Elliptically-Faceted CdSe Nanocrystals.

Extensive prior work has shown that colloidal inorganic nanocrystals coated with organic ligand shells can behave as artificial atoms and, as such, form superlattices with different crystal structures and packing densities. Although ordered superlattices present a high degree of long-range positional order, the relative crystallographic orientation of the inorganic nanocrystals with respect to each other tends to be random. Recent works have shown that superlattices can achieve orientational alignment through combinations of nanocrystal faceting and ligand modification, as well as selective metal particle attachment to particular facets. These studies have focused on the assembly of high-symmetry nanocrystals, such as cubes and cuboctahedra. Here, we study the assembly of elliptically faceted CdSe/CdS core/shell nanocrystals with one distinctive crystallographic orientation along the major elliptical axis. We show that the nanocrystals form an unexpectedly well-ordered translational superlattice, with a degree of order comparable to that achieved with higher-symmetry nanocrystals. Additionally, we show that, due to the particles' faceted shape, the superlattice is characterized by an orientational glass phase in which only certain orientations are possible due to entropically frustrated crystallization. In this phase, the nanocrystals do not exhibit a local orientational ordering but rather have distinct orientations that emerge at different locations within the same domain. The distinct orientations are a result of a facet-to-facet lock-in mechanism that occurs during the self-assembly process. These facet-to-facet alignments force the nanocrystals to tilt on different lattice planes forming different projections that we termed apparent polydispersity. Our experimental realization of an orientational glass phase for multifaceted semiconducting nanocrystals can be used to investigate how this phase is formed and how it can be utilized for potential optical, electrical, and thermal transport applications.

Effects of internal molecular degrees of freedom on the thermal conductivity of some glasses and disordered crystals

The thermal conductivity $\ensuremath{\kappa}(T)$ of the fully ordered stable phase II, the metastable phase III, the orientationally disordered (plastic) phase I, as well as the nonergodic orientational glass (OG) phase, of the glass former cyclohexanol (C${}_{6}$H${}_{11}$OH) has been measured under equilibrium vapor pressure within the 2--200 K temperature range. The main emphasis is here focused on the influence of the conformational disorder upon the thermal properties of this material. Comparison of results with those regarding cyanoclyclohexane (C${}_{6}$H${}_{11}$CN), a chemically related compound, serves to quantify the role played by the terminal groups -OH and -CN on the phonon scattering processes. The picture that emerges shows that motions of such groups do play a minor role as scattering centers, both within the low-temperature orientationally ordered phases as well as in the OG states. The results are analyzed within the Debye-Peierls relaxation time model for isotropic solids comprising mechanisms for long-wave phonon scattering within the OG and orientational ordered low-temperature phases, as well as others arising from localized short-wavelength vibrational modes as pictured by the Cahill-Pohl model. By means of complementary neutron and Raman scattering we show that in the OG state the energy landscapes for both compounds are very similar.

Tetragonal tungsten bronze compounds: relaxor versus mixed ferroelectric–dipole glassbehavior

We demonstrate that recent experimental data (Castel et al 2009 J. Phys.:Condens. Matter 21 452201) on the tungsten bronze compound (TBC)Ba2PrxNd1 − xFeNb4O15 can be well explained in our model predicting a crossover from ferroelectric (x = 0) to orientational(dipole) glass (x = 1), rather then relaxor, behavior. We show that, since a ‘classical’ perovskite relaxor likePb(Mn1/3Nb2/3)O3 is never a ferroelectric, the presence of ferroelectric hysteresis loops in the TBC shows thatthis substance actually transits from ferroelectric to orientational glass phase withx growth. To describe the above crossover theoretically, we use the simple replica-symmetricsolution for the disordered Ising model.

Pressure-induced orientational glass phase in molecular para-hydrogen

We propose a theoretical description of a possible orientational glass transition in solid molecular para-hydrogen and ortho-deuterium under pressure supposing that they are mixtures of J=0 and J=2 states of molecules. The theory uses the basic concepts and methods of standard spin-glass theory. We expect our orientational glass to correspond to the II' phase of the high-pressure hydrogen phase diagram.

Phase diagram of a lattice of vortex molecules in multicomponent superconductors andmultilayer cuprate superconductors

The Abrikosov lattice in the multilayer cuprate superconductorCuBa2Ca3Cu3Oy (Cu-1223) has been experimentally and theoretically demonstrated to be composed ofvortex molecules. Cu-1223 is considered to be a typical multicomponent superconductor.We show that in such a system the rotational freedom around the axis of the vortexmolecular tube generates orientational disorder and the orientational glass (orcrystal) phase, which is never present in conventional vortex lattices consistingof axisymmetric vortices. The emergence of the orientational glass phase andorientational order phase with orthorhombic distortion is a general property ofvortex molecule lattices of the multiband type of multicomponent superconductors.

Cluster model of glass transition in simple liquids

On the basis of microscopic statistical mechanics of simple liquids an orientational interaction between clusters consisting of a particle and its nearest neighbors is estimated. It is shown that the intercluster interaction drastically depends on density, temperature and radius of a cluster. The model of interacting cubic and icosahedral clusters is proposed for description of structural glasses and is solved in mean-field replica symmetric approximation. Under cooling of the Lennard–Jones system icosahedral clusters freeze first forming bond orientational order; at lower temperatures cubic clusters freeze and form orientational glass phase.

Coherent tunnelling dynamics of muonium in a disordered medium

Conventional understanding suggests that strong disorder inevitably causes particle localization: states with the same energy are too far apart in space for tunnelling to be effective. We present experimental evidence to the contrary: we have observed coherent tunnelling of Mu atoms in a highly disordered mixture—75% CH 4+25% Kr—which forms an orientational glass phase at low temperatures.

Coherent quantum diffusion of muonium in a highly disordered medium

We have investigated the quantum diffusion of interstitial muonium atoms in solid mixtures of methane with krypton at temperatures from 10 to 90 K. At sufficiently high concentrations of Kr (above about 25%) the frustration of anisotropic molecular interactions produces an orientational glass phase. Even in this highly disordered medium, muonium exhibits coherent quantum tunneling dynamics with a bandwidth only slightly reduced with respect to that in pure solid methane. The muonium diffusion is shown to be long range rather than local.

Glassy dynamics of akinetically constrained model: a direct comparison withexperiment

The dynamics of the freezing transition of the rotator-phasecrystal of ethanol into its orientational glass phase ismonitored by measurements of molecular rotational components inthe quasielastic neutron scattering spectrum. It is shown thatphenomena experimentally observed at pico- and nanosecond scalescan be mapped onto those shown by a model of infinitely thinhard needles rotating about body-centred cubic latticepositions. The basic signatures that appear on crossing theorientational glass transition are similar in the needle modeland in the neutron scattering data for ethanol. As the modelglass transition is of purely dynamical origin, our findingssupport the idea that the glass transition is purely dynamicaland is not associated with an underlying thermodynamic phasetransition.

Phase Transitions and Water Dynamics of [Mn(H2O)6](ClO4 )2 Studied by Differential Scanning Calorimetry and Neutron Scattering Methods

Abstract DSC measurements performed at 95 -290 K have shown that [Mn(H 2 O) 6 ](CIO 4) 2 possesses, besides a high-temperature phase, existing above 323 K, four low-temperature solid phases. The inelastic incoherent neutron scattering (IINS) spectra and neutron powder diffraction (NPD) pat-terns registered at 20 -290 K have supported the DSC results and provided evidence that the investigated substance possesses even more than five solid phases. The IINS spectra have shown that in the room-temperature phase, water molecules perform fast stochastic reorientation at the picosecond scale. The orientational disorder characteristic for the room-temperature phase can be easily overcooled and frozen. Even by relatively slow cooling at ca. 40 K/hour a metastable, orientational (protonic) glass phase is formed below ca. 160 K. Below ca. 100 K, a structural phase transition was observed by the NPD, however the IINS spectra indicate existence of the pure ordered low-temperature phase only after annealing the sample for a few hours at 100 K. On heating, a structural phase transition takes place at ca. 120 K, and at ca. 225 K water molecules begin fast reorientation.

Quantum diffusion and localization in molecular crystals

The quantum diffusion of interstitial muonium atoms in crystals which undergo orientational order–disorder phase transitions has been studied using the technique of muonium-spin relaxation. In the high-temperature disordered phase, muonium dynamics becomes band-like with decreasing T. In contrast, in the orientational glass phase the coherence is destroyed. At low temperatures, both in the orientationally ordered phase, and in the orientational glass phase muonium undergoes strong localization.

Rotational freezing in plastic crystals: a model system for investigating the dynamics of the glass transition

The dynamics of the rotational freezing transition of the rotator phase (RP) crystal of ethanol into its orientational glass (OG) phase is monitored by measurements of molecular rotational components in the quasielastic neutron scattering spectrum. Such a transition is known to share several features in common with the canonical liquid glass transition (both taking place within the same temperature interval), and therefore some of the results from such a study should be of relevance for our understanding of the latter. We show that the basic features of the RP OG transition can be understood in terms of a hard-needle model by means of a mapping of the aspect ratio onto a temperature scale.

Purely Dynamical Signature of the Orientational Glass Transition

The dynamics of the freezing transition of the rotator phase crystal of ethanol into its orientational glass phase is monitored by measurements of molecular rotational components in the quasielastic neutron scattering spectrum. We demonstrate that phenomena observed at pico- and nanosecond scales can be mapped onto those shown by a model of infinitely thin hard needles rotating around body-centered-cubic lattice positions. As the model glass transition is of purely dynamical origin, our findings support the idea that the glass transition is purely dynamical and not associated with any thermodynamic phase transition.

Indications of an intermediate phase in single crystals of betaine phosphate/phosphite solid solutions

The solid solution of ferroelectric betaine phosphite and antiferroelectric betaine phosphate shows an interesting phase diagram with a transition from a high temperature prototype phase to a common nonpolar antiferrodistortive phase followed towards lower temperatures by transitions into ferroelectric, antiferroelectric or orientaional glass phases in dependence on its composition. Using magnetic resonance techniques such as electron nuclear double resonance (ENDOR) and electron spin-lattice relaxation measurements we are able to show that an intermediate phase exists between the antiferrodistortive and the orientational glass phase at intermediate compositions. This new phase shows a regular proton order within the phosphate-phosphite chains and is supposed to be of quasiferroic or clusterlike nature.

Intrinsic–Field Cooperative Freezing into Orientational Glass–Phase in Solid Hydrogen. Theory and Experiment

We report on the mechanism of orientational freezing in ortho–para–hydrogen mixtures. We argue that instead of the Zeeman–type uncorrelated intrinsic field, which dominats at high temperatures, the molecular ordering field, strongly correlated by the reaction Onsager–type field, drives the cooperative short–range ordering into the glass–like states. This mechanism is in accord with the observed unsaturated orientational order caused by moderation of the ordering field

Low-temperature orientational-glass-like phase transition in glass model

A simplified model of low-temperature orientational-glass-like phase transition from orientationally disordered phase to orientational glass phase in dielectric glass is presented. The model predicts a freezing of the defect orientations in random directions like the spin-glass phase in magnetic materials. A cusp of the orientational defect susceptibility is found. The second-order phase-transition temperature is defined by the strength of the interaction between the orientational degrees of freedom of the tunnelling defects in glass.

Thermal expansion measurements of [formula omitted] mixed crystals

The results of thermal expansion and room temperature powder X-ray diffraction measurements on (C 60) 1−x(C 70) x fullerene alloys are presented. Samples have been prepared by controlled condensation from the vapor phase. Strong anomalies in thermal expansion occur at the fcc to sc phase transition in C 60-rich, and hcp to distorted hcp one in C 70-rich crystals, respectively. The phase transition temperature diminishes with rising mixture ratio both on the C 60-rich ( ∼ −4K 1% C 70 ) and on the C 70-rich ( ∼ −20K 1% C 60 ) side of the phase diagram. This suggests the existence of an orientational glass phase on the C 70-rich side of the phase diagram (in agreement with thermal expansion measurements on sample with 70% C 70, which show no sign of the phase transitions at temperatures above 80 K).

Quantum molecular tunneling of NH4 + ions in mixed ammonium-alkali halide systems

The rotational excitations of NH4 + ions in dilute solution in metal alkali halide lattices have been studied at 4K by inelastic neutron scattering technique. It is found that the ions exist in matrix isolation at very low ammonium ion concentration,c (c → 0), and it transforms to orientational glass phase on increase ofc. It is shown that the onset of the orientational glass phase is moderated through the strain field generated by the substituted impurity in the lattice. The variation of the intensity profiles with increase ofc have been successfully explained by a phenomenological model based on gaussian distributed heights.

Quantitative Assessment of the Effects of Orientational and Positional Disorder on Glassy Dynamics

The microscopic dynamics of several phases of solid ethanol are studied under the same thermodynamic conditions by inelastic neutron scattering. It is found that the vibrational density of states of the orientational glass phase, where the molecules are arranged on an ordered lattice but with disordered orientations, is very similar to that of the structural glass phase, where the molecules are disordered both in position and orientation. Low-temperature specific heat measurements on the same phases strongly support the neutron measurements. We therefore find that positional disorder, even in a stoichiometrically homogeneous system such as ethanol, is not essential for the manifestation of glasslike behavior to an extent comparable with that exhibited by the structural glass.

X‐Ray Study of the System BP1−xBPIx

AbstractAccurate X‐ray diffraction measurements are carried out as function of temperature in antiferroelectric betaine phosphate (BP), ferroelectric betaine phosphite (BPI), and mixed compounds of BP1−xBPIx. In agreement with previous studies, the phases which derive from a high‐temperature phase with P21/c symmetry are clearly detected. However, in contrast with previous assignments, for 0.90 < x ≦ 1.0 a transition occurs to the P21 ferroelectric phase. For 0 ≦ x < 0.20, the two successive transitions are to a P21, phase and subsequently to a Pc phase with a simultaneous doubling of the unit cell. For 0.9 ≦ x < 1.0 the results confirm previous assignments of a single transition to a P21 phase. In the intermediate region 0.20 < x < 0.90 a clear evidence of an isomorphous transition to a glass phase is obtained, where the relative dominance of the ferroelectric or antiferroelectric interaction depends on the concentration values. Furthermore an orientational glass phase near x = 0.80 is identified. The existence of a strong anisotropy in all the studied elements of the family and of a weak coupling between the ordering of the dipoles and the lattice is evidenced. The role of the absent oxygen in the behaviour of the lattice parameters as well as its subsequent influence on the competitive interactions between dipoles are discussed.