Orientational Glass Research Articles

Effect of molecular nitrogen impurity on the photoluminescence of fullerite C60

Polycrystalline samples of fullerite C60 with different concentrations of molecular nitrogen impurity are studied by luminescence spectroscopy at temperatures of 20–230 K. The photoluminescence characteristics of the C60-N2 solutions vary significantly. At T = 20 K there is a significant contribution to the luminescence from emission centers (“deep X-traps”) determined by the concentration of N2 molecules in octahedral voids of the fcc C60 lattice. The experimentally observed differences in the effect of nitrogen impurities on the structural and luminescence characteristics of C60-N2 solutions show that the filling of the lattice voids into the depth (from the surface) of the samples is substantially inhomogeneous. The temperature dependences of the integrated emission intensity from samples with different nitrogen concentrations are studied. N2 molecules are found to have little effect on the formation of fullerite orientational glass. A tendency for the vitrification temperature of these C60-N2 solutions to fall with increasing impurity concentration is observed experimentally.

Superstructures formed by orientationally ordered tetrahedra in the bcc lattice: new diffusionless order-disorder transition in solids

We investigated and clarified the superstructures formed by tetrahedra in the bcc lattice within the framework of second-order transitions. Compliance with both the Landau and Lifshitz conditions was investigated for all possible superstructures and, based on this, we demonstrate that bcc crystals that contain tetrahedra at an inversion center can exhibit a variety of second-order transitions, which are regarded as a new type of diffusionless order-disorder transition with antiferroic orientational orders. Finally, we show that the transition gives rise to a new glassy state. Breaking of the local inversion symmetry may lead to a new orientational glass, which is reminiscent of spin glasses in magnetism.

Molecular disorder effects in the thermal conductivity of solid thiophene

The temperature dependence of the thermal conductivity κ(T) of solid thiophene is measured in a sequence of stable orientationally disordered phases with different degrees of orientational ordering of the molecules: in orientational glass (Vg); in phase V with large angular librational molecular vibrations; in incommensurate phase IV with a static orientational disorder; and in orientationally disordered crystalline phase III with dynamic orientational disorder of the molecules. Measurements are made at the saturated vapor pressure in a temperature range of 2–180 K. It is found that the thermal conductivity of thiophene is practically independent of temperature in phases III and V with dynamic orientational disorder of the molecules. In the orientational glass state and in the incommensurate state, the temperature dependence of the thermal conductivity of thiophene has a form typical of crystals with a long-range orientational order. A distinct hysteresis of the thermal conductivity is found at temperatures slightly below the temperature of the transition from phase IV into phase V.

Low-temperature dynamics of matrix isolated methane molecules in fullerite C60: The heat capacity, isotope effects

The heat capacity of the interstitial solid solution (CH4)0.4C60 has been investigated in the temperature interval 1.4–120 K. The contribution of CH4 molecules to the heat capacity of the solution has been separated. The contributions of CH4 and CD4 molecules to the heat capacity of the solutions (CH4)0.40C60 and (CD4)0.40C60 have been compared. It is found that above 90 K the character of the rotational motion of CH4 and CD4 molecules changes from libration to hindered rotation. In the interval 14–35 K the heat capacities of CH4 and CD4 molecules are satisfactorily described by contributions of the translational and libration vibrations, as well as the tunnel rotation for the equilibrium distribution of the nuclear spin species. The isotope effect is due to mainly, the difference in the frequencies of local translational and libration vibrations of molecules CH4 and CD4. The contribution of the tunnel rotation of the CH4 and CD4 molecules to the heat capacity is dominant below 8 K. The isotopic effect is caused by the difference between both the conversion rates and the rotational spectra of the nuclear spin species of CH4 and CD4 molecules. The conversion rate of CH4 molecules is several times lower than that of CD4 ones. Weak features observed in the curves of temperature dependencies of the heat capacity of CH4 and CD4 molecules near 6 and 8 K, respectively, are most likely a manifestation of first-order polyamorphic phase transitions in the orientational glasses of these solutions.

Thermal Conductivity of Molecular Crystals with Self-Organizing Disorder

The thermal conductivity of some orientational glasses of protonated C2H5OH and deuterated C2D5OD ethanol, cyclic substances (cyclohexanol C6H11OH, cyanocyclohexane C6H11CN, cyclohexene C6H10), and freon 112 (CFCl2)2 have been analyzed in the temperature interval 2-130 K. The investigated substances demonstrate new effects concerned with the physics of disordered systems. Universal temperature dependences of the thermal conductivity of molecular orientational glasses have been revealed. At low temperatures, the thermal conductivity exhibits a universal behavior that can be described by the soft potential model. At relatively high temperatures, the thermal conductivity has a smeared maximum and than decreases with increase in the temperature, which occurs typically in crystalline structures.

Orientational glass in mixtures of elliptic and circular particles: structural heterogeneities, rotational dynamics, and rheology.

Using molecular dynamics simulation with an angle-dependent Lennard-Jones potential, we study orientational glass with quadrupolar symmetry in mixtures of elliptic particles and circular impurities in two dimensions. With a mild aspect ratio (= 1.23) and a mild size ratio (= 1.2), we realize a plastic crystal at relatively high temperature T. With further lowering T, we find a structural phase transition for very small impurity concentration c and pinned disordered orientations for not small c. The ellipses are anchored by the impurities in the planar alignment. With increasing c, the orientation domains composed of isosceles triangles gradually become smaller, resulting in orientational glass with crystal order. In our simulation, the impurity distribution becomes heterogeneous during quenching from liquid, which then produces rotational dynamic heterogeneities. We also examine rheology in orientational glass to predict a shape memory effect and a superelasticity effect, where a large fraction of the strain is due to collective orientation changes.

The phase transition between the orientational glass and plastic crystal in cryovacuum condensate of ethanol

After more than 100 years of research, the nature of glasses (and disordered condensed matter, in general) and glass transition-related phenomena is still an open issue in condensed-matter physics [1] and the chemicophysical sciences [2]. Thin films of cry vacuum condensates of ethanol formed by condensation of the gas on a cooled metal substrate are studied by infrared spectrometry and thermal desorption techniques. The main purpose was to examine dynamic relaxation processes near the glass transition temperature and the transition from orientation ally disordered to orders states in thin ethanol cry condensate films. Based on the analysis of the thermo grams conclusions that the phase transition orientation glass-plastic crystal of ethanol around T = 97 K is reversible and is a phase transition type I.

Heat transfer in different phases of solid cyclohexene

The thermal conductivity of solid cyclohexene C6H10 has been measured in two independent experiments in five different stable and metastable phase states: orientational glass (Ig), orientational glass (IIIg) with a partial order, dynamically orientationally disordered state (III) with a partial order, completely orientationally ordered phase (II) and “plastic” phase (I). The measurements were carried out at saturated vapor pressure in the temperature range 2–120K and at isochoric conditions in “plastic” and orientationally ordered phases on samples of different densities. The isochoric thermal conductivity of “plastic” phase increases smoothly with temperature. It can be attributed to weakening of the translational orientational coupling which, in turn, leads to a decrease in phonon scattering on rotational excitations. The thermal conductivity of cyclohexene measured at saturated vapor pressure exhibits a similar behavior in phases Ig, IIIg, and II. At low temperatures (T<8K) the thermal conductivity tends to T2 dependence, passes through a maximum and decreases further with increasing temperature following the dependence, which is somewhat different from 1/T. It was found that the thermal conductivity can be represented as a sum of two contributions κ(T)=κ1(T)+κ2(T), where κ1(T) is due to propagating phonons whose mean-free path exceeds half the phonon wavelength, and κ2(T) is attributed to localized short-wavelength or “diffusive” vibrational modes.

Two-Step Glass Transition Induced by Attractive Interactions in Quasi-Two-Dimensional Suspensions of Ellipsoidal Particles

We study experimentally the glass transition dynamics in quasi-two-dimensional suspensions of colloidal ellipsoids, aspect ratio α=2.1, with repulsive as well as attractive interactions. For the purely repulsive case, we find that the orientational and translational glass transitions occur at the same area fraction. Strikingly, for intermediate depletion attraction strengths, we find that the orientational glass transition precedes the translational one. By quantifying structure and dynamics, we show that quasi-long-range ordering is promoted at these attraction strengths, which subsequently results in a two-step glass transition. Most interestingly, within experimental certainty, we observe reentrant glass dynamics only in the translational degrees of freedom.

Orientational glass: Full replica symmetry breaking in generalized spin glass-like models without reflection symmetry

We investigate near the point of glass transition the expansion of the free energy corresponding to the generalized Sherrington--Kirkpatrick model with arbitrary diagonal operators U standing instead of Ising spins. We focus on the case when U is an operator with broken reflection symmetry. Such a consideration is important for understanding the behavior of spin-glass-like phases in a number of real physical systems, mainly in orientational glasses in mixed molecular crystals which present just the case. We build explicitly a full replica symmetry breaking (FRSB) solution of the equations for the orientational glass order parameters when the non-symmetric part of U is small. This particular result presents a counterexample in the context of usually adopted conjecture of the absence of FRSB solution in systems with no reflection symmetry.

Molecular dynamics simulation of orientational glass formation in anisotropic particle systems in three dimensions

We propose a simple microscopic model of molecular dynamics simulation to study orientational glass in three dimensions. We present simulation results for mixtures of mildly anisotropic particles and spherical impurities. We realize fcc solids without orientational order in a rotator phase. As the temperature T is lowered, the disordered matrix is gradually replaced by four kinds of orientationally ordered, rhombohedral domains. Two-phase coexistence is realized in a temperature window. The impurities serve to anchor the orientations of the surrounding anisotropic particles, resulting in finely divided domains or medium long-range orientational order. We examine the rotational dynamics of the molecular orientations which is slowed down at low T. We predict the shape memory effect under a stretching cycle due to inter-variant transformation.

Orientational glassification in fullerite C60 saturated with H2: Photoluminescence studies

Using one-photon excitation we studied photoluminescence of C60 saturated with molecular hydrogen over a temperature range from 10 to 230 K. Saturation of samples was done at 30 atm and at temperatures low enough (T &amp;lt; 250 °C) to exclude chemical sorption. The samples were saturated during periods of varied duration τ to reach different occupancy levels. To check the reliability of our luminescence results and their interpretation, our spectra for pure C60 were compared with data known in the art, demonstrating good compatibility. The luminescence spectra were attributed according to the approach of Akimoto and Kan’no by separating the total spectra into two components of different origin. The A-type spectra, associated with exciton transport to deep traps, become prevalent over the B-type emission above 70 K. The integrated intensity I as a function of the temperature T of the luminescence measurements I(T) remained at a constant level up to the orientational vitrification point of about 100 K when the saturation times exceeded a certain value (for one, 50 h for a saturation temperature of 200 °C); then I(T) went down rather steeply with increasing T. However, at longer τ the intensity I(Τ) persisted consistently to higher T (the higher, the longer τ) and then dropped with increasing T. This finding made us reexamine the lattice parameter vs. saturation time dependence for saturation temperatures of 200 and 230 °C. As a result, additional evidence allowed us to infer that after the completion of the single-molecule filling of O-voids (specifically, after roughly 50 h at Tsat = 200 °C) a slower process of double filling sets in. Double filling entails an anisotropic deformation of the octahedral cage, which modifies rotational dynamics more than single filling. Further, we argue that singlet exciton transport to traps (which is responsible for the A-type emission) can be crucially hampered by rotational jumps of one of the molecules, over which a travelling exciton is spread. Such jumps break coherence, and the exciton stops, thereby increasing the probability of emissionless deactivation. If so, then the temperature at which the rotational jumps occur sufficiently frequently may be by inference considered to be the unfreezing point for the orientational glass state (essentially coinciding with the inverse critical point Tg, where the rotational system freezes into the orientational glass). This treatment of Tg differs from that existing in the art, according to which the glass state is destroyed owing to the increased density of phonon states. Keeping to our reasoning, we conclude that the orientational glass state does not disappear but, instead, is conserved almost unchanged under one-molecule filling and persists to appreciably higher temperatures in the case of double filling, which has a stronger effect on exciton dynamics.

Phase transitions in KIO3

The high-pressure behavior of KIO3 was studied up to 30 GPa using single crystal and powder x-ray diffraction, Raman spectroscopy, second harmonic generation (SHG) experiments and density functional theory (DFT)-based calculations. Triclinic KIO3 shows two pressure-induced structural phase transitions at 7 GPa and at 14 GPa. Single crystal x-ray diffraction at 8.7(1) GPa was employed to solve the structure of the first high-pressure phase (space group R3, a = 5.89(1) Å, α = 62.4(1)°). The bulk modulus, B, of this phase was obtained by fitting a second order Birch–Murnaghan equation of state (eos) to synchrotron x-ray powder diffraction data resulting in Bexp,second = 67(3) GPa. The DFT model gave BDFT,second = 70.9 GPa, and, for a third order Birch–Murnaghan eos, BDFT,third = 67.9 GPa with a pressure derivative of . Both high-pressure transformations were detectable by Raman spectroscopy and the observation of second harmonic signals. The presence of strong SHG signals shows that all high-pressure phases are acentric. By using different pressure media, we showed that the transition pressures are very strongly influenced by shear stresses. Earlier work on low- and high-temperature transitions was complemented by low-temperature heat capacity measurements. We found no evidence for the presence of an orientational glass, in contrast to earlier dielectric studies, but consistent with earlier low-temperature diffraction studies.

The effect of molecular impurities CO and CH4 on the structural characteristics of the C60 fullerene around the orientational phase transition

X-ray studies of structural characteristics of С60(С)0.9 and С60(CH4)0.5 solid solutions were carried out in around the orientation phase transition in the C60 fullerene. It was established that the filling of octahedral cavities of the fullerene lattice by molecules of CO and CH4 results in a significant increase of the lattice parameter and the thermal expansion coefficients of the ordered phase and in a decrease in the volume change and phase transition temperature TC. In this case there also occurs a noticeable “smearing” of the phase transformation. Based on these results and on the data in the literature contradictions in the conclusions about the structural peculiarities of the formation of a glass state in the CO–C60 solutions were analyzed and discussed. Linear dependences of orientational transition temperature TC and glass transition temperature Tg on CO concentration in the octahedral cavities were obtained. Concentrations of CO in the previous study of specific heat of solid mixtures were calculated.

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.

Low-temperature heat capacity of fullerite C60 doped with deuteromethane

The heat capacity C of fullerite doped with deuteromethane (CD4)0.4(C60) has been investigated in the temperature interval 1.2–120 K. The contribution ΔCCD4 of the CD4 molecules to the heat capacity C has been isolated. It is shown that at T ≈ 120 K the rotational motion of CD4 molecules in the octahedral voids of the C60 lattice is weakly hindered. When the temperature is lowered to 80 K, the rotational motion of the CD4 molecules changes from weakly hindered rotation to libration. In the range T = 1.2–30 K, ΔCCD4 is described quite accurately by the sum of contributions from the translational and librational vibrations and tunneling rotation of CD4 molecules. The contribution of tunneling rotation to the heat capacity ΔCCD4(T) is dominant below 5 K. The effect of nuclear-spin conversion of the CD4 molecules on the heat capacity has been observed and the characteristic times for nuclear spin conversion between the lowest levels of the A- and T-species of CD4 molecules at T &amp;lt; 5 K have been estimated. A feature observed in ΔCCD4(T) near T = 5.5 K is most likely a manifestation of a first-order phase transition in the orientational glass form of the solution.

Glass Transitions in Quasi-Two-Dimensional Suspensions of Colloidal Ellipsoids

We observed a two-step glass transition in monolayers of colloidal ellipsoids by video microscopy. The glass transition in the rotational degree of freedom was at a lower density than that in the translational degree of freedom. Between the two transitions, ellipsoids formed an orientational glass. Approaching the respective glass transitions, the rotational and translational fastest-moving particles in the supercooled liquid moved cooperatively and formed clusters with power-law size distributions. The mean cluster sizes diverge in power law as they approach the glass transitions. The clusters of translational and rotational fastest-moving ellipsoids formed mainly within pseudonematic domains and around the domain boundaries, respectively.

Secondary relaxations of orientationally disordered mixed crystals at temperatures lower than the glass transition temperature

AbstractLow‐molecular weight cyclic alcohols as cycloheptanol (C7H14O, hereinafter referred to as cC7‐ol) and cyclooctanol (C8H16O, cC8‐ol) are prototypical materials displaying OD phases which, under fast cooling give rise to orientational glasses (OG). In addition to the ubiquitous α‐relaxation of canonical glasses, several secondary relaxations appear for the mentioned systems (β and γ for cC8‐ol and β for cC7‐ol). The intramolecular character of these secondary relaxations for these materials as well as their mixed crystals was highlighted at temperatures close but above the glass transition. For lower temperatures the low values of dielectric strength makes difficult to account for the relaxation times obtained from the permittivity losses and, thus in this work we present a data analysis based on the Kramers–Kronig relations which connect the real and imaginary parts of dielectric permittivity and shows up a new method to make evident the existence of such secondary relaxations as well as to avoid phenomenological equations for determining the relaxation time.

Deuteration effects in the thermal conductivity of molecular glasses

The thermal conductivity κ(T) of pure deuterated ethanol has been measured at the equilibrium vapor pressure of its orientationally-ordered crystal form (T = 2 K − Tm), orientational glass, and glass state (T = 2 K − Tg, Tg is the glass transition temperature) solid phases. The temperature dependence of the conductivity is well described by the sum of two contributions, κ(T) = κI(T) + κII(T), where κI(T) accounts for heat transport by acoustic phonons and κII(T), for heat transfer by localized high-frequency excitations. The thermal conductivities of deuterated and hydrogenated ethanols are compared in the different phases. The mechanisms of phonon scattering in the glasses are analyzed. In these glasses the effect of complete deuteration shows up in the κII(T) term.

Universal critical-like scaling of dynamics in plastic crystals

Many theoretical models for the glassy dynamics have been proposed so far describing the changes in molecular dynamics along the extraordinary slowing down in the vitrification process of a disordered phase on cooling. Many of these theories share the concept of cooperative rearranging regions firstly proposed by Adam and Gibbs. Among them, the dynamical scaling model (DSM) is based on the random diffusion of free volume which creates random walking clusters formed by cooperatively rearranging entities. Within this framework a critical phenomenon relating a hidden phase transition at T C (below T g ) implies the divergence of the relaxation time ( τ) or viscosity ( η) τ, η ∝ ( T − T C ) − ϕ with a universal scaling exponent φ → 9. In this work we apply the DSM model to orientational glasses, obtained from the quenching of orientationally disordered phases (plastic crystals) via the application of the linearized derivative-based transformation of dielectric spectroscopy τ( T) data.