Orientational Glass Research Articles

Universal behavior of low-temperature heat capacity of acrylonitrile-butadiene-styrene thermoplastic polymer and its composite with graphene oxide

The low-temperature dependence of the heat capacity of acrylonitrile-butadiene-styrene (ABS) polymer and its composite with thermally reduced graphene oxide was studied. The existence of a so-called “boson peak” characteristic of orientational and structural glasses was demonstrated. The boson peak appears in the form of a local maximum in the heat capacity curve displayed as C/T3 vs T at Tmax = 3.52 K. It was found that for both ABS polymer and its composite, as well as for a number of other substances of a crystalline and amorphous nature, the manifestation of the anomaly of the boson peak in the heat capacity has a universal character that is described by an empirical function Δ*. The value of Δ* depends on the magnitude of the anomaly in the heat capacity and the temperature of the boson peak manifestation. Thus, this study provides new physical information about the possible causes of the boson peak appearance in disordered materials and indicates the universality of boson peak anomaly for substances with short- and long-range order.

Quantum effects in the low-temperature thermal expansion of fullerite C60 doped with a 4He impurity

The thermal expansion of fullerite C60 doped with a 4He impurity at T ∼2 K has been investigated by the method of low temperature precision dilatometry in the interval T = 2.2−24 K. The character and the derived values of the thermal expansion coefficients were strongly dependent on the concentration of the 4He impurity in fullerite. In the interval T = 2.2−5 K the thermal expansion of the 4Hex−C60 system is negative, which is attributed to the tunnel movement of the 4He atoms in the cavity system of the C60 crystal lattice and at the crystallite surface. The contribution of this process to the thermal expansion decreases as the impurity (4He) concentration increases and the probability of 4He tunneling between the crystal lattice cavities and the impurity-free areas of the grain surface diminishes. In the temperature interval T = 4.5−24 K the thermal expansion of the 4Hex−C60 system is influenced predominantly by the mutual transformations of different orientation glass modifications of fullerite. Owing to their tunnel character the transformations make a negative contribution to the process of thermal expansion entailing a hysteresis and other anomalies observed in this temperature interval. The intensity of the processes provoked by the tunneling-encouraged phase transformations of the orientational glasses of C60 increases with the 4He concentration in fullerite.

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.

Spectroscopic and calorimetric investigation of cyclohexanol in different orientational states

The thermal analysis and infrared (IR) spectroscopy were used to study cyclohexanol irreversible transformations from an orientationally disordered plastic state to metastable III and then to a stable II crystalline phase. It was found that the orientational glass transition in cyclohexanol occurs at Tg ∼ 163.5 K. Solid–solid transition from orientationally disordered phase I to metastable phase III occurs at 195 K. Crystal phase III nucleates at Ta = 203 K and converts into phase I at T = 233.7 K. The stable crystal phase II is formed by nucleation at 213 K. It converts into phase I at T = 250 K. It is shown by IR spectroscopy that the phase transitions in solid cyclohexanol are correlated with conformational changes in the cyclohexanol molecule.

Is pressure the key to hydrogen ordering ice IV?

Hydrochloric-acid-doped ice IV prepared at increasing pressures leads to growing endotherms observed with ambient pressure calorimetry. The endotherms are irreversible suggesting three possible scenarios for their origins: (1) a weakly hydrogen-ordered counterpart of ice IV is formed, but ambient pressure favours hydrogen-disordered ice IV, (2) increased pressure creates increased strain within the crystal structure of the ice, which is released upon heating yielding the endotherms or (3) the endotherms are kinetic overshoot effects related to the underlying orientational glass transition. X-ray diffraction cannot distinguish between these scenarios. Recent controversies regarding the preparation of ice IV are also discussed.

Magnetic properties of NH4Al(SO4)2•12H2O: Evidence of glass behavior based on proton orbitals

The polarity of the magnetic susceptibility χ(T) of ammonium alum NH4Al(SO4)2•12H2O single crystals below its ferroelectric Curie temperature TC is identified to depend only on the cooling rate. To explain this, we consider reorienting distorted NH4+ to carry weak magnetic and electric dipole moments that are linearly coupled. Slow cooling allows the dipole moments to align which enhances χ(T) to become positive. However, due to energy degeneracy of rotational motion and geometric frustration of the NH4+, faster cooling rates render the system to become an orientational glass state causing χ(T) to remain negative. Specific heat measurements also show the presence of excess entropy below a negative anomaly near 53 K. Our findings identify a possible new class of non-equilibrium system with potential applications in non-volatile memory devices.

Impact of Orientational Glass Formation and Local Strain on Photo-Induced Halide Segregation in Hybrid Metal-Halide Perovskites.

Band gap tuning of hybrid metal–halide perovskites by halide substitution holds promise for tailored light absorption in tandem solar cells and emission in light-emitting diodes. However, the impact of halide substitution on the crystal structure and the fundamental mechanism of photo-induced halide segregation remain open questions. Here, using a combination of temperature-dependent X-ray diffraction and calorimetry measurements, we report the emergence of a disorder- and frustration-driven orientational glass for a wide range of compositions in CH3NH3Pb(ClxBr1–x)3. Using temperature-dependent photoluminescence measurements, we find a correlation between halide segregation under illumination and local strains from the orientational glass. We observe no glassy behavior in CsPb(ClxBr1–x)3, highlighting the importance of the A-site cation for the structure and optoelectronic properties. Using first-principles calculations, we identify the local preferential alignment of the organic cations as the glass formation mechanism. Our findings rationalize the superior photostability of mixed-cation metal–halide perovskites and provide guidelines for further stabilization strategies.

Cation Dynamics and Structural Stabilization in Formamidinium Lead Iodide Perovskites.

The vibrational dynamics of pure and methylammonium-doped formamidinium lead iodide perovskites (FAPbI3) has been investigated by high-resolution neutron spectroscopy. For the first time, we provide an exhaustive and accurate analysis of the cation vibrations and underlying local structure around the organic moiety in these materials using first-principles electronic-structure calculations validated by the neutron data. Inelastic neutron scattering experiments on FAPbI3 provide direct evidence of the formation of a low-temperature orientational glass, unveiling the physicochemical origin of phase metastability in the tetragonal structure. Further analysis of these data provides a suitable starting point to explore and understand the stabilization of the perovskite framework via doping with small amounts of organic cations. In particular, we find that the hydrogen-bonding interactions around the formamidinium cations are strengthened as a result of cage deformation. This synergistic effect across perovskite cages is accompanied by a concomitant weakening of the methylammonium interactions with the surrounding framework.

Structure of 1,6-anhydro-β-D-glucopyranose in plastic crystal, orientational glass, liquid and ordinary glass forms: molecular modeling and X-ray diffraction studies

The structures of the plastic crystal (PC), orientational glass (OTG), liquid (LQ) and ordinary glass (OG) phases of 1,6-anhydro-β-D-glucopyranose (levoglucosan) have been investigated using X-ray diffraction and molecular modeling. The experimental diffraction data in the forms of static structure factors and pair distribution functions are analyzed in reciprocal and real spaces and compared with results of model-based simulations. A new approach to modeling the structure of the disordered phases, taking into account the intermolecular scattering contribution in the form of sharp Bragg peaks, the slowly varying intensity associated with intramolecular correlations and the diffusive component resulting from structural disorder, is applied. In the case of the LQ and OG samples, reverse Monte Carlo simulations are also used. The PC and OTG phases show long-range ordering of the hexagonal close-packed (hcp)-type structure up to 120 Å with random orientation of the molecules. Assuming a rigid molecular skeleton, isotropic free rotations of the molecules about their geometrical center in full and limited angular ranges are generated in theoretical models of the structure. It is demonstrated that the adoption of free rotations of the molecules leads to the best fits to experimental data for each studied phase of levoglucosan. The diffraction patterns of the LQ and OG samples show a relatively sharp first peak originating from quasi-Bragg planes of the densely packed face-centered cubic (fcc) type molecular arrangement. Moreover, the slowly varied intensity component of LQ and OG is practically the same as that of PC and OTG, suggesting that the intramolecular structure of these four phases does not change. Interestingly, structural correlations for the disordered LQ and OG states extend surprisingly far, up to about 50 Å. In addition, for all levoglucosan phases investigated, the paracrystalline disorder imposed on the generated models resulted in better compliance with the experimental data.

Metastability, tetragonality and quadrupolar glass states in cubic stabilized zirconia

AbstractFor cubic lime‐stabilized and yttria‐stabilized zirconia ceramic alloys the temperatures at which the onset of metastability and a sharp reduction in ionic conductivity occur are observed to coincide, appearing at approximately 1000°C and 650°C respectively. This behavior is attributed to the development of quadrupolar (orientational) glass states in each alloy below its transition temperature. We suggest that several other unexplained phenomena including tetragonal distortion in these cubic phases may also be understood by reference to the alloys' crystalline structures, which differ slightly. At equilibrium, each possesses a random two‐component crystalline constitution, one component of which is a low‐symmetry quadrupole intergrown with the other and capable of switching its unique axis in a small number of different directions. The quadrupoles interact, thereby imparting a disordered stress on the other component in the lattice. Such a structure is identifiable as a quadrupolar glass, the elastic analog of a spin glass.

Secondary relaxation in the terahertz range in 2-adamantanone from theory and experiments

We applied the recently developed Generalized Langevin Equation (GLE) approach for dielectric response of liquids and glasses to link the vibrational density of states (VDOS) to the dielectric response of a model orientational glass (OG). The dielectric functions calculated based on the GLE, with VDOS obtained in experiments and simulations as inputs, are compared with experimental data for the paradigmatic case of 2-adamantanone at various temperatures. The memory function is related to the integral of the VDOS times a spectral coupling function $\gamma(\omega_p)$, which tells the degree of dynamical coupling between molecular degrees of freedom at different eigenfrequencies. With respect to previous empirical fittings, the GLE-based fitting reveals a broader temperature range over which the secondary relaxation is active. Furthermore, the theoretical analysis provides a clear evidence of secondary relaxation being localized within the THz ($0.5-1$ THz) range of eigenfrequencies, and thus not too far from the low-energy modes involved in $\alpha$-relaxation. In the same THz region, the same material displays a crowding of low-energy optical modes that may be related to the secondary relaxation.

Orientational glass, orientationally disordered crystal and crystalline polymorphism: A further study on the thermal behavior and molecular mobility in levoglucosan

Levoglucosan has very particular features in its phase diagram. On the one hand it has two crystalline phases, one orientationally ordered (ORC), stable at room temperature, which is transformed by heating in an orientationally disordered crystal (ODIC), a plastic crystal. On the other hand, it is difficult to form the structural glass by cooling down from the isotropic liquid, but it is easy to form the orientational glass (O-G) of levoglucosan by cooling from the ODIC phase. The stability of the O-G and ODIC phases allowed the study of the orientational glass transition, O-G → ODIC, by differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC) with the determination of the respective dynamic fragility. In this context a critical analysis of the procedures using the effect of the DSC scanning rate on the temperature location of the DSC glass transition signal was performed.Two polymorphs of levoglucosan, not yet described in the literature, were discovered. These crystalline ORC forms are converted into an ODIC phase by heating, as happens with the most stable polymorph (as received). The three ORCs show ORC → ODIC transitions that differ greatly in temperature, enthalpy, and entropy of transition. We must therefore consider four levoglucosan polymorphs, three ORCs and one ODIC.TSDC was also used to study the secondary relaxations in the levoglucosan orientational glass, and it was possible to identify a β-type mobility (Johari-Goldstein) in addition to fast relaxations. Finally, the two dielectric techniques, dielectric relaxation spectroscopy (DRS) and TSDC, were compared in their ability to provide relevant information on the secondary mobility.

Source of JG-Relaxation in the Entropy of Glass.

Intrinsic mobility of the glassy state is known as Johari-Goldstein (JG)-relaxation. Its source is either rotational and translational motions of a small number of molecules or small-angle orientational motion of all molecules, which may include restricted translational motion of part of their flexible segments. We examine the merits of the two views by using the excess entropy, Sexc, the difference between the entropy of a glass and its crystal state. Its value at the glass-forming temperature, Sexc( Tg), is the sum of (1) Sexc from phonons and anharmonic forces and (2) the configurational entropy from (a) the JG-process and, (b) the unfrozen modes of the α-process. If all molecules participated in the JG-process, the minimum configurational entropy from this process would be R ln(2) [= 5.76 J/(mol K)], and [ Sexc( Tg) - Sexc(0 K)] cannot be less than that. Analysis of data for 33 liquid and 3 liquid crystal glasses and 7 orientational glasses shows that its value is less than or comparable to R ln(2) for 10 liquids and 2 orientational glasses. If contributions from (1) and (2b) were known and subtracted from Sexc( Tg), the [ Sexc( Tg) - Sexc(0 K)] value would be less than R ln(2) for more glasses. Cooperative motions to explain our finding and heterogeneous dynamics to explain the broad JG-relaxation spectra would require sufficiently large local mobility regions. We also argue that all molecules cannot participate in small-angle orientational fluctuations, cite evidence against such fluctuations for the JG-process, and note that nucleation and crystallization in a glass structure indicate such regions. After discussing the consequences for the entropy theory, the available NMR studies, the Eshelby inclusions, the potential energy landscape, and the Gardner transition, we conclude that JG-process likely occurs in local regions in internal equilibrium in a glass structure.

Multiple glass transitions in vapor-deposited orientational glasses of the most fragile plastic crystal Freon 113.

We investigate by fast-scanning nanocalorimetry the formation of Freon 113 films from the vapor phase at deposition temperatures ranging from 50 to 120 K, that is, spanning above and below the transition temperature of the glassy crystal to the plastic crystal (Tgc = 72 K). Analysis of the heat capacity curves indicates that vapor deposition at T < Tgc of the highly fragile Freon 113 yields structural and orientational glasses in the as-deposited state depending on the temperature range of deposition. Interestingly, growing above Tgc produces plastic crystals with a conformational ratio C1/Cs that changes with Tdep above and below 110-120 K, the temperature at which previous works have identified the arrest of the transformations between the C1 and Cs conformers.

Phase behavior and design rules for plastic colloidal crystals of hard polyhedra via consideration of directional entropic forces.

Plastic crystals - like liquid crystals - are mesophases that can exist between liquids and crystals and possess some of the characteristic traits of each of these states of matter. Plastic crystals exhibit translational order but orientational disorder. Here, we characterize the phase behavior in systems of hard polyhedra that self-assemble plastic face-centered cubic (pFCC) colloidal crystals. We report a first-order transition from a pFCC to a body-centered tetragonal (BCT) crystal, a smooth crossover from pFCC to an orientationally-ordered FCC crystal, and an apparent orientational glass transition wherein long-range order fails to develop from a plastic crystal upon an increase in density. Using global order parameters and local environment descriptors, we describe how particle shape influences the development of orientational order with increasing density, and we provide design rules based on the arrangement of facets for engineering plastic crystal behavior in colloidal systems.

Disentangling α and β relaxation in orientationally disordered crystals with theory and experiments.

We use a microscopically motivated generalized Langevin equation (GLE) approach to link the vibrational density of states (VDOS) to the dielectric response of orientational glasses (OGs). The dielectric function calculated based on the GLE is compared with experimental data for the paradigmatic case of two OGs: freon-112 and freon-113, around and just above T_{g}. The memory function is related to the integral of the VDOS times a spectral coupling function γ(ω_{p}), which tells the degree of dynamical coupling between molecular degrees of freedom at different eigenfrequencies. The comparative analysis of the two freons reveals that the appearance of a secondary β relaxation in freon-112 is due to cooperative dynamical coupling in the regime of mesoscopic motions caused by stronger anharmonicity (absent in freon-113) and is associated with the comparatively lower boson peak in the VDOS. The proposed framework brings together all the key aspects of glassy physics (VDOS with the boson peak, dynamical heterogeneity, dissipation, and anharmonicity) into a single model.

Nuclear Magnetic Resonance Spectroscopy

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Instability and thermal conductivity of pressure-densified and elastically altered orientational glass of Buckminsterfullerene.

We report on the temperature, pressure, and time (T, p, and t)-dependent features of thermal conductivity, κ, of partially ordered, non-equilibrium state of C60-OG, the orientational glass of Buckminsterfullerene (at T below the orientational freezing temperature Tog) made more unstable (i) by partially depressurizing its high-p formed state to elastically expand it and (ii) by further pressurizing that state to elastically contract it. The sub-Tog effects observed on heating of C60-OG differ from those of glasses because phonon propagation depends on the ratio of two well-defined orientational states of C60 molecules and the density of the solid. A broad peak-like feature appears at T near Tog in the κ-T plots of C60-OG formed at 0.7 GPa, depressurized to 0.2 GPa and heated at 0.2 GPa, which we attribute to partial overlap of the sub-Tog and Tog features. A sub-Tog local minimum appears in the κ-T plots at T well below Tog of C60-OG formed at 0.1 GPa, pressurized to 0.5 GPa and heated at 0.5 GPa and it corresponds to the state of maximum disorder. Although Buckminsterfullerene is regarded as an orientationally disordered crystal, variation of its properties with T and p is qualitatively different from other such crystals. We discuss the findings in terms of the nature of its disorder, sensitivity of its rotational dynamics to temperature, and the absence of the Johari-Goldstein relaxation. All seem to affect the phenomenology of its glass-like transition.

Spin-One p-Spin Glass: Exact Solution for Large p

We study the low-temperature properties of the p-spin spin glass model in the spin-one (three-state) case for large values of p. We show that the one-step replica symmetry-breaking phase is unstable at a very low temperature, and we calculate the explicit boundary of the stability interval, the Gardner temperature, analytically for large values of p. This temperature for the spin-one model has the same form of dependence on p as in the case of Ising spins (two states). In the one-step replica symmetrybreaking state, a quadrupolar orientational glass coexists with the spin glass and also with a regular quadrupole ordering.

Orientational Glass Formation in Substituted Hybrid Perovskites

Hybrid organic–inorganic perovskites have gained notoriety in the photovoltaic community for their composition-tunable band gaps and long-lived electronic excited states, which are known to be related to the crystalline phase. While it is known that the inorganic and organic components are coupled through structural phase transitions, it remains unclear as to what role each plays in directing the structure of hybrid perovskites such as methylammonium lead halides (CH3NH3PbX3). Here, we present crystallographic and spectroscopic data for the series (CH3NH3)1–xCsxPbBr3. CH3NH3PbBr3 behaves as a plastic crystal in the high temperature cubic phase, and substitution of CH3NH3+ with Cs+ leads to the formation of an orientational glass. While the organic molecule exhibits slow, glassy reorientational dynamics, the inorganic framework continues to undergo crystallographic phase transitions. These crystallographic transitions occur in the absence of thermodynamic signatures in the specific heat from molecular orie...