The anomalous Debye–Waller factor and the fragility of glasses

Unravelling the amorphous structure and crystallization mechanism of GeTe phase change memory materials

The reversible phase transitions in phase-change memory devices can switch on the order of nanoseconds, suggesting a close structural resemblance between the amorphous and crystalline phases. Despite this, the link between crystalline and amorphous tellurides is not fully understood nor quantified. Here we use in-situ high-temperature x-ray absorption spectroscopy (XAS) and theoretical calculations to quantify the amorphous structure of bulk and nanoscale GeTe. Based on XAS experiments, we develop a theoretical model of the amorphous GeTe structure, consisting of a disordered fcc-type Te sublattice and randomly arranged chains of Ge atoms in a tetrahedral coordination. Strikingly, our intuitive and scalable model provides an accurate description of the structural dynamics in phase-change memory materials, observed experimentally. Specifically, we present a detailed crystallization mechanism through the formation of an intermediate, partially stable 'ideal glass' state and demonstrate differences between bulk and nanoscale GeTe leading to size-dependent crystallization temperature.

Temperature Dependence of Structural Relaxation in Glass-Forming Liquids and Polymers

Understanding the microscopic mechanism of the transition of glass remains one of the most challenging topics in Condensed Matter Physics. What controls the sharp slowing down of molecular motion upon approaching the glass transition temperature T_g, whether there is an underlying thermodynamic transition at some finite temperature below T_g, what the role of cooperativity and heterogeneity are, and many other questions continue to be topics of active discussions. This review focuses on the mechanisms that control the steepness of the temperature dependence of structural relaxation (fragility) in glass-forming liquids. We present a brief overview of the basic theoretical models and their experimental tests, analyzing their predictions for fragility and emphasizing the successes and failures of the models. Special attention is focused on the connection of fast dynamics on picosecond time scales to the behavior of structural relaxation on much longer time scales. A separate section discusses the specific case of polymeric glass-forming liquids, which usually have extremely high fragility. We emphasize the apparent difference between the glass transitions in polymers and small molecules. We also discuss the possible role of quantum effects in the glass transition of light molecules and highlight the recent discovery of the unusually low fragility of water. At the end, we formulate the major challenges and questions remaining in this field.

Effect of Chitin Whiskers on the Molecular Dynamics of Carrageenan-Based Nanocomposites

Films of carrageenan (KC) and glycerol (g) with different contents of chitin nanowhiskers (CHW) were prepared by a solution casting process. The molecular dynamics of pure carrageenan (KC), carrageenan/glycerol (KCg) and KCg with different quantities of CHWs as a filler was studied using dielectric relaxation spectroscopy. The analysis of the CHW effect on the molecular mobility at the glass transition, T_g, indicates that non-attractive intermolecular interactions between KCg and CHW occur. The fragility index increased upon CHW incorporation, due to a reduction in the polymer chains mobility produced by the CHW confinement of the KCg network. The apparent activation energy associated with the relaxation dynamics of the chains at T_g slightly increased with the CHW content. The filler nature effect, CHW or montmorillonite (MMT), on the dynamic mobility of the composites was analyzed by comparing the dynamic behavior of both carrageenan-based composites (KCg/xCHW, KCg/xMMT).

Dynamics of the lignin glass transition

Despite lignin being a heterogenous polyphenolic, its glass transition obeys well-established polymer theory concepts.

Quantum effects in dynamics of water and other liquids of light molecules

Nuclear quantum effects in atomic motions are well known at low temperatures [Formula: see text] K, but analyses of structural relaxation in liquids and description of the glass transition traditionally neglect quantum effects at higher temperatures, [Formula: see text] K. Recent studies, however, suggested that nuclear quantum effects in systems of light molecules (e.g., water) might play an important role in structural dynamics and provide non-negligible contributions at such temperatures, and even up to ambient temperature. In this article, we discuss experimental evidences of the quantum effects in glass transition in liquids of light molecules and possible theoretical descriptions of these effects. We show that quantum effects may qualitatively change the temperature behavior of the structural relaxation time in supercooled liquids leading to deviations of some well-established properties of the glass transition when it happens at low temperatures. We also demonstrate that unusual behavior of water dynamics at low temperatures, including apparent fragile-to-strong crossover, can be ascribed to nuclear quantum effects.

Microscopic dynamics perspective on the relationship between Poisson's ratio and ductility of metallic glasses

In metallic glasses a clear correlation had been established between plasticity or ductility with the Poisson's ratio νPoisson and alternatively the ratio of the elastic bulk modulus to the shear modulus, K/G. Such a correlation between these two macroscopic mechanical properties is intriguing and is challenging to explain from the dynamics on a microscopic level. A recent experimental study has found a connection of ductility to the secondary β-relaxation in metallic glasses. The strain rate and temperature dependencies of the ductile-brittle transition are similar to the reciprocal of the secondary β-relaxation time, τβ. Moreover, metallic glass is more ductile if the relaxation strength of the β-relaxation is larger and τβ is shorter. The findings indicate the β-relaxation is related to and instrumental for ductility. On the other hand, K/G or νPoisson is related to the effective Debye-Waller factor (i.e., the non-ergodicity parameter), f0, characterizing the dynamics of a structural unit inside a cage formed by other units, and manifested as the nearly constant loss shown in the frequency dependent susceptibility. We make the connection of f0 to the non-exponentiality parameter n in the Kohlrausch stretched exponential correlation function of the structural α-relaxation function, ϕ(t)=exp[-(t/τα)(1-n)]. This connection follows from the fact that both f0 and n are determined by the inter-particle potential, and 1/f0 or (1 - f0) and n both increase with anharmonicity of the potential. A well tested result from the Coupling Model is used to show that τβ is completely determined by τα and n. From the string of relations, (i) K/G or νPoisson with 1/f0 or (1 - f0), (ii) 1/f0 or (1 - f0) with n, and (iii) τα and n with τβ, we arrive at the desired relation between K/G or νPoisson and τβ. On combining this relation with that between ductility and τβ, we have finally an explanation of the empirical correlation between ductility and the Poisson's ratio νPoisson or K/G based on microscopic dynamical properties.

Connection between slow and fast dynamics of molecular liquids around the glass transition

The mean-square displacement (MSD) was measured by neutron scattering at various temperatures and pressures for a number of molecular glass-forming liquids. The MSD is invariant along the glass-transition line at the pressure studied, thus establishing an "intrinsic" Lindemann criterion for any given liquid. A one-to-one connection between the MSD's temperature dependence and the liquid's fragility is found when the MSD is evaluated on a time scale of ∼4 ns , but does not hold when the MSD is evaluated at shorter times. The findings are discussed in terms of the elastic model and the role of relaxations, and the correlations between slow and fast dynamics are addressed.

Macro- and microdefinitions of fragility of hydrogen-bonded glass-forming liquids

In the present paper, the nature of the fragility of liquids with a developed H-bond network is investigated. It is shown that the effective activation energy of the shear viscosity eta and the fragility parameter defined as m=d log eta/d(Tg/T)|t=t+g are immediately connected with the average number nH(T) of the H-bond per molecule. The consistency of the macroscopic (m) and the microscopic [M=d(u(2)(0)/<u(-->2)>loc)/d(Tg/T)|T=T+8, where <u-->2>loc=<u-->2>anharm - <u-->2>harm] definitions of the fragility is established.

Why liquids are fragile

The fragilities (T(g)-normalized temperature dependence of alpha-relaxation times) of 33 glass-forming liquids and polymers are compared for isobaric, mP, and isochoric, mV, conditions. We find that the two quantities are linearly correlated: mP = (37+/-3) + (0.84+/-0.05)mV. This result has obvious and important consequences, since the ratio mV/mP is a measure of the relative degree to which temperature and density control the dynamics. Moreover, we show that the fragility itself is a consequence of the relative interplay of temperature and density effects near T(g). Specifically, strong behavior reflects a substantial contribution from density (jammed dynamics), while the relaxation of fragile liquids is more thermally activated. Drawing on the scaling law log(tau) = I(T upsilon(gamma)), a physical interpretation of this result in terms of the intermolecular potential is offered.

Effect of chemical structure on the isobaric and isochoric fragility in polychlorinated biphenyls

Pressure-volume-temperature data, along with dielectric relaxation measurements, are reported for a series of polychlorinated biphenyls (PCB), differing in the number of chlorine atoms on their phenyl rings. Analysis of the results reveals that with increasing chlorine content, the relaxation times of the PCB become governed to a greater degree by density rho relative to the effect of temperature T. This result is consistent with the respective magnitudes of the scaling exponent gamma yielding superpositioning of the relaxation times measured at various temperatures and pressures, when plotted versus rho(gamma)/T. While at constant (atmospheric) pressure, fragilities for the various PCB are equivalent, the fragility at constant volume varies inversely with chlorine content. Evidently, the presence of bulkier chlorine atoms on the phenyl rings magnifies the effect which the density has on the relaxation dynamics.

Landscape equivalent of the shoving model

It is shown that the shoving model expression for the average relaxation time of viscous liquids, according to which the activation energy is proportional to the instantaneous shear modulus, follows largely from a classical "landscape" estimation of barrier heights from curvature at energy minima. Although the activation energy in this reasoning involves both instantaneous bulk and shear moduli, the bulk modulus contributes less than 8% to the temperature dependence of the activation energy. This reflects the fact that the physics of the two models are closely related.

Correlation between nonexponential relaxation and non-Arrhenius behavior under conditions of high compression

Photon correlation spectroscopy was used to investigate the behavior of the dynamical properties of 1,1'-di(4-methoxy-5-methyl-phenyl)cyclohexane (BMMPC) at elevated pressures. The fragility of BMMPC measured by the steepness index m(T) is decreasing and the nonexponentiality parameter beta(KWW) is increasing with increasing pressure. This result strongly suggests that the phenomenological correlation between the steepness index and nonexponentionality is also preserved under high compression. The pressure dependence of the structural relaxation times is well characterized by a simple activation volume form. The activation volume continuously increases with decreasing temperature, which is probably due to the increase of cooperativity of the structural relaxation process. Moreover, we found that the glass-transition temperature exhibits a significant dependence on pressure.