Transition from single-molecule to cooperative dynamics in a simple glass former: Raman line-shape analysis

Normal-to-Supercooled Liquid Transition in Molecular Glass-Formers: A Hidden Structural Transformation Fuelled by Conformational Interconversion

Molecular dynamics and transport coefficients change significantly around the so-called Arrhenius crossover in glass-forming systems. In this article, we revisit the dynamic processes occurring in a glass-forming macrocyclic crown thiaether MeBzS2O above its glass transition, revealing two crossover temperatures: TB at 309 and TA at 333 K. We identify the second one as the Arrhenius crossover that is closely related to the normal-to-supercooled liquid transition in this compound. We show that the transformation occurring at this point goes far beyond molecular dynamics (where the temperature dependence of structural relaxation times changes its character from activation-like to super-Arrhenius), being reflected also in the internal structure and diffraction pattern. In this respect, we found a twofold local organization of the nearest-neighbor molecules via weak van der Waals forces, without the formation of any medium-range order or mesophases. The nearest surrounding of each molecule evolves structurally in time due to the ongoing fast conformational changes. We identify several conformers of MeBzS2O, demonstrating that its lowest-energy conformation is preferred mainly at lower temperatures, i.e., in the supercooled liquid state. Its increased prevalence modifies locally the short-range intermolecular order and promotes vitrification. Consequently, we indicate that the Arrhenius transition is fuelled rather by conformational changes in this glass-forming macrocyclic crown thiaether, which is a different scenario from the so-far existing concepts. Our studies combine broadband dielectric spectroscopy (BDS), X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, molecular dynamics (MD) simulations, and density functional theory (DFT) calculations.

Research of the structure and dynamic interactions of particles in the Li0.42K0.58NO3- R (R = α-Al2O3, γ-Al2O3, SiO2) and (LiNO3-LiClO4) - γ-Al2O3 composites in various temperature conditions and phase states

The paper presents a comprehensive study of the Li_0.42K_0.58NO₃- R (R = α-Al₂O₃, γ-Al₂O₃, SiO₂) and (LiNO₃ - LiClO4) - γ- Al₂O₃ composite salt systems by vibrational spectroscopy, X-ray diffractometry, and differential scanning calorimetry. According to the results of Raman spectra and XRD analysis, it has been found that in the case of a sufficiently small filler (15 nm-γ-Al₂O₃ and SiO₂), an amorphous phase appears, both for systems based on Li_0.42K_0.58NO₃ and for the nanocomposite based on (LiNO₃)_0.5-(LiClO₄)_0.5. In the case of nanocomposites based on the Li_0.42K_0.58NO₃ system, this additionally leads to the formation of the metastable KNO₃ (R3m) phase. The temperature changes in the half-widths of the corresponding bands of the Raman spectrum during the transition from a solid-phase to a molten state become less significant as the concentration of the oxide filler increases. At x ≥ 0.7, on the temperature dependences of the half-width of the ν1 (A) band in the vicinity of the melting point, only a small kink characteristic of amorphous systems is noticed. This is confirmed by the results of thermal analysis. It has been found that the solid filler has a different effect on the processes of orientational and vibrational relaxation in the molten phases of the studied NIS, namely, the orientational mobility of NO₃^- decreases, while the relaxation rate of its vibrational excitation increases.

Transition from Arrhenius to non-Arrhenius temperature dependence of structural relaxation time in glass-forming liquids: continuous versus discontinuous scenario

The temperature dependences of α relaxation time τ(α)(T) of three glass-forming liquids (salol, o-terphenyl, and α-picoline) were investigated by a depolarized light scattering technique. A detailed description of τ(α)(T) near T(A), the temperature of the transition from the Arrhenius law at high temperatures to a non-Arrhenius behavior of τ(α)(T) at lower temperatures, was done. It was found that this transition is quite sharp. If the transition is described as switching from the Arrhenius law to the Vogel-Fulcher-Tammann law, it occurs within the temperature range of about 15 K or less. Most of the known expressions for τ(α)(T) cannot describe this sharp transition. Our analysis revealed that this transition can be described either as a discontinuous transition in the spirit of the frustration-limited domain theory [D. Kivelson, G. Tarjus, X. Zhao, and S. A. Kivelson, Phys. Rev. E 53, 751 (1996)], implying a phase transition, or by a phenomenological expression recently suggested [B. Schmidtke, N. Petzold, R. Kahlau, M. Hofmann, and E. A. Rössler, Phys. Rev. E 86, 041507 (2012)], where the activation energy includes the term depending exponentially on temperature.

Rayleigh-Brillouin light-scattering study of a simple glass former: evidence of locally favored structures

The Rayleigh-Brillouin light scattering is studied in the glass-forming picoline with emphasis to the temperature dependence of the Landau-Placzek ratio. It was found that the Landau-Placzek ratio is in good agreement with the theoretical prediction at T>T(A), where TA is the temperature of transition from an Arrhenius-like to a non-Arrhenius behavior for the α-relaxation time dependence on temperature. Below T(A) the Landau-Placzek ratio exceeds significantly the theoretical prediction. The finding is interpreted as the evidence of the locally favored structures, which presence in an equilibrium glass-forming liquid below T(A).