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  • Evolution of Tg in lithium-based glass under pressure

    LiFePO4 is an important base material for generation of new batteries. One of the important developments is its use in the form of a solid glass, which allows an increase in the electrical conductivity after the high-pressure process. Such a treatment allows full control of the vitrification and nanocrystallization processes as well. This report shows the basic reference for the pressure dependence of the glass transition temperature. The unique behavior has been proven with a maximum of Tg (P) already at moderate pressures. The protocol for depicting the resulting evolution is as follows: it enables a reliable extrapolation beyond the experimental domain. The importance of the presented results for the general topic of glass transition physics is also remarkable due to the scant evidence of the existence of systems with clearly inverted vitrification under compression.

    See more here.

    Pressure dependence of the glass temperature in LiFePO4. The dashed curve is related to eq 1. The curve has the following parameters: π = −6.2 GPa, b = 0.12, c = −0.83 GPa. An arbitrary reference temperature and pressure along the vitrification curve can be substituted, including Tg0 = 754 K and Pg0 = 0.001 GPa. The error bars for Tg are also shown.
  • New paper in Soft Matter!

    Liquid crystal is a state of matter combining properties of conventional liquids (isotropic liquids) and solid crystals. Liquid crystals flow like isotropic liquids but they exhibit longrange order of molecules, typical for crystalline solids. This group of materials is probably best known for their use in displays (LCD, LED). As liquid crystals research is largely driven by the display industry, many papers focus on properties connected with the performance of these devices, including threshold voltage (minimum amount of voltage that is necessary to produce any molecular movement) or direct current (DC) electric conductivity (its source are ions, which cause image sticking). In our work we chose a more fundamental approach, we show the temperature evolution of static and dynamic properties and describe them using theoretical models. We put special stress on a topic that is often neglected in literature: the influence of pretransitional fluctuations on tested properties.

    In the paper, we compared the properties of a pure liquid crystal (8OCB) with its nanocolloids doped with nanoparticles (BaTiO3). Measurements were carried out in a wide range of temperatures and covers isotropic liquid, two different liquid crystalline, and solid phases. We investigated such parameters as dielectric constant (which describes the ability to polarize a material), relaxation time (time that takes a perturbed molecule to return to its initial state), or parameters describing the shape of the relaxational curve (e.g. its maximum).

    It was shown that the addition of nanoparticles leads to the permanent orientation of molecules, approximately parallel to the external electric field. This caused even up to a 16% increase in dielectric constant value in comparison to the pure sample.

    The fractional Debye-Stokes-Einstein equation was used to assess coupling/decoupling between orientational (rotational) and translational motions. In the isotropic phase in each sample, orientational processes were faster than translational. The small concentrations of nanoparticles (lower than 1 weight percent) made this effect even stronger. In the low-temperature liquid crystal phase (smectic A) addition of nanoparticles lead to a slowdown of orientational motions, which was not observed before. The decoupling is especially visible in near phase-transition temperature regions. It results from pretransitional fluctuations. They are also evidenced in temperature evolutions of relaxation peak maximum and dielectric constant. To describe the latter, a new equation, proposed by one of the authors (ADR) in her previous work, was applied.

    New evidence for a premelting effect in the solid phase is also shown.

    See more: https://pubs.rsc.org/en/content/articlelanding/2022/sm/d2sm00105e/unauth

  • Thermodynamic character of glass transition is possible!

    In the latest paper entitled „New paradigm for configurational entropy in glass-forming systems” Colleagues from the X-PressMatter Laboratory (NL10, Aleksandra Drozd-Rzoska, Sylwester J. Rzoska, Szymon Starzonek) published in Scientific Reports (Nature-Springer) [https: // www. nature.com/articles/s41598-022-05897-2] showed that the current definition of the configuration entropy for the transition to the glass state is far inadequate. It turned out that using the developed methodology of the Physics of Critical Phenomena, it is possible to precisely determine the Kauzmann temperature and describe the changes in the configuration entropy in the wide glass transition temperature environment using a critical-like function with an exponent n, which assumes identical values for both the specific heat and the configuration entropy (thermodynamics) and dielectric relaxation (dynamics). What is especially worth emphasizing – its value correlates with the local molecular symmetry. The generalized (critical-like) relationship for entropy leads to the „generalized” Vogel-Fulcher-Tammann equation (VFT) for the description of viscosity, relaxation time, diffusion, electrical and thermal conductivity. It also leads to breakthrough consequences for concepts as fundamental in vitrification as brittleness or activation enthalpy. The glass transition is usually indicated as a dynamic phenomenon, which heuristically justifies by far-reaching pre-vitrification changes in the structural relaxation time or a similar evolution for viscosity. This is also confirmed by the dependence of the glass transition temperature on the cooling rate. In the aforementioned work, it was unequivocally demonstrated that the changes in the configuration entropy are described by a power function with a universal exponent depending on symmetry. Is this not like a critical phenomenon? Is this not a proof, that Kauzmann temperature is a specific critical temperature? So, are we not on the way to the Great Unification of critical and pre-vitrification phenomena?

    (left) Dependence of configuration entropy on temperature for selected substances. Fit with a classical function (red, n = 1) and a critical-like function (blue, n ≠ 1). (right) Pre-nitrification dependence of specific configuration heat for selected substances. The inset shows the pre-transient effect of specific heat around the glass transition temperature Tg.