Critical Insight into Pretransitional Behavior and Dielectric Tunability of Relaxor Ceramics.

This model discussion focuses on links between the unique properties of relaxor ceramics and the basics of Critical Phenomena Physics and Glass Transition Physics. It indicates the significance of uniaxiality for the appearance of mean-field type features near the paraelectric-to-ferroelectric phase transition. Pretransitional fluctuations, that are increasing up to the size of a grain and leading to inter-grain, random, local electric fields are responsible for relaxor ceramics characteristics. Their impact yields the pseudospinodal behavior associated with "weakly discontinuous" local phase transitions. The emerging model redefines the meaning of the Burns temperature and polar nanoregions (PNRs). It offers a coherent explanation of "dielectric constant" changes with the "diffused maximum" near the paraelectric-to-ferroelectric transition, the sensitivity to moderate electric fields (tunability), and the "glassy" dynamics. These considerations are challenged by the experimental results of complex dielectric permittivity studies in a Ba0.65Sr0.35TiO3 relaxor ceramic, covering ca. 250 K, from the paraelectric to the "deep" ferroelectric phase. The distortion-sensitive and derivative-based analysis in the paraelectric phase and the surrounding paraelectric-to-ferroelectric transition reveal a preference for the exponential scaling pattern for ε(T) changes. This may suggest that Griffith-phase behavior is associated with mean-field criticality disturbed by random local impacts. The preference for the universalistic "critical & activated" evolution of the primary relaxation time is shown for dynamics. The discussion is supplemented by a coupled energy loss analysis. The electric field-related tunability studies lead to scaling relationships describing their temperature changes.

Two-dimensional phases of confined 5-cyano-biphenyl: Computer simulation study.

The properties of composites of mesogens and two-dimensional (2D) materials are of great interest due to their practical applications in flexible displays, optoelectronics, microelectronics, and novel nanodevices. The properties of such composites are very complex and strongly depend on the interactions between the host material and the mesogen filling. We have performed molecular dynamics simulations for 4-cyano-4^{'}-pentylbiphenyl embedded between graphene and hexagonal 2D boron nitride layers. The structural and dynamical properties of such systems were investigated in terms of the order parameters, density profiles, mean square displacement, and autocorrelation function of the single-molecule dipole moment. Our simulations have shown that the mesogenic molecules form highly stable ordered layered structures and that their dynamics are strongly related to the structural properties. We have investigated not only the effects of the polarization of the host material, but also the effects of the spatial repetition of such composites by using two models of mesogens embedded in 2D layers: the direct sheet and the structure formed by multiplying a single unit of the composite in the direction perpendicular to the substrate surface.

History-dependent phase transition character.

We consider history-dependent behavior in domain-type configurations in orientational order that are formed in configurations reached via continuous symmetry-breaking phase transitions. In equilibrium, these systems exhibit in absence of impurities a spatially homogeneous order. We focus on cases where domains are formed via (i) Kibble-Zurek mechanism in fast enough quenches or by (ii) Kibble mechanism in strongly supercooled phases. In both cases, domains could be arrested due to pinned topological defects that are formed at domain walls. In systems exhibiting polar or quadrupolar order, point and line defects (disclinations) dominate, respectively. In particular, the disclinations could form complex entangled structures and are more efficient in stabilizing domains. Domain patterns formed by fast quenches could be arrested by impurities imposing a strong enough random-field type disorder, as suggested by the Imry-Ma theorem. On the other hand, domains formed in supercooled systems could be also formed if large enough energy barriers arresting domains are established due to large enough systems' stiffness. The resulting effective interactions in established domain-type patterns could be described by random matrices. The resulting eigenvectors reveal expected structural excitations formed in such structures. The most important role is commonly played by the random matrix largest eigenvector. Qualitatively different behavior is expected if this eigenvector exhibits a localized or extended character. In the former case, one expects a gradual, non-critical-type transition into a glass-type structure. However, in the latter case, a critical-like phase behavior could be observed.

Pressure Evolution of Glass Transition Temperature in LiFePO4.

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.

The fluctuation-driven dielectric properties of liquid crystalline 8OCB and its nanocolloids.

Results of broadband dielectric spectroscopy studies in liquid crystalline octyloxycyanobiphenyl (8OCB) and its colloids with BaTiO3 nanoparticles (paraelectric, diameter d = 50 nm) are presented. Studies were carried out in isotropic liquid, nematic, smectic A and solid crystalline phases. They are supported by derivative-based and distortion-sensitive analyses, revealing a set of universal scaling patterns for temperature evolution of static and dynamic dielectric properties. All these yielded evidence for a pretransitional fluctuation impact on the dielectric constant, primary relaxation time, loss curve maximum and translational-orientational decoupling associated with anomalous values of the fractional Debye-Stokes-Einstein (DSE) exponent 0.2 < S < 2.2. The evidence for critical like changes in a distribution of relaxation times is shown. For mesophases, a tiny addition of nanoparticles causes permanent orientation of LC molecules, leading even to a 16% increase of the dielectric constant above a maximal value in pure 8OCB. A split of two regions, dominated by pre-isotropic and pre-smectic fluctuations, is evidenced. Model explanations for detected phenomena, particularly in the isotropic liquid phase, are presented. New evidence for a premelting effect in the solid phase is also shown.

New paradigm for configurational entropy in glass-forming systems.

We show that on cooling towards glass transition configurational entropy exhibits more significant changes than predicted by classic relation. A universal formula according to Kauzmann temperature [Formula: see text] is given: [Formula: see text], where [Formula: see text]. The exponent [Formula: see text] is hypothetically linked to dominated local symmetry. Such a behaviour is coupled to previtreous evolution of heat capacity [Formula: see text] associated with finite temperature singularity. These lead to generalised VFT relation, for which the basic equation is retrieved. For many glass-formers, basic VFT equation may have only an effective meaning. A universal-like reliability of the Stickel operator analysis for detecting dynamic crossover phenomenon is also questioned. Notably, distortions-sensitive and derivative-based analysis focused on previtreous changes of configurational entropy and heat capacity for glycerol, ethanol and liquid crystal is applied.

Novel High-Pressure Nanocomposites for Cathode Materials in Sodium Batteries.

A new nanocomposite material was prepared by high pressure processing of starting glass of nominal composition NaFePO4. Thermal, structural, electrical and dielectric properties of the prepared samples were studied by differential thermal analysis (DTA), X-ray diffraction (XRD) and broadband dielectric spectroscopy (BDS). It was demonstrated that high-pressure-high-temperature treatment (HPHT) led to an increase in the electrical conductivity of the initial glasses by two orders of magnitude. It was also shown that the observed effect was stronger than for the lithium analogue of this material studied by us earlier. The observed enhancement of conductivity was explained by Mott's theory of electron hopping, which is more frequent in samples after pressure treatment. The final composite consisted of nanocrystalline NASICON (sodium (Na) Super Ionic CONductor) and alluaudite phases, which are electrochemically active in potential cathode materials for Na batteries. Average dimensions of crystallites estimated from XRD studies were between 40 and 90 nm, depending on the phase. Some new aspects of local dielectric relaxations in studied materials were also discussed. It was shown that a combination of high pressures and BDS method is a powerful method to study relaxation processes and molecular movements in solids. It was also pointed out that high-pressure cathode materials may exhibit higher volumetric capacities compared with commercially used cathodes with carbon additions.

Long-Range Static and Dynamic Previtreous Effects in Supercooled Squalene-Impact of Strong Electric Field.

This article presents evidence for the long-range previtreous changes of two static properties: the dielectric constant (ε) and its strong electric field related counterpart, the nonlinear dielectric effect (NDE). Important evidence is provided for the functional characterizations of ε(T) temperature changes by the 'Mossotti Catastrophe' formula, as well as for the NDE vs. T evolution by the relations resembling those developed for critical liquids. The analysis of the dynamic properties, based on the activation energy index, excluded the Vogel-Fulcher-Tammann (VFT) relation as a validated tool for portraying the evolution of the primary relaxation time. This result questions the commonly applied 'Stickel operator' routine as the most reliable tool for determining the dynamic crossover temperature. In particular, the strong electric field radically affects the distribution of the relaxation times, the form of the evolution of the primary relaxation time, and the fragility. The results obtained in this paper support the concept of a possible semi-continuous phase transition hidden below Tg. The studies were carried out in supercooled squalene, a material with an extremely low electric conductivity, a strongly elongated molecule, and which is vitally important for biology and medicine related issues.

Pretransitional Effects of the Isotropic Liquid-Plastic Crystal Transition.

We report on strong pretransitional effects across the isotropic liquid-plastic crystal melting temperature in linear and nonlinear dielectric response. Studies were carried out for cyclooctanol (C8H16O) in the unprecedented range of temperatures 120 K < T < 345 K. Such pretransitional effects have not yet been reported in any plastic crystals. Results include the discovery of the experimental manifestation of the Mossotti Catastrophe behavior, so far considered only as a hypothetical paradox. The model interpretations of experimental findings are proposed. We compare the observed pretransitional behavior with the one observed in octyloxycyanobiphenyl (8OCB), typical liquid crystal (LC), displaying a reversed sequence of phase transitions in orientational and translational degrees of order on varying temperature. Furthermore, in its nematic phase, we demonstrate first-ever observed temperature-driven crossover between regions dominated by isotropic liquid and smectic A pretransitional fluctuations. We propose a pioneering minimal model describing plastic crystal phase behavior where we mimic derivation of classical Landau-de Gennes-Ginzburg modeling of Isotropic-Nematic-Smectic A LC phase behavior.

Dynamics and Pretransitional Effects in C60 Fullerene Nanoparticles and Liquid Crystalline Dodecylcyanobiphenyl (12CB) Hybrid System.

The report shows the strong impact of fullerene C60 nanoparticles on phase transitions and complex dynamics of rod-like liquid crystal dodecylcyanobiphenyl (12CB), within the limit of small concentrations. Studies were carried out using broadband dielectric spectroscopy (BDS) via the analysis of temperature dependences of the dielectric constant, the maximum of the primary loss curve, and relaxation times. They revealed a strong impact of nanoparticles, leading to a ~20% change of dielectric constant even at x = 0.05% of C60 fullerene. The application of the derivative-based and distortion-sensitive analysis showed that pretransitional effects dominate in the isotropic liquid phase up to 65 K above the clearing temperature and in the whole Smectic A mesophase. The impact of nanoparticles on the pretransitional anomaly appearance is notable for the smectic-solid phase transition. The fragility-based analysis of relaxation times revealed the universal pattern of its temperature changes, associated with scaling via the "mixed" ("activated" and "critical") relation. Phase behavior and dynamics of tested systems are discussed within the extended Landau-de Gennes-Ginzburg mesoscopic approach.

The Bioaccessibility of Antioxidants in Black Currant Puree after High Hydrostatic Pressure Treatment.

The aim of the study was to investigate the effect of high-pressure processing (HPP) and thermal processing (TP) on the bioaccessibility of vitamin C and anthocyanins as well as changes in the antioxidant capacity (AC) using ABTS+• and DPPH• tests on blackcurrant (Ribes nigrum L.) puree during the steps in the digestive process. The puree was subjected to HPP at 200, 400, and 600 MPa for 5 min (room temperature) or TP at 85 °C for 10 min. The controls were untreated puree (P) and fruit crushed in a mortar (M). All the samples were digested in a static in vitro digestion model, including the mouth, stomach, and small intestine, and subjected to dialysis. The vitamin C, anthocyanin, and antioxidant capacity were monitored at each step of the digestion process. The potential bioaccessibility of the antioxidants studied was calculated in relation to the undigested sample. TP and HPP enabled a high content of vitamin C, anthocyanins, and AC to be maintained. After simulated digestion in the small intestine, a significant decrease was observed in the vitamin C and anthocyanins (approximately 98%) content. However, a high stability (approximately 70%) of both compounds was noted at the gastric stage. HPP and TP significantly affected the potential bioaccessibility of vitamin C and anthocyanins, although the bioaccessibility of both compounds in the samples treated using HPP was higher than when using TP. Moreover, the potential bioaccessibility of vitamin C after HPP treatment (400 and 600 MPa) was higher than the bioaccessibility calculated for the M and P control samples. TP and HPP treatment negatively affected anthocyanin bioaccessibility after dialysis. The most favorable pressure was 400 MPa, as it allowed maintaining the best antioxidant activity after digestion.

Multifold pressure-induced increase of electric conductivity in LiFe0.75V0.10PO4 glass.

We investigated the impact of high pressure and high-temperature annealing on lithium-vanadium-iron-phosphate (LiFe0.75V0.10PO4) glass materials, proposed for the use in cathodes for high-performance batteries. The treatment was carried out below the glass transition temperature (Tg ≈ 483 °C) at P = 1 GPa pressure, in an argon atmosphere. It led to the multifold electrical conductivity increase. Broadband dielectric spectroscopy (BDS) measurements before and after the process revealed the strong DC-conductivity increase across the whole studied frequency range by two orders of magnitude. The phenomenon is explained using Mott's theory of polaron hopping in disordered solids containing transition metal oxides. The pressure evolution of the glass transition temperature and the crystallisation temperature above Tg is shown.

Polymer matrix ferroelectric composites under pressure: Negative electric capacitance and glassy dynamics.

This report presents the results of high-pressure and broadband dielectric spectroscopy studies in polyvinylidene difluoride (PVDF) and barium strontium titanate (BST) microparticles composites (BST/PVDF). It shows that the Arrhenius behaviour for the temperature-related dynamics under atmospheric pressure is coupled to Super-Arrhenius/Super-Barus isothermal pressure changes of the primary relaxation time. Following these results, an explanation of the unique behaviour of the BST/PVDF composite is proposed. Subsequently, it is shown that when approaching the GPa domain the negative electric capacitance phenomenon occurs.

Nanoparticle-controlled glassy dynamics in nematogen-based nanocolloids.

Results of broad-band dielectric spectroscopy studies in liquid crystal (pentylcyanobiphenyl, 5CB)-based nanocolloids are presented. They reveal the strong impact of BaTiO_{3} nanoparticles on dynamics and uniaxial ordering. Studies were carried out in an extreme range of temperatures (∼150 K), including the supercooled nematic phase. For the latter, the unique "pretransitional" effect for dielectric constant on approaching solid state is reported. The distortion-sensitive analysis revealed super-Arrhenius dynamics but associated with critical-like behavior. In the isotropic phase, translational-orientational decoupling, unusual for the high temperature dynamic domain, was detected. It can be directly link to heterogeneities-prenematic fluctuations. The model linking the classical Landau-de Gennes approach with Imry-Ma arguments has been developed to discuss experimental results.

Impact of polarized nanotube surface on ultrathin mesogen film properties: Computer simulation study.

We studied properties of monolayer films of n-cyanobiphenyl (with n=5,...,8) series of mesogens anchored on the surface of single walled boron nitride nanotube. In order to assess the impact of substrate polarization on the ordering effects we compare translational and reorientational dynamics of the films with the characteristics of analogous carbon and silicon carbide nanotube based systems. We observed significant increase of the ordering degree accompanied by increased thermal stability. This ordering is less selective than those induced by the silicon carbide nanotube, which were previously reported. The antiparallel orientation of the nearest neighboring mesogens is predominant, while the system does not exhibit any long-range spatial correlations which indicates that the size of the domains is constrained to this region. These features might be of potential importance in the design of novel optoelectronic devices.

Unique dynamic crossover in supercooled x,3-dihydroxypropyl acrylate (x = 1, 2) isomers mixture.

The previtreous dynamics in the glass-forming monomer, glycerol monoacrylate (GMA), was tested using the broadband dielectric spectroscopy (BDS). The measurements revealed a clear dynamic crossover at the temperature [Formula: see text] K and the time scale [Formula: see text] ns for the primary (structural) relaxation time and no hallmarks for the crossover for the DC electric conductivity [Formula: see text]. This result was revealed via the derivative-based and distortions-sensitive analysis [Formula: see text] vs. [Formula: see text] , where [Formula: see text] stands for the apparent activation energy. Subsequent tests of the fractional Debye-Stokes-Einsten relation [Formula: see text] showed that the crossover is associated with [Formula: see text] [Formula: see text] (for [Formula: see text]. The crossover coexists with the emergence of the secondary beta relaxation, which smoothly develops deeply into the solid amorphous phase below the glass temperature [Formula: see text].

Impact of ferroelectric and superparaelectric nanoparticles on phase transitions and dynamics in nematic liquid crystals.

Results of broadband dielectric spectroscopy (BDS) studies of pure liquid crystalline (4-pentyloxy-4-biphenylcarbonitryle) 5OCB and its nanocolloids with BaTiO_{3} nanoparticles (NPs) under varying pressure and temperature are presented. The notable impact of NPs on phase transitions and dynamics was found. Particularly strong impact on pretransitional behavior was observed for relatively low concentrations of NPs, which can be related to the NPs-induced disorder. There are also notable differences between pressure and temperature paths of studies for nanocomposites, absent for the pure LC compound. For instance, tests focused on the translational orientational decoupling via the fractional Debye-Stokes-Einstein relation yielded S=0.71 and S=0.3 for the temperature and pressure paths, respectively: S=1 is for the complete coupling. The possible theoretical frame of observed phenomena is also proposed.

Dielectric Spectroscopy of Pressurized Saccharomyces cerevisiae.

Results of broadband dielectric spectroscopy (BDS) in Saccharomyces cerevisiae (baker's yeast), in situ as the function of pressure are presented. They show a clear evidence of a threshold to the new pattern of the pressure evolution of the static dielectric permittivity and DC electric conductivity already for Pt ≈ 200MPa at T = 5 oC and Pt ≈ 300MPa at T = 25 oC. BDS monitoring versus pressure tests up to P = 400MPa revealed particularly notable changes of properties after 30 minutes of compressing. Finally, the correlation between the amount of the spectrophotometric maximum absorbance and the DC electric conductivity was found. All these indicate significance of BDS as the tool for testing of pressure properties of cells assemblies, model foods etc., in situ under high pressures.

Fractional Debye-Stokes-Einstein behaviour in an ultraviscous nanocolloid: glycerol and silver nanoparticles.

One of the major features of glass forming ultraviscous liquids is the decoupling between translational and orientational dynamics. This paper presents studies of this phenomenon in glycerol, an accepted molecular glass former, concentrating on the impact of two exogenic factors: high pressures (P) up to the extreme 1.5 GPa and silver (Ag) nanoparticles (NPs). The analysis is focused on the fractional Debye-Stokes-Einstein (FDSE) relationship: σ(T,P)(τ(T,P))(S) = const, linking DC electric conductivity (σ) and primary (alpha, structural) relaxation time (τα). In glycerol and its nanocolloid (glycerol + Ag NPs) at atmospheric pressure only negligible decoupling (S ∼ 1) was detected. However, in the compressed nanocolloid, a well-defined transformation (at P = 1.2 GPa) from S ∼ 1 to the very strongly decoupled dynamics (S ∼ 0.5) occurred. For comparison, in pressurized 'pure' glycerol the stretched shift from S ∼ 1 to S ∼ 0.7 took place. This paper also presents the general discussion of FDSE behavior in ultraviscous liquids, including the new link between the FDSE exponent, fragility and the apparent activation enthalpy and volume.

Fragility and basic process energies in vitrifying systems.

The concept of 'fragility' constitutes a central point of the glass transition science serving as the 'universal' metric linking previtreous dynamics of qualitatively distinct systems. Finding the fundamental meaning of fragility is the 'condicio sine qua' for reaching the long expected conceptual breakthrough in this domain. This report shows that fragility is determined by the ratio between two fundamental process energies, viz. the activation enthalpy and activation energy. The reasoning, avoiding any underlying physical model, is supported by the experimental evidence ranging from low molecular weight liquids and polymers to plastic crystals and liquid crystals. All these lead to the new general scaling plot for dynamics of arbitrary glass former. The limited adequacy of broadly used so far semi-empirical relationships between fragility and the activation energy is shown. Results presented remain valid for an arbitrary complex system and collective phenomena if their dynamics is described by the general super-Arrhenius relation.