Search

Electrical measurements were carried out on nanocrystals Yb2-xVxO3+x (YbVO) (where x=0.10, 0.15, 0.20, 0.30, and 0.40) obtained in the high-energy ball milling process, which crystallite size increased from 23.3 to 42.4 nm with increasing vanadium content. Electrical conductivity and thermoelectric power studies showed insulating properties in the extrinsic region (77-300 K) and n-type semiconductivity in the intrinsic one (350-400 K) with an activation energy of 1.3 eV. Broadband dielectric spectroscopy studies showed a low relative dielectric permittivity (εr<16) and loss tangent (tan(δ)<0.05) weakly dependent on temperature and frequency with the exception of a wide maximum between 200 and 300 K, suggesting an antiferroelectric-paraelectric phase transition. The most interesting result is the observation of a strong increase in the thermoelectric power factor with an increase in unit cell volume and nanograin size in a highly thermally activated Arrhenius region. [ABSTRACT FROM AUTHOR]

In this work, we examined the effect of the length of alkyl chain attached to the benzene ring on the self-assembling phenomena for a series of phenyl alcohol (PhA) derivatives, from phenylmethanol (benzyl alcohol) to 7-phenyl-1-heptanol, by means of X-Ray Diffraction (XRD), Differential Scanning Calorimetry (DSC), Fourier Transform Infrared (FTIR) spectroscopy, and Broadband Dielectric Spectroscopy (BDS) methods. XRD data in the reciprocal and real spaces showed a gradual increase in the local order with the elongation of the alkyl chain. However, the position and full width at half maximum of the main diffraction peak exhibited a non-systematic behavior. To better understand this fact, PhAs were subjected to FTIR spectroscopic studies. These investigations revealed that the association degree and the activation energy of dissociation increase as the alkyl chain length grows. On the other hand, BDS data showed a non-monotonic variation in the Kirkwood correlation factor with increasing length of the alkyl chain, indicating a competition between interactions of the non-polar and polar parts of the molecules in the studied PhAs. Finally, it was also found that the molar surface entropy for PhAs increases with the number of methylene groups, approaching values reported for alkanes, which indicates suppression of the surface order for PhAs with a long alkyl chain. This variability of the various parameters as a function of the length of the side chain shows that the interplay between soft interactions has a strong impact on the local structure and intra and intermolecular dynamics of the studied PhAs. [ABSTRACT FROM AUTHOR]

The fragility parameter has been acknowledged as one of the most important characteristics of glassforming liquids. We show that the mystery of the dramatic change in molecular dynamics of systems approaching the glass transition can be better understood by the high pressure study of fragility parameters defined in different thermodynamic conditions. We formulate and experimentally confirm a few rules obeyed by the fragility parameters, which are also rationalized by the density scaling law and its modification suggested for associated liquids. In this way, we successfully explore and gain a new insight into the pressure effect on molecular dynamics of van der Waals liquids, polymer melts, ionic liquids, and hydrogen-bonded systems near the glass transition. [ABSTRACT FROM AUTHOR]

4-methyl-3-heptanol, a monohydroxy alcohol with a relatively small dielectric Debye process, is studied in wide ranges of temperature (143 K < T < 308 K) and pressure (0.1 MPa < p < 864 MPa). When monitored under isochronous conditions, i.e., focusing on constant relaxation times, as well as under isothermal conditions, the Debye process gains significant intensity upon pressure application. This behavior contrasts with that of the previously studied octanol 2-ethyl-1-hexanol, which features a large Debye process. These experimentally observed, clearly distinguishable pressure evolutions are discussed to reflect differences in the formation of hydrogen-bonded supramolecular structures. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the effect of pressure on the molecular dynamics of protic ionic liquid lidocaine hydrochloride, a commonly used pharmaceutical, by means of dielectric spectroscopy and pressure-temperature-volume methods. We observed that near Tg the pressure dependence of conductivity relaxation times reveals a peculiar behavior, which can be treated as a manifestation of decoupling between ion migration and structural relaxation times. Moreover, we discuss the validity of thermodynamic scaling in lidocaine HCl. We also employed the temperature-volume Avramov model to determine the value of pressure coefficient of glass transition temperature, dTg/dP|P = 0.1. Finally, we investigate the role of thermal and density fluctuations in controlling of molecular dynamics of the examined compound. [ABSTRACT FROM AUTHOR]

The complex relative permittivity of a non-crystallizable secondary alcohol, 5-methyl-2-hexanol, is measured over a wide range of temperatures and pressures up to 1750 MPa (17.5 kbar). The data at atmospheric pressure (P = 0.101 MPa) are analyzed in terms of three processes, and the results are in complete agreement with that of O. E. Kalinovskaya and J. K. Vij [J. Chem. Phys. 112, 3262 (2000)]. Process I is of the Debye type and process II is of the Davidson-Cole type, whereas process III is identified as the Johari-Goldstein relaxation process. For pressures of ∼500 MPa and higher, processes I and II are seen to merge into each other to form a single dominant process which unambiguously cannot be resolved into more than one process. The dielectric relaxation strength of process I decreases slightly initially with pressure and when the two processes have merged at elevated pressures, the total relaxation strength increases with increase in pressure. Process III is better resolvable at higher pressures especially above Tg in the supercooled liquid state for the reason that the separation in the time scales between the dominant and the JG relaxation process increases at elevated pressures. Surprisingly we find a change in the slope in the plot of log τJG vs. 1/T for P = 1750 MPa. The results for the relaxation time of alcohols are compared with the Kirkwood correlation factor, g, and it is found that higher is the g, lower is the relaxation time for process I, and it is more of the Debye type. On a reduction in g brought about by an increase in pressure at lower temperatures, the dominant process becomes non-Debye though extensive hydrogen bonding is still present. The dielectric strength of the merged processes increases with increase in pressure. The values of the steepness index, m = |d log τ/d(Tg/T)|T = Tg for processes I and II are different for P = 0.1 MPa. However the value of m, for the composite process, which is a merger of processes I and II, for P = 1750 MPa is almost the same for process II at P = 0.1 MPa. From the results of the activation volume, activation enthalpy, and a comparison of the relaxation times with the g factor, we conclude that both processes I and II are significantly affected by hydrogen bonding and both contribute to the structural relaxation. [ABSTRACT FROM AUTHOR]

Broadband dielectric measurements on leucrose were performed under ambient and high pressure. We showed that in this disaccharide, there are two secondary relaxation modes, a slower one sensitive to pressure and a faster one that is not. This finding clearly indicates that the faster secondary relaxation originates from the intramolecular motion. This conclusion contradicted previous interpretations of this mode observed for trehalose and maltitol, systems very closely related to leucrose. In addition, pressure sensitivity of the slower relaxation confirms our recent interpretation about the character of this process. Furthermore, we discovered that unlike the faster relaxation, the slower secondary relaxation is sensitive to the thermodynamic history of measurements. Finally, monitoring the changes in maximum loss of the slower secondary relaxation measured at the same pressure and temperature conditions for glasses obtained via different thermodynamic routes enabled us to draw a conclusion about the density of the formed glasses. Our observations may be helpful in establishing a new method of suppressing crystallization of amorphous drugs. [ABSTRACT FROM AUTHOR]

Despite extensive efforts in experimental and computational studies,the microscopic understanding of dynamics of biologicalmacromolecules remains a great challenge. It is known that hydratedproteins, DNA and RNA, exhibit a so-called “dynamic transition.” It appears as a sharp rise of their mean-squared atomic displacements ≤r2≥ at temperatures above 200–230 K. Even after a long history of studies, this sudden activation of biomolecular dynamics remains a puzzle and many contradicting models have been proposed. By combining neutron and dielectric spectroscopy data, we were able to follow protein dynamics over an extremely broad frequency range. Our results show that there is no sudden change in the dynamics of the protein at temperatures around ∼200–230 K. The protein’s relaxation time exhibits a smooth temperature variation over the temperature range of 180–300 K. Thus the experimentally observed sharp rise in ≤r2≥ is just a result of the protein’s structural relaxation reaching the limit of the experimental frequency window. The microscopic mechanism of the protein’s structural relaxation remains unclear. [ABSTRACT FROM AUTHOR]

The primary α and the secondary Johari–Goldstein (JG) β relaxations of supercooled glass-forming neat epoxy resin and 2-picoline in mixture with tristyrene are monitored by broadband dielectric relaxation spectroscopy at ambient pressure and elevated pressures. For different combinations of pressure and temperature that maintain the α-relaxation time constant, the frequency dispersion of the α relaxation is unchanged, as previously found in other glass-formers, but remarkably the JG β-relaxation time remains constant. This is more clear evidence of a strong connection between the α- and JG β-relaxation times, a fact that should be taken into account in the construction of a viable theory of glass transition. [ABSTRACT FROM AUTHOR]

Dielectric relaxation spectra of two closely related glass formers, dipropylene glycol [H–(C3H6O)2–OH] and dipropylene glycol dimethyl ether [CH3–O–(C3H6O)2–CH3], were measured at ambient and elevated pressures in the supercooled and the glassy states are presented. Hydrogen bonds formed in dipropylene glycol are removed when its ends are replaced by two methyl groups to become dipropylene glycol dimethyl ether. In the process, the primary relaxation, the excess wing, and the resolved secondary relaxation of dipropylene glycol are all modified when the structure is transformed to become dipropylene glycol dimethyl ether. The modifications include the pressure and temperature dependences of these relaxation processes and their interrelations. Thus, by comparing the dielectric spectra of these two closely related glass formers at ambient and elevated pressures, the differences in the relaxation dynamics and properties in the presence and absence of hydrogen bonding are identified. [ABSTRACT FROM AUTHOR]

A combined positronium annihilation lifetime spectroscopy (PALS) and dielectric spectroscopy (DS) study on a typical van der Waals glass-former diethyl phthalate (DEP) was performed and the results were compared. From phenomenological point of view, the mutual relationships between the characteristic PALS temperatures, the glass temperature TgPALS, and the crossover temperatures Tb1L and Tb2L on the ortho-positronium (o-Ps) lifetime versus the temperature plot, have been discussed with respect to the characteristic DS temperatures, the glass temperature TgDS and the dynamic crossover temperature TBST, concerning the crossover behavior of primary α-relaxation times. Next, simultaneous application of the extended free volume (EFV) model by Cohen-Grest on the temperature dependence of both the mean free volume hole size data as extracted from PALS and the dielectric α-relaxation time revealed a good agreement between the experimental Tb1L and the characteristic EFV temperatures T0DS and T0PALS at which a free volume percolation should occur. These results indicate the important role of free volume in control of the primary (α) dynamics of supercooled DEP. [ABSTRACT FROM AUTHOR]

In some polymers, in addition to the usual structural α relaxation, a slower α′ relaxation is observed with a non-Arrhenius temperature dependence. In order to understand better the molecular origin of this α′ relaxation in poly(methylphenylsiloxane) (PMPS) we have studied, for the first time, the pressure dependence of its relaxation time, together with the usual temperature dependence, by means of dynamic light scattering (DLS). For the same material the α relaxation was also studied by means of DLS and dielectric spectroscopy (DS) in broad temperature and pressure ranges. We find that the temperature dependence of both α and α′ relaxation times, at all pressures studied, can be described by a double Vogel-Fulcher-Tammann (VFT) law. The pressure dependence of the characteristic temperatures Tg (glass transition temperature) and T0 (Vogel temperature) as well as the activation volumes for both α and α′ processes are very similar, indicating, that both relaxation processes originate from similar local molecular dynamics. Additionally, for both α and α′ relaxations the combined temperature and pressure dependences of the relaxation times can be described using a parameter Γ=ρn/T with the same value of the exponent n. [ABSTRACT FROM AUTHOR]

Broadband dielectric measurements for anhydrous tripropylene glycol (3PG) and 96, 92, 84, 80, 74, 71, and 68 wt % 3PG-water mixtures are performed in the frequency range of 10-2–107 Hz and in the temperature range of 123–243 K. We examined the effect of adding water into anhydrous 3PG on relaxation dynamics. Apart from the two well-known relaxation processes, i.e., α and β for anhydrous 3PG we observed new relaxation peak (β′) for all aqueous mixtures of 3PG. In addition we found the critical mole fraction of water xw=0.67 in which relaxation dynamics changes its behavior. According to the Sudo approach [S. Sudo et al., J. Non-Cryst. Solids 307–310, 356 (2002)], the behavior of relaxation processes was interpreted assuming the existence of three kinds of cooperative domains (CDs): containing only 3PG molecules, including only water molecules, and including both 3PG and water molecules, which molecules of each kind CD are bound by hydrogen bonds. [ABSTRACT FROM AUTHOR]

Broadband dielectric measurements were carried out at isobaric and isothermal conditions up to 1.75 GPa for reconsidering the relaxation dynamics of decahydroisoquinoline, previously investigated by Richert et al. [R. Richert, K. Duvvuri, and L.-T. Duong, J. Chem. Phys. 118, 1828 (2003)] at atmospheric pressure. The relaxation time of the intense secondary relaxation τβ seems to be insensitive to applied pressure, contrary to the α-relaxation times τα. Moreover, the separation of the α- and β-relaxation times lacks correlation between shapes of the α-process and β-relaxation times, predicted by the coupling model [see for example, K. L. Ngai, J. Phys.: Condens. Matter 15, S1107 (2003)], suggesting that the β process is not a true Johari–Goldstein (JG) relaxation. From the other side, by performing measurements under favorable conditions, we are able to reveal a new secondary relaxation process, otherwise suppressed by the intense β process, and to determine the temperature dependence of its relaxation times, which is in agreement with that of the JG relaxation. [ABSTRACT FROM AUTHOR]

We investigated dielectric relaxation of a tri-propylene glycol system under high compression. By increasing temperature and pressure we observed that a new relaxation process emerges from the low frequency tail of the structural peak. This new peak starts to be visible at about 0.5 GPa and becomes clearly evident at 1.7 GPa. However, this additional peak merges again with the structural one as the glass transition is approached, since it has a weaker temperature dependence. This finding enriches the relaxation scenario of molecular glass formers confirming that the application of very high hydrostatic pressure can favor the detection of new relaxation or otherwise unresolved processes in supercooled liquid systems. [ABSTRACT FROM AUTHOR]

The α-relaxation process in polymethylphenylsiloxane was studied over a broad temperature and pressure range by dielectric spectroscopy. In the vicinity of the glass temperature, the shape of the dielectric loss peak is independent of both temperature and pressure. The steepness index (fragility), describing the temperature dependence of the relaxation times, is also independent of pressure (and of molecular weight as well). Thus, the correlation between fragility and nonexponentiality of the relaxation function is maintained under conditions of high compression. The combined temperature and pressure dependences of the relaxation time conformed to the Avramov equation. This model offers a means to relate the relaxation behavior to thermodynamic properties of the material. [ABSTRACT FROM AUTHOR]

The effect of molecular structure on the dynamical properties of supercooled di-n-butyl phthalate (DBP) and its isomer, diisobutyl phthalate (DiBP) is examined. The results of the dielectric alpha- and beta-relaxations and the ionic conductivity are discussed.

The electrical measurements of ZnSb2-x TaxO6 phases with x = 0:0; 0:1; 0:25; 0:75; 1:6 have revealed insulating behavior with strongly decreasing electrical conductivity when tantalum x content is increased. As a consequence, high values of relative permittivity "r and loss tangent tan(δ) were observed, that decreased with increase of temperature and frequencies, for samples with low Ta content, below x = 0:75. In turn, for samples richer in tantalum, εr reaches 14, and tan(δ) becomes as low as 0.02 for x = 1:6. These effects have been interpreted either by the framework of the relaxation processes, or by the spatial charge polarization leading to the low energy loss of materials. [ABSTRACT FROM AUTHOR]

The (1 − x )Pb(Zr 0.70 Ti 0.30 )O 3 – x BiMn 2 O 5 ceramics (PZT-BM), where x = 0, 0.02, 0.055, 0.11, 0.15, 0.22 and 1, were studied. We determined how addition of nonpolar BM influenced electrical properties of the ferroelectric PZT ceramics. Impedance spectroscopy measurements in broad frequency and temperature ranges were performed and several contributions to impedance response were identified. A crossover to the relaxor state was observed in the PZT-BM ceramics by doping with Bi and Mn ions. The relaxation times for the electric conductivity relaxation and dipole relaxation were estimated from electric modulus representation of the data. Activation energy values of the conductivity process, estimated for T > 510 K, decreased from 0.82 to 0.37 eV when BM content increased. The occurrence of the high-frequency dipole relaxation was assigned to the charge transfer of Ti 3+ /Ti 4+ , Zr 3+ /Zr 4+ and Mn 3+ /Mn 4+ ions. Occurrence of the ferroelectric relaxor features were deduced from the non-Arrhenius dependence of the relaxation times. Superposition of the relaxor features and electric relaxations provides high value permittivity (ε′ > 1000) in wide temperature range (~ 250–573 K). This effect corresponds to the disorder and precipitation of ions that were shown using x-ray photoelectron spectroscopy and the time of flight–secondary ion mass spectrometry. [ABSTRACT FROM AUTHOR]

Metal–organic frameworks (MOFs), in which metal clusters are coupled by organic moieties, exhibit inherent porosity and crystallinity. Although these systems have been examined for vast potential applications, the elementary proton conduction in anhydrous MOFs still remains elusive. One of the approaches to deal with this problem is the utilization of protic organic molecules, to be accommodated in the porous framework. In this work we report the temperature-dependent crystal structure and proton conduction in [C2H5NH3][Na0.5Fe0.5(HCOO)3] metal–organic frameworks using X-ray diffraction and broadband dielectric spectroscopic techniques. The detailed analysis of the crystal structure reveals disorder of the terminal ethylene groups in the polar phase (space group Pn). The structural phase transition from Pn to P21/n at T≈ 363 K involves the distortion of the metal formate framework and ordering of EtA+ cations due to the reduction of the cell volume. The dielectric data have been presented in the dynamic window of permittivity formalism to understand the ferroelectric phase transition. The relaxation times have been estimated from the Kramers–Kronig transformation of the dielectric permittivity. A Grotthuss type mechanism of the proton conduction is possible at low temperatures with the activation energy of 0.23 eV. This type of experimental observation is expected to provide new prospective on the fundamental aspect of elementary proton transfer in anhydrous MOFs. [ABSTRACT FROM AUTHOR]

We report the synthesis, crystal structure, dielectric, vibrational and emission spectra of heterometallic MOFs, [C2H5NH3][Na0.5Cr0.5(HCOO)3] (EtANaCr), [C2H5NH3][Na0.5Al0.5(HCOO)3] (EtANaAl) and [C2H5NH3][Na0.5Al0.475Cr0.025(HCOO)3] (EtANaAlCr). These compounds crystallize in non-centrosymmetric monoclinic polar structures (space group Pn) and undergo order–disorder phase transitions upon heating to the monoclinic centrosymmetric structure (space group P21/n) at 369 (EtANaAl) and 373 K (EtANaCr). In principle, they are ferroelectric below these temperatures. In the high-temperature phase, ethylammonium (EtA+) cations are dynamically disordered over two symmetrically independent positions while upon cooling they begin to order. The ordering is accompanied by distortion of the metal formate framework. The hydrogen bonds (HBs) between the NH3+ group and NaO6 octahedral units are more robust than between the NH3+ group and CrO6 (AlO6) octahedral units and this feature explains a much stronger distortion of the former units and a weak effect of a trivalent cation type on the phase transition temperature. The dielectric studies have confirmed the occurrence of phase transitions of dipolar character and dipole relaxation processes. The optical studies show that EtANaCr and EtANaAlCr exhibit efficient Cr(iii)-based emission characteristics for intermediate-ligand field strength. [ABSTRACT FROM AUTHOR]

Primary alcohols have been an active area of research since the beginning of the 20th century. The main problem in studying monohydroxy alcohols is the molecular origin of the slower Debye relaxation, whereas the faster process, recognized as structural relaxation, remains much less investigated. This is because in many primary alcohols the structural process is strongly overlapped by the dominating Debye relaxation. Additionally, there is still no answer for many fundamental questions concerning the origin of the molecular characteristic properties of these materials. One of them is the role of molecular architecture in the formation of hydrogen-bonded structures and its potential connection to the relaxation dynamics of Debye and structural relaxation processes. In this article, we present the results of ambient and high-pressure dielectric studies of monohydroxy alcohols with similar chemical structures but different carbon chain lengths (2-ethyl-1-butanol and 2-ethyl-1-hexanol) and positions of the OH- group (2-methyl-2-hexanol and 2-methyl-3-hexanol). New data are compared with previously collected results for 5-methyl-2-hexanol. We note that differences in molecular architecture have a significant influence on the formation of hydrogen-bonded structures, which is reflected in the behavior of the Debye and structural relaxation processes. Intriguingly, studying the relaxation dynamics in monohydroxy alcohols at high pressures of up to p = 1700 MPa delivers a fundamental bridge to understand the potential connection between molecular conformation and its response to the characteristic properties of these materials. [ABSTRACT FROM AUTHOR]

The fundamental aspects of the relaxation dynamics in niccolite-type, mixed valence metal–organic framework, multiferroic [(CH3)2NH2][Fe3+Fe2+(HCOO)6] single crystals have been reported using dielectric relaxation spectroscopy covering eight decades in frequency (10−2≤f≤ 106) in the temperature range 120 K ≤T≤ 250 K. The compound shows antiferroelectric to paraelectric phase transition near T = 154 K with the relaxor nature of electric ordering. The temperature dependent dielectric response in modulus representation indicates three relaxation processes within the experimental window. The variable range hopping model of small polarons explains the bulk non-Debye type conductivity relaxation. The fastest relaxation with activation energy Ea = 0.17 eV is related to progressive freezing of the reorientation motions of DMA+ cations. X-ray diffraction data revealed that complete freezing of orientational and translational motions of DMA+ cations occurs well below phase transition temperature. These experimental observations are fundamentally important for the theoretical explanation of relaxation dynamics in niccolite-type metal–organic frameworks. [ABSTRACT FROM AUTHOR]

Broadband dielectric spectroscopy measurements of Nb6VSb3O25 showed that in both the real (ε') and imaginary (ε") components of permittivity there is visible relaxation process strongly obscured by dc conductivity. Application of the electric modulus representation of the data enables to study temperature evolution of this relaxation together with conductivity relaxation. It was showed that the activation energies for both processes are close. Low-frequency loss tangent increases strongly with temperature, suggesting that in the compound under study additional energy losses are associated with the conduction of electric current, as determined by the Joule- Lenz law. [ABSTRACT FROM AUTHOR]

Secondaryrelaxations with properties closely related to the α-relaxationhave fundamental importance in glass-forming substances includingpolymers. To distinguish these secondary relaxations from those involvingintramolecular degrees of freedom, they are called the Johari–Goldstein(JG) β-relaxations. Acting as the precursor of the α-relaxation,the JG β-relaxation is supposedly ubiquitous in all glass-formers,a thesis supported by experiments on a variety of glass-formers. Notwithstanding,the JG β-relaxation has not been identified definitively inthe hydroxyl-terminated polypropylene glycols (PPGs) with variousmolecular weights, despite these polymers have been intensively studiedexperimentally in the last several decades. The difficulty of findingthe JG β-relaxation is due to the presence of a faster intramolecularγ-relaxation and a slower relaxation originating from residualwater. This is demonstrated in two recent papers by Gainaru et al. Macromolecules2010, 43, 1907, and Kaminskiet al. Macromolecules2013, 46, 1973. In this paper, we show the presence of the JG β-relaxationin the PPGs from the dielectric relaxation data by using the timehonored criterion derived from the coupling model in conjunction withthe observed anomalous temperature dependence of the γ-relaxationcaused by merging with the JG β-relaxation, and new experimentaldata obtained by applying pressure on PPG4000 before and after dryingthe sample to remove the residual water. From the results, we concludethat the behavior of the PPGs is no different from the other glass-formersas far as the omnipresence of the JG β-relaxation is concerned. [ABSTRACT FROM AUTHOR]

High pressure viscosity and dielectric measurements were carried out on two monohydroxy alcohols, 2-ethyl-1-hexanol and 5-methyl-2-hexanol, at room temperature. Analysis of the dielectric relaxation times versus viscosity revealed the breakdown of the Einstein-Debye relation above some characteristic pressure. The failure of the Einstein-Debye relation is a manifestation of pressure induced changes of supramolecular hydrogen bonded structures which occur in these liquids. [ABSTRACT FROM AUTHOR]

Broadband dielectric spectroscopy measurements revealed an anomalously large relative permittivity value (ε r = 884) for MnPr2W2O10, a smaller value (ε r = 156) for CoPr2W2O10 and the smallest value (ε r = 22) for CdPr2W2O10 at low frequency (ν = 0.1 Hz) and above room temperature in the insulating and paramagnetic state. Below 273 K, the relative permittivity (ε r ∼ 24) did not depend significantly on frequency for all the tungstates under study. Electrical resistivity, thermoelectric power, electron paramagnetic resonance, magnetic susceptibility and magnetization provided experimental evidence that the studies tungstates were paramagnetic insulators with low n-type conduction. Only in the case of MnPr2W2O10 was a ferrimagnetic order below 45 K observed. These effects are discussed within the framework of Maxwell–Wagner polarization, chemical covalent bonds and porosity mechanism. [ABSTRACT FROM AUTHOR]

Although the presence of hydrogen bonds determines most properties of associated materials, their role in relaxation dynamics of liquids remains unclear. Very recently Nakanishi and Nozaki [M. Nakanishi and R. Nozaki, Phys. Rev. E 84, 011503 (2011)] proposed a simplified model for the description of the molecular dynamics of H-bonding network and tested its validity for several polyols. The authors concluded that relaxation dynamics is controlled mainly by the number of hydroxyl groups, whereas the role of molecular architecture can be neglected. This conclusion, as demonstrated herein, fails in the case of pentiols. When we take into account the role of molecular architecture for development of H-bonded structures, it is still possible to satisfactorily describe molecular dynamics in polyols with the use of the Nakanishi and Nozaki model. [ABSTRACT FROM AUTHOR]

We present a comprehensive study of data for the dielectric relaxation of ten glass-forming organic liquids at high-pressure along isotherms, showing that the primary (α) high-frequency relaxation is well-characterized by the minimum slope and the width of the loss peak. The advantage of these two parameters is that they are model independent. For some materials with β processes in the mHz and kHz range, the high-frequency slope tends to be [image omitted] with pressure increase. In addition, the two parameters capture the relaxation shape invariance at a given relaxation time but different combinations of pressure and time. [ABSTRACT FROM AUTHOR]

Abstract: Dielectric and light scattering spectra of two linear polymers, polyisoprene (PIP) and polystyrene (PS), were analyzed in broad temperature and frequency range above the glass transition temperature, T g. The crossover temperature, T C, was estimated using two approaches: (i) derivative analysis of relaxation times proposed by Stickel and (ii) Mode-Coupling Theory approach. Both estimates provide consistent values. T C varies with molecular weight (MW) in both polymers, while the ratio T C/T g changes significantly with MW in PS only. It appears that the segmental relaxation time at T C has value τ(T C)∼10−7 s for both polymers independent of MW and similar to the value reported for many non-polymeric glass-forming systems. No sign of the dynamic crossover has been observed in the chain relaxation around T C of the segmental dynamics. [Copyright &y& Elsevier]