525 research outputs found
Johari-Goldstein relaxation far below Tg: Experimental evidence for the Gardner transition in structural glasses?
Experimental evidence for the Gardner transition, theoretically predicted to
arise deep in the glassy state of matter, is scarce. At this transition, the
energy landscape sensed by the particles forming the glass is expected to
become more complex. In the present work, we report the dielectric response of
two typical glass formers with well-pronounced Johari-Goldstein beta relaxation
following this response down to unprecedented low temperatures, far below the
glass transition. As the Johari-Goldstein process is believed to arise from the
local structure of the energy landscape, its investigation seems an ideal tool
to seek evidence for the Gardner transition. Indeed, we find an unusual
broadening of the beta relaxation below TG ~ 110 K for sorbitol and TG ~ 100 K
for xylitol, in excess of the expected broadening arising from a distribution
of energy barriers. Thus, these results provide hints at the presence of the
Gardner transition in canonical structural glass formers.Comment: 6 pages, 3 figures + 2 pages, 3 figures in Supplemental Materia
Ionic conductivity and relaxation dynamics in plastic-crystals with nearly globular molecules
We have performed a dielectric investigation of the ionic charge transport
and the relaxation dynamics in plastic-crystalline 1-cyano-adamantane (CNA) and
in two mixtures of CNA with the related plastic crystals adamantane or
2-adamantanon. Ionic charge carriers were provided by adding 1% of Li salt. The
molecules of these compounds have nearly globular shape and, thus, the
so-called revolving-door mechanism assumed to promote ionic charge transport
via molecular reorientations in other PC electrolytes, should not be active
here. Indeed, a comparison of the dc resistivity and the reorientational
alpha-relaxation times in the investigated PCs, reveals complete decoupling of
both dynamics. Similar to other PCs, we find a significant mixing-induced
enhancement of the ionic conductivity. Finally, these solid-state electrolytes
reveal a second relaxation process, slower than the alpha-relaxation, which is
related to ionic hopping. Due to the mentioned decoupling, it can be
unequivocally detected and is not superimposed by the reorientational
contributions as found for most other ionic conductors.Comment: 9 pages, 7 figure
Role of counterions in the adsorption and micellization behavior of 1:1 ionic surfactants at fluid interfaces─demonstrated by the standard amphiphile system of alkali perfluoro-n-octanoates
In our latest communication, we proved experimentally that the ionic surfactant’s surface excess is exclusively determined by the size of the hydrated counterion.[Lunkenheimer, Langmuir, 2017, 33, 10216−1022410.1021/acs.langmuir.7b00786]. However, at this stage of research, we were unable to decide whether this does only hold for the two or three lightest ions of lithium, sodium, and potassium, respectively. Alternatively, we could also consider the surface excess of the heavier hydrated alkali ions of potassium, rubidium, and cesium, having practically identical ion size, as being determined by the cross-sectional area of the related anionic extended chain residue. The latter assumption has represented state of art. Searching for reliable experimental results on the effect of the heavier counterions on the boundary layer, we have extended investigations to the amphiphiles’ solutions of concentrations above the critical concentration of micelle formation (cmc).We provided evidence that the super-micellar solutions’ equilibrium surface tension will remain constant provided the required conditions are followed. The related σcmccmccmc+ > Na+ > K+ > (NH4)+ > Rb+ > Cs+. Therefore, we have to extend our model of counterion effectiveness put forward in our previous communication. It represents a general principle of the counterion effect
Dielectric and conductivity relaxation in mixtures of glycerol with LiCl
We report a thorough dielectric characterization of the alpha relaxation of
glass forming glycerol with varying additions of LiCl. Nine salt concentrations
from 0.1 - 20 mol% are investigated in a frequency range of 20 Hz - 3 GHz and
analyzed in the dielectric loss and modulus representation. Information on the
dc conductivity, the dielectric relaxation time (from the loss) and the
conductivity relaxation time (from the modulus) is provided. Overall, with
increasing ion concentration, a transition from reorientationally to
translationally dominated behavior is observed and the translational ion
dynamics and the dipolar reorientational dynamics become successively coupled.
This gives rise to the prospect that by adding ions to dipolar glass formers,
dielectric spectroscopy may directly couple to the translational degrees of
freedom determining the glass transition, even in frequency regimes where
usually strong decoupling is observed.Comment: 8 pages, 7 figure
Relaxation dynamics and colossal magnetocapacitive effect in CdCr2S4
A thorough investigation of the relaxational dynamics in the recently
discovered multiferroic CdCr2S4 showing a colossal magnetocapacitive effect has
been performed. Broadband dielectric measurements without and with external
magnetic fields up to 10 T provide clear evidence that the observed
magnetocapacitive effect stems from enormous changes of the relaxation dynamics
induced by the development of magnetic order.Comment: 4 pages, 4 figure
Debye relaxation and 250 K anomaly in glass forming monohydroxy alcohols
A previous dielectric, near-infrared (NIR), and nuclear magnetic resonance
study on the hydrogen-bonded liquid 2-ethyl-1-hexanol [C. Gainaru et al., Phys.
Rev. Lett. 107, 118304 (2011)] revealed anomalous behavior in various static
quantities near 250 K. To check whether corresponding observations can be made
for other monohydroxy alcohols as well, these experimental methods were applied
to such substances with 5, 6, 7, 8, and 10 carbon atoms in their molecular
backbone. All studied liquids exhibit a change of behavior near 250 K which is
tentatively ascribed to effects of hydrogen bond cooperativity. By analyzing
the NIR band intensities, a linear cluster size is derived that agrees with
estimates from dielectric spectroscopy. All studied alcohols, except
4-methyl-3-heptanol, display a dominant Debye-like peak. Furthermore, neat
2-ethyl-1-butanol exhibits a well resolved structural relaxation in its
dielectric loss spectrum which so far has only been observed for diluted
monohydroxy alcohols.Comment: 39 pages including 12 figure
Orbital-Order Driven Ferroelectricity and Dipolar Relaxation Dynamics in Multiferroic GaMoS
We present the results of broadband dielectric spectroscopy of GaMoS,
a lacunar spinel system that recently was shown to exhibit non-canonical,
orbitally-driven ferroelectricity. Our study reveals complex relaxation
dynamics of this multiferroic material, both above and below its Jahn-Teller
transition at T K. Above T, two types of
coupled dipolar-orbital dynamics seem to compete: relaxations within
cluster-like regions with short-range polar order like in relaxor
ferroelectrics and critical fluctuations of only weakly interacting dipoles,
the latter resembling the typical dynamics of order-disorder type
ferroelectrics. Below the Jahn-Teller transition, the onset of orbital order
drives the system into long-range ferroelectric order and dipolar dynamics
within the ferroelectric domains is observed. The coupled dipolar and orbital
relaxation behavior of GaMoS above the Jahn-Teller transition markedly
differs from that of the skyrmion host GaVS, which seems to be linked
to differences in the structural distortions of the two systems on the
unit-cell level.Comment: 6 pages, 3 figures + Supplemental Material (2 pages, 2 figures
Broadband dielectric spectroscopy on benzophenone: alpha relaxation, beta relaxation, and mode coupling theory
We have performed a detailed dielectric investigation of the relaxational
dynamics of glass-forming benzophenone. Our measurements cover a broad
frequency range of 0.1 Hz to 120 GHz and temperatures from far below the glass
temperature well up into the region of the small-viscosity liquid. With respect
to the alpha relaxation this material can be characterized as a typical
molecular glass former with rather high fragility. A good agreement of the
alpha relaxation behavior with the predictions of the mode coupling theory of
the glass transition is stated. In addition, at temperatures below and in the
vicinity of Tg we detect a well-pronounced beta relaxation of Johari-Goldstein
type, which with increasing temperature develops into an excess wing. We
compare our results to literature data from optical Kerr effect and depolarized
light scattering experiments, where an excess-wing like feature was observed in
the 1 - 100 GHz region. We address the question if the Cole-Cole peak, which
was invoked to describe the optical Kerr effect data within the framework of
the mode coupling theory, has any relation to the canonical beta relaxation
detected by dielectric spectroscopy.Comment: 11 pages, 7 figures; revised version with new Fig. 5 and some smaller
changes according to referees' demand
Plastic-crystalline solid-state electrolytes: Ionic conductivity and orientational dynamics in nitrile mixtures
Many plastic crystals, molecular solids with long-range, center-of-mass
crystalline order but dynamic disorder of the molecular orientations, are known
to exhibit exceptionally high ionic conductivity. This makes them promising
candidates for applications as solid-state electrolytes, e.g., in batteries.
Interestingly, it was found that the mixing of two different
plastic-crystalline materials can considerably enhance the ionic dc
conductivity, an important benchmark quantity for electrochemical applications.
An example is the admixture of different nitriles to succinonitrile, the latter
being one of the most prominent plastic-crystalline ionic conductors. However,
until now only few such mixtures were studied. In the present work, we
investigate succinonitrile mixed with malononitrile, adiponitrile, and
pimelonitrile, to which 1 mol% of Li ions were added. Using differential
scanning calorimetry and dielectric spectroscopy, we examine the phase behavior
and the dipolar and ionic dynamics of these systems. We especially address the
mixing-induced enhancement of the ionic conductivity and the coupling of the
translational ionic mobility to the molecular reorientational dynamics,
probably arising via a "revolving-door" mechanism.Comment: 9 pages, 7 figures; revised version as accepted for publication in J.
Chem. Phy
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