Roland Böhmer

Nonresonant Spectral Hole Burning in the Slow Dielectric Response of Supercooled Liquids

Large-amplitude, low-frequency electric fields can be used to burn spectral holes in the dielectric response of supercooled propylene carbonate and glycerol. This ability to selectively modify the dielectric response establishes that the non-Debye behavior results from a distribution of relaxation times. Refilling of the spectral hole was consistent with a single recovery time that coincided with the peak in the distribution. Moreover, refilling occurred without significant broadening, which indicates negligible direct exchange between the degrees of freedom that responded to the field. Nonresonant spectral hole burning facilitates direct investigation of the intrinsic response of systems that exhibit nonexponential relaxation.

Relaxation in the glass former acetylsalicylic acid studied by deuteron magnetic resonance and dielectric spectroscopy

Supercooled liquid and glassy acetylsalicylic acid was studied using dielectric spectroscopy and deuteron relaxometry in a wide temperature range. The supercooled liquid is characterized by major deviations from thermally activated behavior. In the glass the secondary relaxation exhibits the typical features of a Johari-Goldstein process. Via measurements of spin-lattice relaxation times the selectively deuterated methyl group was used as a sensitive probe of its local environments. There is a large difference in the mean activation energy in the glass with respect to that in crystalline acetylsalicylic acid. This can be understood by taking into account the broad energy barrier distribution in the glass.

Water’s second glass transition

Significance Water is not only the most important liquid for life on Earth, but also one of the most anomalous liquids. These anomalies become most evident in the supercooled state at subzero temperatures. We show from dielectric and calorimetric studies that water in the deeply supercooled regime, below –120 °C, can even exist as two distinct, ultraviscous liquids at ambient pressure, a low- (LDL, 0.92 g/cm3) and high-density liquid (HDL, 1.15 g/cm3), which can both remain in the metastable, equilibrium liquid state for many hours above their calorimetric glass transition temperatures of –137 °C (136 K) and –157 °C (116 K). LDL is identified as the strongest of all liquids, and also HDL is a strong liquid at record low temperature. The glassy states of water are of common interest as the majority of H2O in space is in the glassy state and especially because a proper description of this phenomenon is considered to be the key to our understanding why liquid water shows exceptional properties, different from all other liquids. The occurrence of water’s calorimetric glass transition of low-density amorphous ice at 136 K has been discussed controversially for many years because its calorimetric signature is very feeble. Here, we report that high-density amorphous ice at ambient pressure shows a distinct calorimetric glass transitions at 116 K and present evidence that this second glass transition involves liquid-like translational mobility of water molecules. This “double Tg scenario” is related to the coexistence of two liquid phases. The calorimetric signature of the second glass transition is much less feeble, with a heat capacity increase at Tg,2 about five times as large as at Tg,1. By using broadband-dielectric spectroscopy we resolve loss peaks yielding relaxation times near 100 s at 126 K for low-density amorphous ice and at 110 K for high-density amorphous ice as signatures of these two distinct glass transitions. Temperature-dependent dielectric data and heating-rate–dependent calorimetric data allow us to construct the relaxation map for the two distinct phases of water and to extract fragility indices m = 14 for the low-density and m = 20–25 for the high-density liquid. Thus, low-density liquid is classified as the strongest of all liquids known (“superstrong”), and also high-density liquid is classified as a strong liquid.

Nonresonant dielectric hole burning spectroscopy of supercooled liquids

The nonexponential response of propylene carbonate and glycerol near their glass transitions could be selectively altered using nonresonant spectral hole burning (NSHB) experiments. This observation provides evidence of the existence of a distribution of relaxation times in these supercooled liquids. NSHB is based on a pump, wait, and probe scheme and uses low-frequency large amplitude electrical fields to modify the dielectric relaxation. The temporal evolution of the polarization of the sample is then measured subsequent to a small voltage step. By variation of a recovery time inserted between pump and probe, the refilling of the spectral features could be monitored and was found to take place on the time scale set by the peak in the distribution. The recovery time and pump frequency dependences of the spectral modifications were successfully simulated using a set of coupled rate equations.

Rotational motion in the molecular crystals meta− and ortho−carborane studied by deuteron nuclear magnetic resonance

Spin-lattice and spin-spin-relaxation times, one- and two-dimensional spectra as well as two- and four-time correlation functions were measured for the molecular crystals ortho- and meta-carborane using deuteron nuclear magnetic resonance. It is found that in their noncubic phases these crystals exhibit highly anisotropic motions. In order to allow for a quantitative description of the motional geometry of the carboranes several stochastic models are formulated. By comparison of the model calculations with the experimental results it is found that the dynamics of these quasi-icosahedrally shaped molecules is governed by a composite reorientation process. Here the molecules perform threefold jumps around a molecule-fixed axis which itself can be tilted in four different directions with respect to a crystal-fixed axis. The tilt angle increases significantly with increasing temperature. On the basis of measurements of four-time stimulated-echo functions, implications for dynamic heterogeneity also in comparison with that of supercooled liquids are discussed.

Reorientations in supercooled glycerol studied by two-dimensional time-domain deuteron nuclear magnetic resonance spectroscopy

The method of stimulated echoes was used to investigate the reorientational mechanism in the selectively deuterated glass-former glycerol, C3D5(OH)3 about 15 K above its calorimetric glass temperature. The reorientation process is fully isotropic. This enables an accurate determination of the decay constant, T1Q, of the quadrupolar spin order in the regime of ultraslow motion. The knowledge of this time constant has made it possible to reliably determine the rotational correlation function. The experimentally obtained evolution time-dependent correlation functions are compared with those from a simulation procedure involving a distribution of molecular jump angles. It is found that in glycerol small angles in the 2°–3° range dominate. They are accompanied by a small, but significant, fraction of larger jump angles.

Local and Global Relaxations in Glass Forming Materials

Many of the things that we utilize in daily life are made from amorphous materials. In addition to vitreous silica and its derivatives which a non-expert would probably associate with the term glass, a large number of non-crystalline polymers, amorphous semiconductors, and even some amorphous metals also have important technical applications. Most viscous molecular liquids and glasses like e.g. glucose, which continue to attract a great deal of scientific interest, have no immediate technological relevance, but their ability to vitrify on cooling or concentration may be essential for the preservation of life under adverse (sub-zero or arid) conditions.

Nuclear-Magnetic-Resonance Measurements Reveal the Origin of the Debye Process in Monohydroxy Alcohols

Monohydroxy alcohols show a structural relaxation and at longer time scales a Debye-type dielectric peak. From spin-lattice relaxation experiments using different nuclear probes, an intermediate, slower-than-structural dynamics is identified for n-butanol. Based on these findings and on translational diffusion measurements, a model of self-restructuring, transient chains is proposed. The model is demonstrated to explain consistently the so-far puzzling observations made for this class of hydrogen-bonded glass forming liquids.

Radio-frequency dielectric measurements at temperatures from 10 to 450 K

A coaxial air line was constructed to connect a radio-frequency impedance analyzer and a temperature-stabilized sample holder. It is suitable for dielectric measurements in the frequency range 1 MHz-1 GHz and at temperatures between 10 and 450 K. The dielectric dispersion of Fe-doped BaTiO_3 and Na-doped KCN is presented. The results demonstrate the capability of this setup when investigating materials with high as well as with low dielectric constants.

Paraelectric and ferroelectric phases of betaine phosphite: structural, thermodynamic, and dielectric properties

Abstract The paraelectric and ferroelectric phases of betaine phosphite [BPI: (CH3)3 NCH2COO H3PO3] single crystals have been studied. The structure of BPI was determined at room temperature using x-ray and elastic neutron diffraction. Details of the ferroelectric transition and various low temperature properties were investigated using broad-band dielectric spectroscopy and calorimetric experiments. We compare the results obtained for single crystals of different origin which exhibit different ferroelectric phase transition temperatures.