Trace Determination and Pressure Estimation of Fluorine F2 Caused by Irradiation Damage in Minerals and Synthetic Fluorides.

Irradiated alkali and earth alkali halides can form metal colloids and halogen molecules, which stay trapped inside the crystal. In this paper we provide 19 F NMR spectroscopic evidence of trapped F2 fluids in heavy ion-bombarded synthesized LiF crystals as well as in a variety of the mineral Villiaumite (NaF). This is the 2nd mineral in which F2 is unambiguously detected in nature. The trace quantification of the latter is in the order of magnitude of 10-6  mol g-1 . Pressures and densities of the F2 fluids are estimated based on the theory of nuclear spin relaxation in dilute gases.

Solid state field-cycling NMR relaxometry: instrumental improvements and new applications.

The paper reviews recent progress in field cycling (FC) NMR instrumentation and its application to solid state physics. Special emphasis is put on our own work during the last 15years on instrumentation, theory and applications. As far as instrumentation is concerned we report on our development of two types of electronical FC relaxometers, a mechanical FC relaxometer and a combination of FC and one-dimensional microimaging. Progress has been achieved with respect to several parameters such as the accessible field and temperature range as well as the incorporation of sample spinning. Since an appropriate analysis of FC data requires a careful consideration of relaxation theory, we include a theory section discussing the most relevant aspects of relaxation in solids which are related to residual dipolar and quadrupolar interactions. The most important limitations of relaxation theory are also discussed. With improved instrumentation and with the help of relaxation theory we get access to interesting new applications such as ionic motion in solid electrolytes, structure determination in molecular crystals, ultraslow polymer dynamics and rotational resonance phenomena.

NMR studies on the temperature-dependent dynamics of confined water.

We use (2)H NMR to study the rotational motion of supercooled water in silica pores of various diameters, specifically, in the MCM-41 materials C10, C12, and C14. Combination of spin-lattice relaxation, line-shape, and stimulated-echo analyses allows us to determine correlation times in very broad time and temperature ranges. For the studied pore diameters, 2.1-2.9 nm, we find two crossovers in the temperature-dependent correlation times of liquid water upon cooling. At 220-230 K, a first kink in the temperature dependence is accompanied by a solidification of a fraction of the confined water, implying that the observed crossover is due to a change from bulk-like to interface-dominated water dynamics, rather than to a liquid-liquid phase transition. Moreover, the results provide evidence that α process-like dynamics is probed above the crossover temperature, whereas ß process-like dynamics is observed below. At 180-190 K, we find a second change of the temperature dependence, which resembles that reported for the ß process of supercooled liquids during the glass transition, suggesting a value of Tg ≈ 185 K for interface-affected liquid water. In the high-temperature range, T > 225 K, the temperature dependence of water reorientation is weaker in the smaller C10 pores than in the larger C12 and C14 pores, where it is more bulk-like, indicating a significant effect of the silica confinement on the α process of water in the former 2.1 nm confinement. By contrast, the temperature dependence of water reorientation is largely independent of the confinement size and described by an Arrhenius law with an activation energy of Ea ≈ 0.5 eV in the low-temperature range, T < 180 K, revealing that the confinement size plays a minor role for the ß process of water.

Ultra-slow dynamics in low density amorphous ice revealed by deuteron NMR: indication of a glass transition.

The postulated glass-liquid transition of low density amorphous ice (LDA) is investigated with deuteron NMR stimulated echo experiments. Such experiments give access to ultra-slow reorientations of water molecules on time scales expected for structural relaxation of glass formers close to the glass-liquid transition temperature. An involved data analysis is necessary to account for signal contributions originating from a gradual crystallization to cubic ice. Even if some ambiguities remain, our findings support the view that pressure amorphized LDA ices are of glassy nature and undergo a glass-liquid transition before crystallization.

Limits of metastability in amorphous ices: 2H-NMR relaxation.

The high-frequency reorientation dynamics of O-(2)H bonds is investigated in various amorphous ices including eHDA (expanded high density amorphous ice), LDA-II (low density amorphous ice II) and HGW (hyperquenched glassy water) using (2)H-NMR spin-lattice relaxation as a local probe. Both low density forms, HGW and LDA-II, show similar spin-lattice relaxation but differ in the thermal stability with respect to the transition into crystalline cubic ice I(c). HGW already transforms slightly above 135 K whereas LDA-II crystallizes at 150 K. eHDA is distinguishable from other high density amorphous ices in its thermal stability and spin-lattice relaxation. Its relaxation times are much larger compared to those of VHDA (very high density amorphous ice) and uHDA (unrelaxed high density amorphous ice). eHDA does not show annealing effects, transforms sharply into LDA-II above 123 K and provides higher thermal stability as compared to other high density forms.

Limits of metastability in amorphous ices: the neutron scattering Debye-Waller factor.

Recently, it became clear that relaxation effects in amorphous ices play a very important role that has previously been overlooked. The thermodynamic history of amorphous samples strongly affects their transition behavior. In particular, well-relaxed samples show higher thermal stability, thereby providing a larger window to investigate their glass transitions. We here present neutron scattering experiments using fixed elastic window scans on relaxed forms of amorphous ice, namely expanded high density amorphous ice (eHDA), a variant of low density amorphous ice (LDA-II) and hyperquenched glassy water (HGW). These amorphous ices are expected to be true glassy counterparts of deeply supercooled liquid water, therefore fast precursor dynamics of structural relaxation are expected to appear below the calorimetric glass transition temperature. The Debye-Waller factor shows a very weak sub-T(g) anomaly in some of the samples, which might be the signature of such fast precursor dynamics. However, we cannot find this behavior consistently in all samples at all reciprocal length scales of momentum transfer.

Energy efficient iron based electronic field cycling magnet.

A new concept for an energy efficient electromagnet for fast field cycling NMR applications as well as its construction and first test results are presented. The magnet, which provides a rectangular sample space of 17 x 25 mm, has an iron yoke and pole pieces optimised with respect to the B(0) homogeneity. The maximum field is 0.66 T at a current of 320 A; its field inhomogeneity for a cylindrical sample (length 7 mm, diameter 6mm) is about 50 ppm. The power dissipation during polarisation at 0.55 T is as low as 1.4 kW. The magnet is powered by a commercially available power supply and can be rapidly switched with a slew rate of 0.55 T/ms. The system has shown a stability of 50 ppm/h.

On the problem of field-gradient NMR measurements of intracrystalline diffusion in small crystallites--water in NaA zeolites as an example.

Necessary conditions for measuring intracrystalline diffusion in small crystal size systems via field-gradient NMR are discussed. As an illustrative case self-diffusion coefficients of water adsorbed in NaA zeolites (average crystal diameter about 1 microm) have been measured by 1H-NMR stimulated echoes in static magnetic field gradients of up to 180 T/m in the temperature range of 254-344 K. Obtaining intracrystalline diffusion coefficients necessitates a sufficiently high spatial resolution only provided by such large field gradients.