A. F. Privalov

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.

1H-14N cross-relaxation in trinitrotoluene - a step toward improved landmine detection

Motivated by the lack of efficient detection techniques for metal-free trinitrotoluene (TNT) containing landmines, 1 H– 14 N cross-relaxation experiments on TNT have been performed using field-cycling spectroscopy in an electronically switchable volume coil. Using 1 H NMR detection (indirect method), the 14 N quadrupole spectra in several small (about 0.5 g) TNT samples of different producers are determined. The experiment is considered as a step toward a double resonance TNT landmine detection scheme using surface coils.

NMR field-cycling at ultralow magnetic fields.

The paper describes some significant technical improvements of a home built NMR field cycling relaxometer [O. Lips, A. Privalov, S. Dvinskikh, F. Fujara, J. Magn. Reson. 149 (2001) 22-28] now allowing for fast switching of polarization fields (up to more than 1T) to evolution fields down to the sub-μT range. The most important instrumental details such as the description of an involved 3-dimensional resistive coil setup are given. Fields below about 5 μT can only be stabilized by incorporation of an active field drift and fluctuation compensation tool. In this way, the smallest 1H Larmor frequency obtained and measured so far has been 12 Hz.

Analysis of 1H?14N polarization transfer experiments in molecular crystals

A general theoretical description of polarization transfer processes in multi-spin systems containing dipole as well as quadrupole spins is formulated on the background of the Liouville–von Neumann equation. The density operator formalism is used to describe the evolution of an arbitrary spin system due to quadrupole, Zeeman and dipole–dipole interactions. This approach is applied to interpret previously published 1H–14N cross-relaxation NMR experiments for measuring the 14N quadrupole coupling constants of paranitrotoluene (PNT) and trinitrotoluene (TNT) (Nolte et al 2002 J. Phys. D: Appl. Phys. 35 939) and new experiments on urea and urotropine. It is demonstrated that according to the complexity of the analysed spin system an appropriate number of spins has to be taken into consideration for a correct description of the cross-relaxation spectra. The work is a part of an extended project aiming for a method which should permit detection of TNT explosive in anti-personnel landmines.

Transport properties of LaF3 fast ionic conductor studied by field gradient NMR and impedance spectroscopy

The self-diffusion coefficient and the ionic conductivity in LaF3 single crystals are measured over a temperature range from 300 to 1400 K. Three thermally activated processes are found in this range. After heating, both the diffusion coefficient and the conductivity of the as-grown samples undergo strong irreversible changes at T , 1100 K: An activation energy of Ea ¼ 1: 2e V and a Haven ratio of about 0.1 are determined in the intrinsic region. The mechanism of the ion transport and the effect of thermal treatment are discussed. q 2003 Elsevier Science Ltd. All rights reserved.

Dynamic processes in the superionic conductor LaF3 at high temperatures as studied by spin-lattice relaxation dispersion

By nuclear magnetic resonance spin-lattice relaxation dispersion in LaF3, measured in the frequency range from 60 kHz to 284 MHz, dynamic processes with correlation times in the range from 10-5 to 10-10 s are studied. This allows us to trace fluorine dynamics in the temperature range from 303 to 1400 K. Two motional modes, a fast one and a slow one, are identified. Both motional processes are found not to be of Bloembergen-Purcell-Pound type but obey a log-Gaussian distribution of correlation times, thus reflecting the potential energy landscape in the superionic state. Below 1000 K, the activation energy of fluorine diffusion is 0.36 eV for the fast ions, and 0.57 eV for the slow ones. At higher temperatures, the activation energies change drastically. Above 20 MHz, where most relaxation studies have been performed so far, an additional contribution to relaxation, which is probably induced by paramagnetic centres, is found to be dominant.

Field cycling methods as a tool for dynamics investigations in solid state systems: recent theoretical progress.

In this paper physical mechanisms and theoretical treatments of polarization transfer and field-dependent relaxation in solid state systems, containing mutually coupled spins of spin quantum numbers I=12 (spins 12) and S1 (quadrupolar spins), are presented. First, theoretical descriptions of these effects are given in detail for an illustrative, simple system. Next, it is shown how to generalize the theories to much more complex spin systems. The polarization transfer and relaxation effects are illustrated by several examples. Typical misunderstandings regarding their physical origins are clarified. This paper reviews recent theoretical descriptions of the polarization transfer and relaxation phenomena. Its goal is to popularize the proper theoretical treatments with the intention to establish them as standard tools for analyzing field cycling data.

Simultaneous measurement of very small magnetic fields and spin-lattice relaxation

A field cycling (FC) NMR experiment is presented which allows for the simultaneous determination of very small magnetic fields down to about 3 μT and the concomitant measurement of nuclear spin-lattice relaxation times in these fields. The technique will enable broadband spin-lattice relaxation dispersion experiments down to about 100 Hz (1)H Larmor frequency. Limitations of its applicability are discussed.

Low-cost high-temperature NMR probe head

The design of a simple high-temperature nuclear magnetic resonance (NMR) probe head for narrow-bore magnets is presented. It covers the temperature range from 20 to 1300°C, necessitating a heating power of below 100 W. Several probe heads of this design, manufactured for NMR solenoids with bores from 30 to 54 mm have shown good stability and long life times.

Combining 7Li NMR field-cycling relaxometry and stimulated-echo experiments: A powerful approach to lithium ion dynamics in solid-state electrolytes

We use (7)Li NMR to study lithium ion dynamics in a (Li2S)-(P2S5) glass. In particular, it is shown that a combination of (7)Li field-cycling relaxometry and (7)Li stimulated-echo experiments allows us to cover a time window extending over 10 orders of magnitude without any gaps. While the (7)Li stimulated-echo method proved suitable to measure correlation functions F2(t) of lithium ion dynamics in solids in recent years, we establish the (7)Li field-cycling technique as a versatile tool to ascertain the spectral density J2(ω) of the lithium ionic motion in this contribution. It is found that the dynamic range of (7)Li field-cycling relaxometry is 10(-9)-10(-5)s and, hence, it complements in an ideal way that of (7)Li stimulated-echo experiments, which amounts to 10(-5)-10(1)s. Transformations between time and frequency domains reveal that the field-cycling and stimulated-echo approaches yield results for the translational motion of the lithium ions that are consistent both with each other and with findings for the motional narrowing of (7)Li NMR spectra of the studied (Li2S)-(P2S5) glass. In the (7)Li field-cycling studies of the (Li2S)-(P2S5) glass, we observe the translational ionic motion at higher temperatures and the nearly constant loss at lower temperatures. For the former motion, the frequency dependence of the measured spectral density is well described by a Cole-Davidson function. For the latter phenomenon, which was considered as an universal phenomenon of disordered solids in the literature, we find an exponential temperature dependence.