Fluorine dynamics in BaF2 superionic conductors investigated by NMR

In this work the dynamics of fluorine in solid-state electrolytes having BaF2-structure was investigated using three different NMR-methods: field cycling relaxometry, lineshape analysis, and static field gradient NMR. For this purpose a pure BaF2 crystal, as well as crystals doped with trivalent impurities (LaF3), were studied as a function of temperature. The main goal of this investigation was to utilize the structure information provided by neutron scattering and MAS NMR data in order to study dynamic properties on different time and length scales with the techniques mentioned above. Investigations of macroscopic dynamical processes performed by diffusion measurements reveal two different temperature regions, labeled I and II, with different activation energies depending on the concentration of LaF3. Region I, at high-temperature, contains thermally activated defects. The dynamics in this region can be described by diffusion of both, these activated and already existing defects. The low-temperature region (II), only appears at increasing admixture concentrations. It is most likely dominated by the motions of already existing defects. Another important result was observed during the igh-temperature treatment of the pure and lightly doped BaF2 samples, which influenced the dynamics. This aging effect manifests itself as a small change of the diffusion coefficient and can be explained as creation of new defects in the sample. Information about the microscopic dynamic were obtained by the lineshape analysis. Acquired data have shown strong changes of the lineshape with rising concentration of admixtures. Using MAS NMR it was possible to identify two lines in Ba0.9La0. 1F2.1 having different chemical shift, and to refer them to the modified crystal structure. On this basis a model for the fluorine lineshape has been developed, taking into account three motional processes characterized by their correlation times. It includes jump diffusion of the fluorine ions among equivalent sites within two crystallographically distinct sublattices, and inter-lattice exchange processes. The theoretical description has been based on a detailed treatment of fluorine-fluorine dipole-dipole interactions relevant for the shape of the fluorine spectra. Analysis of the lineshape data for low concentrations of admixtures revealed that motion occurs in the F1 lattice, but with increasing concentration of La3+ this method reaches the limits of its applicability. Thus, it was of great interest to introduce a third technique, field cycling relaxometry. By measuring frequency and temperature-dependent spin lattice relaxation times, it was possible to gain information about fluorine dynamics on microscopic length scales. As was done in the case of the lineshape analysis, two models were used to interpret the data, taking into account the different fluorine subsystems. The differences between the approaches consist of several modifications: on the one hand, a BPP-model for relaxation for pure and low concentration of La3+ was incorporated; on the other hand an ”extension” to the BPP theory was used, explicitly taking into account the two different subsystems F1 and F2. An attempt was also made to analyze the data for pure BaF2 and low admixture concentration samples with a non-exponential correlation function. It turned out, at least in the temperature range of 500K-800K, that the uncorrelated motion model (BBP) is more useful to describe the data. In the case of increasing dopant concentration, two different regions could be observed for the temperature-dependent correlation times. These results were in good accordance with measurements of the self diffusion coefficient. It should also be mentioned that a second-order effect, the static quadrupole interaction, could have influenced the results for the corresponding correlation times. Finally, it is important to emphasize the experience gained during this investigation by using different NMR methodologies. Good, fast ionic conductors inevitably exhibit large numbers of structural defects, and many dynamical processes, especially at elevated temperatures. In the early days many different systems of known structures were studied and reported in literature. In this work, a further extension of these studies has been presented through the application of new technical developments (FC NMR). Although the information provided by NMR on the BaF2-system is limited, this study demonstrates the feasibility of elucidating important features of this system by exploiting element specific information concerning dynamics on different time and length scales, relative to a reference temperature or composition.