Gumann, Patryk (2008)
Fluorine dynamics in BaF2 superionic conductors investigated by NMR.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung
Kurzbeschreibung (Abstract)
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.
Typ des Eintrags: |
Dissertation
|
Erschienen: |
2008 |
Autor(en): |
Gumann, Patryk |
Art des Eintrags: |
Erstveröffentlichung |
Titel: |
Fluorine dynamics in BaF2 superionic conductors investigated by NMR |
Sprache: |
Englisch |
Referenten: |
Fujara, Prof. Dr. Franz ; Stuehn, Prof. Dr. Bernd |
Publikationsjahr: |
8 Oktober 2008 |
Ort: |
Darmstadt |
Verlag: |
Technische Universität |
Datum der mündlichen Prüfung: |
17 Dezember 2007 |
URL / URN: |
urn:nbn:de:tuda-tuprints-11366 |
Kurzbeschreibung (Abstract): |
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. |
Alternatives oder übersetztes Abstract: |
Alternatives Abstract | Sprache |
---|
In dieser Arbeit wurde die Fluordynamik in Festelektrolyten der BaF2-Struktur mittels dreier unterschiedlicher NMR-Methoden untersucht: Field Cycling Relaxometrie, Linienformanalysen und Statische Feldgradienten-NMR. Fuer dieses Zielwurden sowohl reines BaF2 als auchmit den dreiwertigen Fremdstoff La3+ dotierte Kristalle, als unktion der Temperatur studiert. Das Hauptziel dieser Untersuchung war die durch Neutronenstreuung und MAS-NMR-Daten gewonnenen Strukturinformationen zu nutzen, um das dynamische Verhalten auf unterschiedlichen Zeit- und Laengenskalen mittels oben genannter Techniken zu etrachten. Aus den Diffussionsmessungen zum Studium makroskopischer Dynamik ergaben sich —in Abhaengigkeit vom Dotierungsgrad mit LaF3— zwei Temperaturbereiche I und II mit nterschiedlichen Aktivierungsenergien. Die Diffusion mit groesserer Aktivierungsenergie wurde bei allen Proben im Bereich hoher Temperaturen beobachtet, dies laesst auf das Vorhandensein thermisch aktivierter Defekte schließen. Die Dynamik kann hier als Bewegung dieser aktivierten und bereits bestehender Defekte beschrieben werden. Der Bereich II (tiefe Temperaturen) tritt erst dann auf, wenn der Dotierungsgrad erhoeht wird. Die Dynamik wird hoehstwahrscheinlich von der Bewegung bereits bestehender Defekte dominiert. Eine weitere wichtige Beobachtung wurde waehrend der Hochtemperatur-Behandlung der reinen und schwach dotierten BaF2-Proben gemacht: Die hohen Temperaturen riefen Aenderungen in der Dynamik hervor. Dieser Alterungseffekt zeigt sich durch kleine Aenderungen des Diffusionskoeffizienten und kann dadurch erklaert werden, dass bei erhoehten Temperaturen neue Defekte erzeugt werden. Die Linienform-Analyse erfasst mikroskopische Bewegung. Die zugehoerigen Messdaten zeigen starke Aenderungen der Linienform mit zunehmendem Dotierungsgrad. Durch die MAS NMR-Messungen war es moeglich, in Ba0.9La0.1F2.1 zwei Linien mit unterschiedlicher chemischer Verschiebung zu identifizieren und diese mit der modifizierten Kristallstruktur zu korrelieren. Basierend auf dieser Grundlage wurde ein Modell fuer die Fluor-Linienform entwickelt, das auf den drei durch ihre Korrelationszeiten charakterisierten Arten von Bewegungsprozessen beruht. Es beinhaltet die Sprungdiffusion der Fluor-Ionen zwischen aequivalenten Gitterplaetzen innerhalb der beiden kristallographischen Untergitter, sowie Austauschprozesse zwischen den beiden Gittern. Die theoretische Auswertung basiert auf einer detaillierten Betrachtung der Fluor-Fluor Dipol-Dipol-Wechselwirkungen, die fuer die Linienform der Fluor-Spektren verantwortlich sind. Analysen der Linienform-Messergebnisse fuer niedrige Dotierungsgrade haben gezeigt, dass im F1-Gitter Bewegung auftritt. Allerdings stoesst diese Methode mit zunehmender Konzentration von La3+ an die Grenzen ihrer Anwendbarkeit, daher war es von großem Interesse, eine dritte Messmethode, naemlich die Fieldcycling-Relaxometrie, einzubeziehen. Durch die Messung frequenz- und temperatur abhaengiger Spin-Gitter- Relaxationszeiten war es moeglich, die Fluor-Dynamik auf mikroskopischer Skala aufzuklaeren. Analog zur Linienformanalyse wurden zur Interpretation der Daten zwei Modelle herangezogen, die die unterschiedlichen Fluor-Subsysteme in Betracht ziehen. Fuer reines BaF2 und bei niedrigen Konzentrationen von La3+ wurde das BPP-Relaxationsmodell angewendet, bei hohen Konzentrationen empfiehlt sich hingegen eine Erweiterung der BPP-Theorie, die die beiden Subsysteme F1 und F2 explizit beruecksichtigt. Diese Modelle unkorrellierter Bewegung zeigen insbesondere im Temperaturbereich von 500K-800K eine sehr gute Uebereinstimmung mit den Daten.Weiterhin wurde der Versuch unternommen, die Daten fuer reines BaF2 und niedrige La3+-Dotierungsgrade anhand einer nichtexponentiellen Korrelationsfunktion zu analysieren. Dieser Ansatz erwies sich im gesamten Temperaturbereich als ungeeignet zur Beschreibung der Messdaten. Bei den nach der BPP-Theorie berrechneten Korrelationszeiten konnten mit zunehmendem Dotierungsgrad zwei unterschiedliche Temperaturbereiche beobachtet werden. Dieses aus den Fieldcycling-Messungen gewonnene Resultat zeigt eine gute Uebereinstimmung mit den Ergebnissen zum Selbstdiffusionskoeffizienten. Weiterhin soll hier erwaehnt werden, dass ein Effekt zweiter Ordnung, die statische Quadrupol-Wechselwirkung, die Werte der Fieldcycling-Korrelationszeiten nicht unerheblich beeinflusst haben koennte. In den Anfaengen der NMR-Untersuchungenwurden verschiedene Systeme bekannter Strukturen untersucht und die Resultate in der Literatur festgehalten. In dieser Arbeit werden solche Studien durch das Einbeziehen neuer technischer Entwicklungen (Fieldcycling-NMR) erweitert. Obwohl der aus NMR-Messungen erhaeltliche Informationsgehalt ueber das BaF2-System begrenzt ist, hat diese Studie gezeigt, dass die Aufklaerung wichtiger Merkmale dieses Systems durch die Nutzung elementspezifischer Informationen bez ueglich der Dynamik auf unterschiedlichen Zeitund Laengenskalen moeglich wird. | Deutsch |
|
Freie Schlagworte: |
superionic conductors, Fluorine NMR, BaF2 |
Sachgruppe der Dewey Dezimalklassifikatin (DDC): |
500 Naturwissenschaften und Mathematik > 530 Physik |
Fachbereich(e)/-gebiet(e): |
05 Fachbereich Physik |
Hinterlegungsdatum: |
17 Okt 2008 09:23 |
Letzte Änderung: |
26 Aug 2018 21:25 |
PPN: |
|
Referenten: |
Fujara, Prof. Dr. Franz ; Stuehn, Prof. Dr. Bernd |
Datum der mündlichen Prüfung / Verteidigung / mdl. Prüfung: |
17 Dezember 2007 |
Export: |
|
Suche nach Titel in: |
TUfind oder in Google |
|
Frage zum Eintrag |
Optionen (nur für Redakteure)
|
Redaktionelle Details anzeigen |