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Single-Crystal X-Ray Diffraction at Extreme Conditions in Mineral Physics and Material Sciences

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Single-Crystal X-Ray Diffraction at Extreme Conditions in Mineral Physics and Material Sciences Single-crystal X-ray diffraction at extreme conditions in mineral physics and material sciences DISSERTATION zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) an der Bayreuther Graduiertenschule für Mathematik und Naturwissenschaften (BayNAT) der Universität Bayreuth vorgelegt von Elena Bykova aus Amursk (Russland) Bayreuth, 2015 This doctoral thesis was prepared at the Bavarian Research Institute of Experimental Geochemistry and Geophysics and the Laboratory of Crystallography of the University of Bayreuth from 03/2011 until 12/2014 and was supervised by Prof. Dr. Leonid Dubrovinsky and Prof. Dr. Natalia Dubrovinskaia. This is a full reprint of the dissertation submitted to obtain academic degree of Doctor of Natural Sciences (Dr. rer. nat.) and approved by the Bayreuth Graduate School of Mathematical and Natural Sciences (BayNAT) of the University of Bayreuth. Date of submission: 23.01.2015 Date of defense: 29.05.2015 Acting director: Prof. Dr. Franz Xaver Schmid Doctoral committee: Prof. Dr. Leonid Dubrovinsky (1st reviewer) Prof. Dr. Sander van Smaalen (2nd reviewer) Prof. Dr. Daniel Frost (chairman) Prof. Dr. Jürgen Senker Zusammenfassung Die Einkristallröntgenstrahlbeugung (X-ray diffraction, XRD) ist eine leistungsfähige und zerstörungsfreie Methode zur eindeutigen Identifizierung kristalliner Phasen, zur Bestimmung ihrer Kristallstrukturen (Gitterparameter, Raumgruppe, fraktionelle Koordinaten und Besetzung der Atome), sowie, falls notwendig, ihrer Phasenzusammensetzung. Die vorliegende Arbeit behandelt die Anwendung der Einkristallröntgenstrahlbeugung in der Hochdruck- Hochtemperatur-Forschung (high pressure high temperature, HPHT) unter Verwendung von laserbeheizten Diamantstempelzellen (diamond anvil cells, DACs), wobei hier auch die methodologischen Aspekte und Arbeitsschritte der Einkristallröntgenstrahlbeugungsexperimente beschrieben werden, also Kristall- bzw. Probenauswahl, Befüllung und Montage der DACs, Durchführung der eigentlichen Experimente, Datenverarbeitung und zuletzt Strukturbestimmung und/oder Strukturverfeinerung. Das große Potential und neuen Anwendungsmöglichkeiten der Hochdruckkristallographie für die Material- und Geowissenschaften werden an verschiedenen Beispielen diskutiert, an Übergangsmetallboriden, Metall-dotierten Borphasen, Silikaten und Oxiden. Im Einzelnen wurden dafür die Kristallstrukturen von Co5B16, MnB4, Al-dotiertem β-Bor, 3+ Knorringit und Fe -haltigem Bridgmanit verfeinert und das Hochdruckverhalten von FeB4, Fe2B7, FexB50 und FeOOH untersucht. Des Weiteren wurden detaillierte strukturelle Untersuchungen an einer Reihe von Hochdruck-Eisenoxiden durchgeführt, die es erlaubten, eine jahrzehntelange Kontroverse zu klären. Die vorliegende Arbeit führte auch zur Entdeckung neuer Phasen, einschließlich dem gemischten Eisenoxid Fe5O7 mit einer sehr ungewöhnlichen Stöchiometrie. Ein Schwerpunkt der hier vorgestellten Arbeit liegt in der Bestimmung der Kristallstrukturen von Al-dotierten, rhomboedrischen β-Bor (AlB44.8(5) oder AlB37.8(5)) und von FexB50. Für Erstere wurden die Positionen und Besetzungen der Zwischengitter-Al- und B-Atome bestimmt. Das Modell von Al-dotiertem β-Bor zur Beschreibung der Fehlordnung ist dem Modell des bekannten SiB30.17C0.35 ähnlich. In FexB50 besetzt das Metall die tetraedrische Position, wobei die Besetzung zwischen 50 und 65% variiert. Ein nächster Themenbereich sind die Kristallstrukturen neuer Übergangsmetallboride, nämlich MnB4, FeB4, Co5B16 und Fe2B7. Die Tetraboride werden allgemein aus Säulen von kantenverknüpften MB12-Polyedern aufgebaut, in denen die Metallatome eindimensionale Ketten bilden. Während die Metall-Metall-Abstände im orthorhombischen FeB4 einheitlich sind, 4 führt in MnB4 die Peierlsverzerrung zur Paarbildung der Mn-Atome und dies zu einer Verringerung der Kristallstruktursymmetrie nach monoklin. Wenn das Metall-Bor-Verhältnis größer ist als 1:4, dann enthalten die Boride nicht nur 12-fach koordinierte Metallatome MB12, sondern auch 10- und 9-fach koordinierte Metallatome (CoB9 in Co5B16, FeB10 in Fe2B7). Aufgrund des Bormangels wird die “Metall”-Packung dichter, was sich nun in gemeinsamen Flächen der Koordinationspolyeder wiederspiegelt. Die für diese Arbeit untersuchten Übergangsmetallboride zeigen kurze B–B Bindungen, welche ihre mechanischen Eigenschaften beeinflussen: das FeB4 und Fe2B7 sind entlang bestimmter kristallographischer Richtungen genauso fest und starr wie Diamant. Die Analyse der Hochdruck- Einkristallröntgenstrahlbeugungsdaten lässt vermuten, dass die Ursache dieser Inkompressibilität in der Steifigkeit der orientierten kurzen B–B Bindungen liegt. Diese einzigartige räumliche Anordnung der Atome befördert FeB4 in die Klasse der superharten Materialien mit einer Nanoeindruckshärte von 62(5) GPa. Die Struktur von FexB50 ist aus B12-Ikosaedern aufgebaut und hat große Hohlräume, wodurch sie effektiver schrumpfen kann als die Bor-Polymorphe (α-, - und -Bor), die ebenfalls chemisch gebundene B12-Ikosaeder enthalten. Die hier gemessenen Daten bestätigen die früheren Beobachtungen zur Komprimierung der Bor-Phasen, dass die intra-ikosaedrischen Bindungen steifer sind als die inter-ikosaedrischen Bindungen. Genau strukturelle Untersuchungen am Granat Knorringit, der bei 26 GPa und 1800 °C in einem Multistempelapparat synthetisiert wurde, zeigen, dass die Zusammensetzung als Mg3(Cr1.58Mg0.21Si0.21)Si3O12 beschrieben werden kann und Knorringit dadurch 21 mol-% von Majorit, dem MgSiO3-Endglied der Mischkristallreihe enthält. Die Verteilung von Eisen im Al-freien, Fe3+-haltigem Mg-Perovskit (Bridgmanit) wurde mittels Einkristallröntgenstrahlbeugung kombiniert mit Mößbauer Spektroskopie bestimmt, die 2+ 3+ chemische Zusammensetzung ist (Mg0.946(17)Fe 0.045(4)Fe 0.011(1))Si0.997(16)O3. Aus der Einkristallröntgenstrahlbeugung konnte gezeigt werden, dass Eisen nicht die Si-Position (die sogenannten B-Sites) besetzt. Die Methode der Hochdruck-Einkristallröntgenstrahlbeugung wurde auch herangezogen, um das Verhalten von FeOOH bei Drücken oberhalb von 70 GPa zu untersuchen. Unter Normalbedingungen besitzt diese Verbindung eine Wasserstoffbrückenbindung entlang von 5 Kanälen, die von verzerrten FeO3(OH)3-Oktaedern gebildet werden. Unterhalb 16 GPa schrumpfen diese Kanäle (und als Folge auch die Wasserstoffbrückenbindung) stärker als die einzelnen Fe–O Bindungen in den Oktaedern; oberhalb 16 GPa schrumpfen beide Bindungen einheitlich. Bei etwa 45 GPa reduziert eine Spin-Überkreuzung in Fe3+ das Elementarzellvolumen drastisch (um etwa 11%) und verursacht eine Symmetrisierung der Wasserstoffbrückenbindungen, was aus der Analyse der interatomaren Abstände in den Fe(O…H)3(OH)3-Gruppen abgeleitet wurde. Eine Symmetrisierung der Wasserstoffbrückenbindungen in Verbindung mit der High-Spin zu Low-Spin Überkreuzung in Fe3+ wurde hier zum ersten Mal mittels Hochdruck-Einkristallröntgenstrahlbeugung nachgewiesen. Die Hochdruck-Hochtemperatur-Einkristallröntgenstrahlbeugung wurde eingesetzt, um nach HPHT-Polymorphen von Fe2O3 und Fe3O4 im Megabar-Druckbereich zu suchen und um das Verhalten der Eisenoxide in plattentektonisch abgetauchten Bändererzen (banded iron formations, BIFs) im unteren Erdmantel zu studieren. Es konnte gezeigt werden, dass oberhalb 29 GPa Fe3O4 die Kristallstruktur von CaTi2O4 annimmt, die bis mindestens 70(1) GPa und 2400(100) K stabil bleibt. Dadurch wurde eine 50-jährige Kontroverse über die Struktur des Fe2O3- Polymorphs, das oberhalb von etwa 50 GPa stabil ist, geklärt. Im Speziellen wurde gezeigt, dass diese Phase eine Doppelperovskit-Typ Struktur in trikliner Symmetrie hat. Darüber hinaus erzwingt die Kompression oberhalb 67 GPa eine Umwandlung zu einer anderen Hochdruckphase mit orthorhombischer Zelle (Raumgruppe Aba2). Diese Phase geht bei Temperaturerhöhung bei etwa 67 GPa in ein HPHT-Fe2O3-Polymorph mit CaIrO3-Struktur (Postperovskit, -Fe2O3) über. Bei Druckentlastung zu 41 GPa und weiterer Temperaturerhöhung bis 1800 K wandelt sich diese Phase dann in die Rh2O3-II Struktur um. Die faszinierendste Beobachtung ist, dass bei Bedingungen des unteren Erdmantels das -Fe2O3 Sauerstoff freigibt und sich weiter zu einer neuen Fe5O7-Verbindung zersetzt. Fe2O3 und Fe3O4 bilden bis zu 85 Gewichts-% der BIFs, die aufgrund der Subduktion lithosphärischer Platten bis in den unteren Erdmantel transportiert werden können. Deshalb kann das Fe2O3 der abgetauchten BIFs als Quelle für ein sauerstoffreiches Fluid im tiefen Erdinneren dienen, mit einer erheblicher Menge an Sauerstoff (bis zu acht Mal die Sauerstoffmenge in der heutiger Atmosphäre!), was zu einer deutlichen Heterogenität in der Sauerstofffugazität in unterschiedlichen Teilen des Erdmantels führt. 6 Summary Single crystal X-ray diffraction (XRD) is a powerful non-destructive method which allows unambiguously identify crystalline phases, determine a crystal structure (unit cell parameters, a space group, atomic coordinates and atomic occupancies) and, if required, a phase composition. This thesis deals with applications of single-crystal XRD in high pressure and high temperature (HPHT) research using laser-heated diamond anvil cells (DACs). The thesis describes methodological aspects of our single-crystal XRD experiments which involve crystals selection, DACs preparation, maintaining experiments, data processing, and structure solutions and/or refinements. We demonstrate a great potential and novel opportunities provided by high- pressure crystallography in materials- and geo-sciences on the examples of studies of transition
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