Open Access

Amir-Abbas Haghighirad

• In this thesis we report on the high pressure synthesis, crystal growth, structural characterisation and magnetic properties of the cubic vanadate pyrochlores A2V2O7 (with A = Y, Er and Dy). We have found that high pressure is requisite for the stabilization of the selected compounds. For this purpose, a multianvil high pressure apparatus was built in our laboratory and a new multianvil inset (i.e., a ceramic pressure medium and the interior parts) was developed. The multianvil press is based on a hydraulic press with a maximum force of 7.73 MN (corresponds to 788 tons), a Walker type module and a specially designed hydraulic and electric control. Pressure calibration of the multianvil setup was performed by high pressure fixed points (i.e. solid-solid transformation of Bi I-II (2.55 GPa) and Bi II-III (3.15 GPa)). A maximum pressure of 6 GPa was attained using hardened metal anvils (tungsten carbide) with truncation edge length (TEL) of 14 mm and a sample volume of ~ 70 mm3. Heating of the sample in our current multianvil setup (TEL = 14 mm) was achieved by resistive heating of a graphite furnace. Temperatures up to 1500 °C could be obtained at pressures up to 6 GPa. By systematic variation of the synthesis conditions (for instance the operation temperature or the choice of the crucible material) under high pressure and taking into account the well known ternary compounds, when accessing the phase diagram, the cubic vanadate pyrochlores A2V2O7 (with A = Y, Er and Dy) were synthesized successfully. It was found that the oxygen partial pressure is crucial for the formation of the desired pyrochlore phase. Gas-tight platinum crucibles were used as container material for the synthesis of the vanadate pyrochlores. We have investigated, that pressures of the order of 5.0 GPa and temperatures of approximately 1200 °C are necessary for the stabilization of the monophasic samples of the vanadate pyrochlores. Lu2V2O7 could be synthesized under ambient pressure conditions and is used in our studies for comparison purposes. A special graphite furnace was developed for the high pressure crystal growth of the vanadate pyrochlores. For the first time, A2V2O7 (with A = Y, Er and Dy) single crystals with a maximum size of 0.4 mm were grown by using the grain growth method at high pressure and high temperature conditions. The samples (i.e., powders and single crystals) were characterised by single crystal Xray diffraction, X-ray powder diffraction method, Laue method and scanning electron microscopy (SEM). Complementary to the X-ray diffraction methods, infrared absorsoption spectroscopy was used to distinguish between the fluorite and pyrochlore structure. It has been shown that all samples crystallize in a well-ordered cubic structure with the space group F d 3m. The vanadium (+4) content in the samples was determined by oxidative weight gain in air using a thermogravimetric (TG) balance. A structural phase transformation of cubic to tetragonal was observed by differential thermal analysis (DTA) in conjunction with high temperature diffractometry. The magnetic characterisation of the vanadate pyrochlores A2V2O7 (Y, Lu, Er and Dy) was performed by Katarina Removic-Langer in the laboratory of Prof. Dr. M. Lang. All materials studied are ferromagnetic. The ferromagnetic critical temperatures are between 70 and 73 K. In case of Er2V2O7 and Dy2V2O7 an additional increase in the magnetization was observed below 20 K. The increase in the magnetization below 20 K exhibited by Er2V2O7 and Dy2V2O7 originates from the interactions between the two magnetic sublattices (i.e., the rare earth- and the vanadium sublattice).
• Diese Arbeit beschäftigt sich mit der Pulversynthese und der Kristallzüchtung von Pyrochlor-Vanadat-Verbindungen A2V2O7 (A = Y, Er und Dy) unter Verwendung hoher Drücke und Temperaturen. Nach einer allgemeinen Einführung zum Thema Druck als thermodynamische Größe, werden die Interessanten physikalischen Eigenschaften der erwähnten Verbindungen berichtet. (Kapitel 1) Pyrochlor-Oxide (chemische Formel A2B2O7) kristallisieren in kubischer Kristallstruktur (Raumgruppe F d 3 m), wobei A und B Metalle sind. Die Pyrochlor-Struktur ist aufgebaut aus den Koordinationspolyedern AO8 und BO6, die in den meisten Fällen verzerrt sind. Die Pyrochlor Struktur existiert für drei Varianten, wenn Kationen mit verschiedenen Valenz-Zuständen kombiniert werden, d.h., A3+B4+, A2+B5+ und Kombinationen der gemischten Valenz-Zustände. Durch die Flexibilität der Pyrochlor-Struktur sind interessante physikalische Eigenschaften beobachtet worden. (Kapitel 5) Speziell in der Festkörperphysik sind die Pyrochlor-Oxide interessant wegen der Wechselwirkungen der magnetischen Ionen (A und/oder B) in der Kristallstruktur. Im Allgemeinen ist bekannt, dass Pyrochlore zusätzlich zum Kristallgitter eine magnetische Untergitter-Struktur aufweisen (die sogenannten Pyrochlor-Gitter), die durch geometrische und topologische Bedingungen zur Frustration führen können. Frustration charakterisiert ein System, wenn dessen konkurrierende Wechselwirkungen in einem möglichen Grundzustand nicht befriedigt werden können. Besonders interessant sind dabei Materialien, die keine langreichweitige magnetische Ordnung besitzen und selbst bei niedrigsten Temperaturen einen exotischen Grundzustand bilden wie z.B., Spinglas, Spinflüssigkeit, Spineis und Spin-1/2-Materialien. Auch die Orbitalen Freiheitsgrade (d.h. Ausrichtung und Besetzung von 3d-Wellenfunktionen) können zur Frustration führen. (Kapitel 1) ...