Experimental Investigations of Correlated Electron Systems: Alkali Fullerides and Sesquioxides

The work reported in this thesis systematically investigates the structural, electronic and/or magnetic properties of fcc AxCs3-xC60 fullerides (A = K and Rb) and of sesquioxide Cs4O6 under ambient and non-ambient temperature and pressure conditions, employing X-ray and neutron powder diffraction, muon spin relaxation, and SQUID magnetometry. In the AxCs3-xC60 alkali fullerides, unconventional superconductivity can emerge through tuning of intermolecular distances. While conventional BCS-like response is found in the underexpanded regime of the electronic phase diagram, significant deviations appear in the overexpanded region adjacent to the Mott boundary where strong electron correlations are prominent. In this work, the solid-state synthesis of fcc KxCs3−xC60 (0.12 ≤ x ≤ 2) superconducting materials, with intermolecular distances controlled via adjusting the K+/Cs+ dopant ratio, is reported. Whilst the structural, electronic and magnetic properties of fcc KxCs3−xC60 are reminiscent of those previously reported for fcc RbxCs3−xC60, i.e. the Mott boundary can be shifted to ambient pressure and the metal-insulator crossover temperature can be tuned by such chemical pressurisation of Cs3C60, cation-specific effects are of importance on the electronic properties of fcc AxCs3−xC60. This work also provides strong evidence for correlated behaviour in the overexpanded regime through examining the presence or not of the Hebel-Slichter coherence peak and extracting the superconducting gap magnitude. Crystallographically, the alkali sesquioxides A4O6 (A = Rb, Cs), at high and low temperature, had been reported to adopt a cubic structure which generates a single crystallographic position for dioxygen, implying charge disorder: A4(O2(4/3)-)3. Conversely, spectroscopy studies at 5 K found evidence for two localized valence states of dioxygen, indicating charge ordering: (A+)4(O2−)2(O22−). This issue is addressed with the first systematic investigation of the temperature- and cooling-protocol-dependent structural evolution of Cs4O6, revealing the existence of a valence-ordered state at low temperature and also that valence disorder-order transitions can be induced by temperature, pressure and X-ray illumination

Synthesis and Structural Characterization of Y-doped Pyramidal ZnO Powders

The present study focuses on the structural changes in ZnO powder induced by doping of a rare earth metal of Y. For this aim, we synthesized four ZnO samples with different Y-content using the combustion reaction method. X-ray powder diffraction (XRPD) technique and scanning electron microscopy (SEM) results confirm that the as-investigated structural parameters and morphology of the ZnO structure were affected directly by the concentration of Y dopant. For each Y-doped sample, randomly-oriented pyramidal morphology and the formation of a minority phase of Y2O3 were observed. A gradual increase in both lattice parameters and unit cell volume was detected with increasing Y content. All samples were found to be thermally stable in the temperature interval of 25-950 °C

Synthesis and Structural Characterization of Y-doped Pyramidal ZnO Powders

The present study focuses on the structural changes in ZnO powder induced by doping of a rare earth metal of Y.For this aim, we synthesized four ZnO samples with different Y-content using the combustion reaction method.X-ray powder diffraction (XRPD) technique and scanning electron microscopy (SEM) results confirm that the asinvestigated structural parameters and morphology of the ZnO structure were affected directly by the concentrationof Y dopant. For each Y-doped sample, randomly-oriented pyramidal morphology and the formation of a minorityphase of Y2O3 were observed. A gradual increase in both lattice parameters and unit cell volume was detected withincreasing Y content. All samples were found to be thermally stable in the temperature interval of 25-950 °C

Y katkılı piramit ZnO Tozlarının Sentez ve Karakterizasyonu

The present study focuses on the structural changes in ZnO powder induced by doping of a rare earth metal of Y. For this aim, we synthesized four ZnO samples with different Y-content using the combustion reaction method. X-ray powder diffraction (XRPD) technique and scanning electron microscopy (SEM) results confirm that the as-investigated structural parameters and morphology of the ZnO structure were affected directly by the concentration of Y dopant. For each Y-doped sample, randomly-oriented pyramidal morphology and the formation of a minority phase of Y2O3 were observed. A gradual increase in both lattice parameters and unit cell volume was detected with increasing Y content. All samples were found to be thermally stable in the temperature interval of 25-950 °C