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Öğe Elusive Valence Transition in Mixed-Valence Sesquioxide Cs4O6(Amer Chemical Soc, 2019) Colman, Ross H.; Okur Kutay, Havva Esma; Kockelmann, Winfried; Brown, Craig M.; Sans, Annette; Felser, ClaudiaCs4O6 is a mixed-valence molecular oxide with a cubic structure, comprising valency-delocalized O-2(4/3-) units and with properties highly sensitive to cooling protocols. Here we use neutron powder diffraction to authenticate that, while upon deep quenching the cubic phase is kinetically arrested down to cryogenic temperatures, ultraslow cooling results in an incomplete structural transition to a contracted tetragonal phase. Two dioxygen anions in a 1:2 ratio are identified, providing evidence that the transition is accompanied by charge and orbital order and stabilizes a Robin-Day Class II mixed-valence state, comprising O-2(2-) and O-2(-) anions. The phenomenology of the phase change is consistent with that of a martensitic transition. The response of the low-temperature phase assemblage to heating is complex, involving a series of successive interconversions between the coexisting phases. Notably, a broad interconversion plateau is present near 260 K, signifying reentrant kinetic arrest tetragonal phase upon heating because of the combined effects of increased steric hindrance for molecular rotation and melting of charge and orbital order. The geometrically frustrated pyrochlore lattice adopted by the paramagnetic S = 1/2 O-2(-) units provides an intimate link between the crystal and magnetic properties of charge-ordered Cs4O6, naturally accounting for the absence of magnetic order.Öğe Pressure-Induced Charge Disorder-Order Transition in the Cs4O6 Sesquioxide(Amer Chemical Soc, 2020) Okur Kutay, Havva Esma; Colman, Ross H.; Ohish, Yasuo; Sans, Annette; Felser, Claudia; Jansen, MartinCs4O6 adopts two distinct crystal structures at ambient pressure. At temperatures below similar to 200 K, its ground state structure is tetragonal, incorporating two symmetry-distinct dioxygen anions, diamagnetic peroxide, O-2(2-), and paramagnetic superoxide, O-2(-), units in a 1:2 ratio, consistent with the presence of charge and orbital order. At high temperatures, its ground state structure is cubic, comprising symmetry-equivalent dioxygen units with an average oxidation state of -4/3, consistent with the adoption of a charge-disordered state. The pressure dependence of the structure of solid Cs4O6 at 300 K and at 13.4 K was followed up to similar to 12 GPa by synchrotron X-ray powder diffraction. When a pressure of similar to 2 GPa is reached at ambient temperature, an incomplete phase transition that is accompanied by a significant volume reduction (similar to 2%) to a more densely packed highly anisotropic tetragonal structure, isostructural with the low-temperature ambient-pressure phase of Cs4O6, is encountered. A complete transformation of the cubic (charge-disordered) to the tetragonal (charge-ordered) phase of Cs4O6 is achieved when the hydrostatic pressure exceeds 6 GPa. In contrast, the pressure response of the Cs4O6 cubic/tetragonal phase assemblage at 13.4 K is distinctly different with the cubic and tetragonal phases coexisting over the entire pressure range (to similar to 12 GPa) accessed in the present experiments and with only a small fraction of the cubic phase converting to tetragonal. Pressure turns out to be an inefficient stimulus to drive the charge disorder-order transition in Cs4O6 at cryogenic temperatures, presumably due to the high activation barriers (much larger than the thermal energy at 13.4 K) associated with the severe steric hindrance for a rotation of the molecular oxygen units necessitated in the course of the structural transformation.Öğe Structural and electronic properties of the overexpanded quaternary superconducting fulleride K0.25Rb0.25CS2.5C60(Pergamon-Elsevier Science Ltd, 2019) Okur Kutay, Havva Esma; Prassides, KosmasThe structural and magnetic properties of the overexpanded quatemary K0.25Rb0.25Cs2.5C60 fulleride are investigated. Rietveld analysis of the high-resolution synchrotron X-ray powder diffraction data shows that K0.25Rb0.25Cs2.5C60 adopts a cubic structure with face-centered (fcc) symmetry. Magnetization measurements at ambient pressure reveal that the material is a superconductor with a transition temperature, T-c, of 29.4 K. Temperature dependence of the magnetic susceptibility does not show a simple temperature-independent Pauli susceptibility term instead, it shows a cusp at a certain temperature implying an insulator-to-metal crossover on cooling. Complementary high-pressure magnetization measurements to a pressure of 10.2 kbar show a non-monotonic response of TT with a dome-shaped scaling with pressure and a maximum value of 32.4 K at 3.9 kbar. The experimental results are consistent with quaternary K0.25Rb0.25Cs2.5C60 following the established unconventional behavior of overexpanded fullerides. However, our results also unveil well-defined detrimental cation specific effects on the electronic properties associated with the introduction of increased disorder in the tetrahedral interstices of the fcc fulleride structure.Öğe Structural and electronic response of overexpanded superconducting fullerides close to the Mott insulator boundary(World Scientific Publ Co Pte Ltd, 2018) Menelaou, M.; Takabayashi, Y.; Okur Kutay, Havva Esma; Zadik, R. H.; Prassides, K.The ternary fulleride, Rb0.25Cs2.75C60, is the most expanded member of the family of face-centered cubic (fcc) structured superconducting fullerides ever accessed with superconductivity surviving at ambient pressure closest to the Mott insulator boundary. Here, we study the evolution of its structural and electronic properties with temperature. At ambient temperature, Rb0.25Cs2.75C60 lies in the Mott-Jahn-Teller (MJT) insulating part of the phase diagram. High-resolution synchrotron X-ray diffraction shows that its structure remains strictly cubic at all temperatures, but the transition to the metallic state at similar to 50 K - evident in the evolution of the magnetic susceptibility with temperature - is accompanied by a lattice collapse, Delta V/V-0 of 0 : 5%. Bulk superconductivity then emerges on further cooling with a T-c of 25.9 K. The results permit the extension of the electronic phase diagram of A(3)C(60) fullerides as close as possible to the metal-insulator (M-I) crossover.Öğe Structural and optical characterization of Sm-doped ZnO nanoparticles(Indian Acad Sciences, 2019) Okur Kutay, Havva Esma; Bulut, N.; Ates, T.; Kaygili, O.Micro-structural changes in zinc oxide (ZnO) nanoparticles induced by the substitution of Zn2+ in ZnO by a rare earth (RE) metal ion, Sm3+, are investigated. Both pristine and Sm-doped ZnO with a nominal doping concentration of 1, 2 and 4% of Sm using a simple wet-chemical synthetic route followed by calcination at a high temperature of 900 degrees C, are synthesized. Structural investigations are primarily conducted using X-ray powder diffraction (XRPD) and scanning electron microscopy techniques. Evolution of structural parameters (unit cell parameters, average crystallite size, crystallinity percentage, lattice strain, stress, energy density and atomic packing factor) upon Sm doping is investigated together with Rietveld refinement and Le Bail analysis techniques. XRPD data confirmed that the synthesized nanostructures crystallize in a wurtzite hexagonal structure, the dopant Sm is incorporated into the Zn lattice and the annealing treatment plays a crucial role in determining the structural and optical properties of RE-metal-doped nanoparticles. Values of the optical band gap energy estimated from optical absorbance measurements reveal a widening of the band gap.Öğe Synthesis and structural analysis of nonstoichiometric ternary fulleride K1.5Ba0.25CsC60(Scientific Technical Research Council Turkey-Tubitak, 2020) Okur Kutay, Havva EsmaThe existence of cation-vacancy sites in fullerides might lead to long-range ordering and generate a new vacancy-ordered superstructure. The purpose of this work is to search whether or not long-range ordering of vacant tetrahedral sites, namely superstructure emerges in nonstoichiometric K-15 Ba0.25CsC60 fulleride. Therefore, K1.5Ba0.25CsC60 with cation-vacancy sites is synthesized using a precursor method to avoid inadequate stoichiometry control and formation of impurity phases within the target composition. For this purpose, first, phase-pure K6C60, BA(6)C(60) and Cs-6 C-60 precursors are synthesized. Stoichiometric quantities of these precursors are used for further reaction with C-60 to afford K1.5Ba0.25CsC60. Rietveld analysis of the high-resolution synchrotron X-ray powder diffraction data of the precursors and K1.5Ba0.25CsC60 confirms that K6C60, Ba6C60 and Cs6C60 are single-phase and they crystallize in a body-centered-cubic structure (Im (3) over bar) as reported in the literature. The analysis also shows that K1.5Ba0.25CsC60 phase can be perfectly modeled using a face-centered cubic structure. No new peaks appear which could have implied the appearance of a superstructure. This suggests that there is no long-range ordered arrangement of vacant tetrahedral sites in K1.5Ba0.25CsC60.Öğe Synthesis and Structural Characterization of Y-doped Pyramidal ZnO Powders(2020) Kebiroğlu, Hanifi; Kaygılı, Ömer; Bulut, Niyazi; Okur Kutay, Havva Esma; Ercan, Ismail; Ercan, Filiz; Yahıa, I.S.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 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.
