Search results for "tetragonal crystal"
showing 10 items of 216 documents
High-pressure X-ray diffraction study of EuWO4 to 12 GPa
2005
In-situ high-pressure X-ray diffraction studies were performed on EuWO4 to 12 GPa. We found that EuWO4 transforms from the tetragonal I41/a structure to the monoclinic I2/a structure at 8.5(5) GPa, being this phase transition re- versible. No other structural change is detected up to 12 GPa. The EOS of EuWO4 is determined from the pressure–volume data. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Contribution of the synchrotron diffraction study of the oxidation of uranium dioxide at 250○C
2004
The structural evolution of UO 2 during its oxidation into U 3 O 8 at 250°C in air was studied by in-situ synchrotron X-ray diffraction on the D2AM-CRG beamline at ESRF. The aim of this study is to determine the phases which are likely to appear during a long term storage of used nuclear fuel. Our results are in disagreement with the literature where the existence of the secondary cubic phase is not reported, and an α-U 3 O 7 tetragonal phase (c/a < 1) is also mentioned but definitely not observed. These previous interpretations are possibly due to a poor instrumental resolution, inducing a sensible broadening of the diffraction peaks. Particularly, the fact that the instrumental resolution…
Polymorphism in Strontium Tungstate SrWO 4 under Quasi-Hydrostatic Compression
2016
The structural and vibrational properties of SrWO4 have been studied experimentally up to 27 and 46 GPa, respectively, by angle-dispersive synchrotron X-ray diffraction and Raman spectroscopy measurements as well as using ab initio calculations. The existence of four polymorphs upon quasi-hydrostatic compression is reported. The three phase transitions were found at 11.5, 19.0, and 39.5 GPa. The ambient-pressure SrWO4 tetragonal scheelite-type structure (S.G. I41/a) undergoes a transition to a monoclinic fergusonite-type structure (S.G. I2/a) at 11.5 GPa with a 1.5% volume decrease. Subsequently, at 19.0 GPa, another structural transformation takes place. Our calculations indicate two possi…
Structural Characterization of Aurophilic Gold(I) Iodide under High Pressure
2019
[EN] The effects of pressure on the crystal structure of aurophilic tetragonal gold iodide have been studied by means of powder X-ray diffraction up to 13.5 GPa. We found evidence of the onset of a phase transition at 1.5 GPa that is more significant from 3.8 GPa. The low- and high-pressure phases coexist up to 10.7 GPa. Beyond 10.7 GPa, an irreversible process of amorphization takes place. We determined the axial and bulk compressibility of the ambient-pressure tetragonal phase of gold iodide up to 3.3 GPa. This is extremely compressible with a bulk modulus of 18.1(8) GPa, being as soft as a rare gas, molecular solids, or organometallic compounds. Moreover, its response to pressure is anis…
Stability of non-stoichiometric laof phases: x-ray diffraction investigation
1998
Summary The X-ray diffraction patterns of the polycrystalline LaO 1−x F 1+2x (0.05≤x≤0.3) phases were measured at room temperature and analyzed by the Rietveld profile refinement method. All compounds crystallize in the tetragonal PbFCl-type structure with P4/nmm as the space group. No distortion to lower orthorhombic symmetry was observed. The tetragonal lattice parameters a and c increase with increasing excess of fluoride. The La-oxygen (La-fluorine) distances first decrease (increase) but then these trends are reversed. The Global Instability Index calculated from the bond valence model increases towards the higher fluoride content of the compounds indicating decreasing stability
Toward tailorable surfaces: a combined theoretical and experimental study of lanthanum niobate layered perovskites.
2014
A comprehensive theoretical investigation of the MLaNb2O7 (M = H, Li, Na, K, Rb, and Cs) series of ion-exchangeable layered perovskite is presented. These perovskites are in particular interesting in view of their potential applications as inorganic supports for the design of new hybrid inorganic-organic proton conductors. In particular, their structural and electronic properties have been investigated by periodic calculations in the framework of Density Functional Theory, using different exchange-correlation functionals. A general very good agreement with the available experimental (XRD, NPD, and EXAFS) data has been found. The structure of the protonated HLaNb2O7 form has also been furthe…
Local structural investigation of hafnia-zirconia polymorphs in powders and thin films by X-ray absorption spectroscopy
2019
Björn Matthey (Fraunhofer IKTS, Dresden) is acknowledged for providing HfO2 and ZrO2 powders on short notice after DESY’s renowned customs office punished us. Parts of this research were carried out at Petra III at DESY, a member of the Helmholtz Association (HGF). The experiments on single Si:HfO2 thin film samples were performed at the CLAESS beamline at ALBA Synchrotron with the collaboration of ALBA staff. We would like to thank Edmund Welter for assistance (in using beamline P65) and DESY for enabling this research for proposal no. 20160591 and for travel support. T.S. acknowledges the German Research Foundation (DFG) for funding this work in the frame of the project “Inferox” (project…
Influence of Twinning Microstructure of Crystals with Low Tetragonality on a X-Ray Diffraction
2001
The intensity distributions of the X-rays scattered in the tetragonal single crystal, which represent a complex of the twin domains separated by the coherent parallel boundaries, are simulated. The calculations are performed by using the Monte Carlo method within the framework of a kinematical approach. The thickness distributions of the twin domains are defined according to the geometrical, Gaussian and log normal functions. ‘Critical’ effects of the X-ray scattering are found, namely there is transformation of the tetragonal doublet into singlet or multiplet. As demonstrated, each of characteristics of the tetragonal doublet profile depends on a few parameters of the twin microstructure o…
Synthesis and host properties of tetragonal Li2Mn2O4 and Li2Co0.4Mn1.6O4
2000
Abstract This paper presents synthesis and electrochemical properties of tetragonal lithium manganese dioxide, Li2Mn2O4 and its cobalt doped analogue Li2Co0.4Mn1.6O4. The materials are compared as host materials for lithium insertion and the behavior during the initial lithium extraction as well as on repeated cycling is presented. These materials show an initial lithium extraction capacity between 200 and 270 mAh/g. On repeated cycling, they are converted into spinel-like lattices with reversible capacities in the range 82–90 mAh/g. As they are chemically compatible with the manganese spinel, they will be well suited as additives compensating for the capacity loss during the initial formin…
Defect Luminescence Study in Tetragonal GeO2 Crystals
2000
The perovskite-or rutile-like GeO2 crystals were grown by the “top-seed” method from a fused germanium dioxide solution with sodium bicarbonate. The luminescence was studied under cathode-, x-ray, and photo-excitation. There are two luminescence bands at 550 nm and 400 nm with slow (100–200 µs) and fast (<20 ns) decay, respectively. The luminescence can be excited either by intra-center and electron-hole recombination processes. An analogous luminescence, however with strong non-exponential decay, is observed in glassy materials obtained from the frozen solution. Sodium-germanate glasses melted under oxygen deficient condition also possess similar luminescence, therefore the luminescence is…