Search results for "HEE"
showing 10 items of 1797 documents
On the ferroelastic nature of the scheelite-to-fergusonite phase transition in orthotungstates and orthomolybdates
2009
Abstract New evidence supporting the ferroelastic nature of the pressure-induced scheelite-to-fergusonite phase transition in ABO 4 orthotungstates and orthomolybdates (A = Ca, Sr, Ba, Pb, Eu and B = W, Mo) has been obtained from the analysis of Raman spectroscopy data. In the studied scheelite-type compounds, one external translational mode of B g symmetry in the scheelite phase softens up to the transition pressure and then becomes a hard A g mode in the fergusonite high-pressure phase. However, other scheelite-type compounds not undergoing the scheelite-to-fergusonite transition do not show softening of the B g mode. The reported results have allowed us to establish a relationship betwee…
Quasi-hydrostatic X-ray powder diffraction study of the low- and high-pressure phases of CaWO4 up to 28 GPa
2014
We have studied CaWO4 under compression using Ne as pressure-transmitting medium at room temperature by means of synchrotron X-ray powder diffraction. We have found that CaWO4 beyond 8.8 GPa transforms from its low-pressure tetragonal structure (scheelite) into a monoclinic structure (fergusonite). The high-pressure phase remains stable up to 28 GPa and the low-pressure phase is totally recovered after full decompression. The pressure dependence of the unit-cell parameters, as well as the pressure volume equation of state, has been determined for both phases. Compared with previous studies, we found in our quasi-hydrostatic experiments a different behavior for the unit-cell parameters of th…
In-situ high-pressure Raman scattering studies in PbWO4 up to 48 GPa
2016
The effect of pressure on the Raman spectrum of PbWO4 has been investigated up to 48 GPa in a diamond-anvil cell using neon as pressure-transmitting medium. Changes are detected in the Raman spectrum at 6.8 GPa as a consequence of a structural phase transition from the tetragonal scheelite structure to the monoclinic PbWO4-III structure. Two additional phase transitions are detected at 15.5 and 21.2 GPa to the previously unknown crystalline phases IV and V. The last one remains stable up to 43.3 GPa. At 47.7 GPa all Raman modes disappear, which could be caused by a pressure-induced amorphization. All structural changes are reversible, being the scheelite phase recovered at ambient pressure.…
Lattice dynamics ofYVO4at high pressures
2010
We report an experimental and theoretical lattice-dynamics study of yttrium orthovanadate $({\text{YVO}}_{4})$ up to 33 GPa together with a theoretical study of its structural stability under pressure. Raman-active modes of the zircon phase are observed up to 7.5 GPa, where the onset of an irreversible zircon-to-scheelite phase transition is detected, and Raman-active modes in the scheelite structure are observed up to 20 GPa, where a reversible second-order phase transition occurs. Our ab initio total-energy calculations support that the second-order phase transition in ${\text{YVO}}_{4}$ is from the scheelite to the monoclinic M-fergusonite structure. The M-fergusonite structure remains u…
Low temperature structural transformations on the (001) surface of SrTiO 3 single crystals
2020
This work was supported by the Ministry of Education and Science of Ukraine under the contract M/51–2019 within the framework of the Program of Ukrainian–Latvian Scientific and Technical Cooperation and Latvian–Ukranian Grant LV-UA/2018/2. Authors are indebted to L.L. Rusevich, G. Zvejnieks, V.P. Gnezdilov and A. Glamazda for stimulating discussions.
Polymorphism of praseodymium orthovanadate under high pressure
2021
Zircon-type $\mathrm{PrV}{\mathrm{O}}_{4}$ has been studied at high pressures and room temperature by means of synchrotron powder x-ray diffraction. At room temperature, we observed the previously known zircon-to-monazite phase transition at 5.5(4) GPa and a second phase transition from monazite to a monoclinic structure at 12.7(8) GPa, which we identified as a $\mathrm{PbW}{\mathrm{O}}_{4}$-III-type phase. This conclusion is supported by our ab initio calculations, which also predict a scheelite-type phase to be stable at high pressure. Motivated by this finding, we subjected zircon-type $\mathrm{PrV}{\mathrm{O}}_{4}$ samples to high pressure (7 GPa) and temperature (600, 800, and 1000 \if…
Lattice dynamics of zircon-type NdVO4 and scheelite-type PrVO4 under high-pressure
2021
Abstract Zircon-type NdVO4 and scheelite-type PrVO4 have been studied by means of Raman spectroscopy up to approximately 20 GPa. In the first compound, zircon-scheelite and scheelite-fergusonite phase transitions are reported at 6.4(3) and 19.6(4) GPa, respectively. In the case of scheelite-type PrVO4, a reversible phase transition to a PbWO4-III structure is observed at 16.8(5) GPa. In both cases, a scheelite-type structure is recovered in a metastable state at low pressures. The pressure evolution of the Raman modes is also reported. Our experimental findings are supported by ab initio calculations, which allowed us to discuss the role of mechanic and dynamical instabilities in the phase …
Multitechnique Analysis of the Hydration in Three Different Copper Paddle-Wheel Metal-Organic Frameworks
2019
The structural instability in a humid environment of the majority of metal-organic frameworks (MOFs) is a challenging obstacle for their industrial-scale development. Recently, two water-resistant MOFs have been synthetized, STAM-1 and STAM-17-OEt. They both contain copper paddle wheels, like the well-known water-sensitive HKUST-1, but different organic linkers. The crystal lattice of both the MOFs undergoes a phase transition upon interaction with water molecules. Their unusual flexibility allows the controlled breaking of some interpaddle wheel Cu-O interactions in the so-called crumple zones, with a mechanism called hemilability, which is considered to have a crucial role for the stabili…
Codeposition of Fe 3 O 4 nanoparticles sandwiched between g-C3N4 and TiO2 nanosheets: Structure, characterization and high photocatalytic activity fo…
2019
Novel ternary nanocomposite photocatalysts based on g-C3N4 /Fe3O4 /TiO2 nanosheet were synthesized using simple solid combustion, hydrothermal and wetness impregnation methods. The g-C3N4 nanosheet (2D)/ Fe3O4 /TiO2 nanosheet (2D) triad-interface nanocomposite arranged in the form of Fe 3 O 4 nanoparticle was sandwiched and effectively dispersed on the surface between g-C3N4 and TiO2 nanosheets. The synthesized composites were characterized by some specific techniques such as X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, field emission scanning electronic microscopy (FE-SEM), transmission electron microscope (TEM), vibrating sample magnetometer (VSM), specific su…
Rajallinen työmuisti S2-oppijoiden puheenymmärtämisvaikeuksien selittäjänä
2019
Tässä artikkelissa käsittelen työmuistin rajallisuuden ja aikuisten suomenoppijoiden puheenymmärtämisvaikeuksien yhteyttä tarkastelemalla oppijoiden kuuntelutehtäväsuorituksia ja niitä seuraavia stimulated recall -haastatteluja. Lisäksi selvitän, millaisilla muistitoiminnoilla oppijat kompensoivat työmuistin rajallisuutta ja helpottavat ymmärtämistä. Tutkimukseen osallistui 12 arabiankielistä alkeis- ja perustason aikuista suomenoppijaa, ja aineisto koostuu oppijoiden kuuntelutehtäväsuoritusten ja stimulated recall -haastattelujen videotallenteista. Työmuistin rajallisuuden vaikutus ymmärtämisvaikeuksissa tuli esiin erityisesti silloin, kun yhteen sanaan tai ilmaukseen keskittyminen aiheutt…