Search results for "Phase Transition"
showing 10 items of 1281 documents
In situ and time resolved study of the - transition in nanometric particles
2007
In situ and real-time study of the {gamma} to {alpha}-Fe{sub 2}O{sub 3} transition is carried out on the H10 beamline at LURE (France). {gamma}-Fe{sub 2}O{sub 3} particles are synthesized by soft chemistry. These particles have an average diameter evaluated by X-ray diffraction of 9{+-}1nm and a specific surface area of 116m{sup 2}g{sup -1}. The size of produced {alpha}-Fe{sub 2}O{sub 3} particles is determined by in situ and time resolved X-ray diffraction measurements at different temperatures. An amazing evolution of size with time is revealed: an abrupt doubling of the {alpha}-Fe{sub 2}O{sub 3} particle size is observed whatever the heating temperature. Some assumptions are given in ord…
Landau theory applied to phase transitions in calcium orthotungstate and isostructural compounds
2007
The pressure-driven tetragonal-to-monoclinic phase transition in CaWO4 and related scheelite-structured orthotungstates is analysed in terms of spontaneous strains. Based upon our previous high-pressure x-ray diffraction results and the Landau theory, it is suggested that the scheelite-to-fergusonite transition is of second order in nature.
High-pressure structural behaviour of HoVO4: combined XRD experiments and ab initio calculations.
2014
We report a high-pressure experimental and theoretical investigation of the structural properties of zircon-type HoVO4. Angle-dispersive x-ray diffraction measurements were carried out under quasi-hydrostatic and partial non-hydrostatic conditions up to 28 and 23.7 GPa, respectively. In the first case, an irreversible phase transition is found at 8.2 GPa. In the second case, the onset of the transition is detected at 4.5 GPa, a second (reversible) transition is found at 20.4 GPa, and a partial decomposition of HoVO4 was observed. The structures of the different phases have been assigned and their equations of state (EOS) determined. Experimental results have also been compared to theoretica…
High-pressure phase transitions and compressibility of wolframite-type tungstates
2010
This paper reports an investigation on the phase diagram and compressibility of wolframite-type tungstates by means of x-ray powder diffraction and absorption in a diamond-anvil cell and ab initio calculations. The diffraction experiments show that monoclinic wolframite-type MgWO4 suffers at least two phase transitions, the first one being to a triclinic polymorph with a structure similar to that of CuWO4 and FeMoO4-II. The onset of each transition is detected at 17.1 and 31 GPa. In ZnWO4 the onset of the monoclinic-triclinic transition has been also found at 15.1 GPa. These findings are supported by density-functional theory calculations, which predict the occurrence of additional transiti…
Two-dimensional electrical ordering in physisorbed monolayers
1990
Abstract Monolayers of polar molecules CF3H, CF3Cl and CF2Cl2 have been physisorbed on the graphite (001) surface and studied by X-ray diffraction and dielectric measurements. The temperature-coverage phase diagrams of the quasitwodimensional crystalline phases are presented. The structures of the paraelectric and the electrically ordered phases have been determined. The phase transitions are discussed.
Ferroelasticity and glass-like behavior in alkali halide-alkali cyanide mixed crystals
1991
Abstract Single crystal neutron diffraction studies in (KBr)1-x(KCN)x are summarized. Mixed crystals with CN− concentrations x > 0.6 exhibit ferroelastic phase transitions from a high-temperature plastic phase into a low-temperature elastically ordered phase in which the CN− orientations show long range orientational order and the center of mass lattice exhibits shear distortions. For concentrations x ≤ 0.6 orientational disorder is frozen-in and transitions into an orientational glass state occur. Close to the critical concentration xc ∼ 0.6 the diffraction profiles at the transition temperatures are dominated by diffuse-scattering contributions. These results are compared to model calcula…
Structure and properties of tris(tetramethylammonium) nonabromodiarsenate(III), [(CH3)4N]3[As2Br9]
2007
The new tetramethylammonium bromoarsenate(III) crystal [(CH3)4N]3 [As2Br9] has been synthesized and its phase behaviour established by differential scanning calorimetry and dilatometry. Three phase transitions are found: at 346/346 K , at 165/171 K and at 157/165 K on cooling/heating. Single-crystal x-ray diffraction studies of phases I (at 370 K) and II (at 298 K) have shown that the structure comprises discrete [As2Br9]3− anions and disordered [(CH3)4N]+ cations. Phase II crystallizes in the polar space group P31c; its polar nature was confirmed by pyroelectric measurements. Phase I has higher symmetry, P63mc or . Dielectric dispersion measurements revealed a relaxation process in the cry…
Preparation and Thermal Behavior of Freely-Suspended and Transferred Films Composed of a Single Compound, the Liquid Crystal 707PP
1991
We have recently introduced a new method for the preparation of ultrathin smectic films on solid supports by transferring freely-suspended liquid-crystalline films [1, 2]. Here we report on the preparation, structural properties and thermal behavior of freely-suspended and transferred films of 2-(4'-Heptyloxyphenyl)-5-heptylpyridine (707PP). Freely-suspended and transferred films were investigated using optical microscopy and X-ray diffraction. Both types of films exhibit reversible phase transitions comparable to bulk values.
X-ray diffraction study of CF3Cl monolayers adsorbed on graphite (001)
1988
X-ray diffraction experiments have been performed on mono- and submonolayers of CF3Cl physisorbed on the (001) surfaces of exfoliated graphite. Five quasi-twodimensional solid phases including the commensurate 2×2 phase, an electrically ordered phase and a disordered incommensurate phase have been observed. A phase diagram is proposed and the various phase transitions are discussed.
Synchrotron X-ray Diffraction and Reflection Studies of a Polymacromonomer Monolayer at the Air−Water Interface: Transition from Aligned Single Mole…
2004
Monolayers of polymacromonomers with poly(vinyl)pyridine side chains of different lengths, polyPVP20.8 and polyPVP46.7, are studied at the air/water interface. Combination of reflection data and diffraction peaks allows the establishment of a structural model. In the expanded phase at pressures below approximately 15 mN/m, the side chains are adsorbed to the air/water interface, causing polymer backbone stretching and alignment of the single molecules. The in-plane peaks are due to the lateral electron density variation above and below the adsorption layer. The diameter of the flattened cylindrical molecules corresponds to twice the contour length of the side chains (25 and 12.5 nm, respect…