Search results for "Phase Transition"

Microfluidic Synthesis of Highly Shape-Anisotropic Particles from Liquid Crystalline Elastomers with Defined Director Field Configurations

2011

In this article, we present the synthesis of highly shape-anisotropic, micrometer-sized particles from liquid crystalline elastomers, which have the ability to reversibly change their shape in response to a certain external stimulus. For their preparation, we utilized a microfluidic setup. We succeeded in preparing sets of particles with differing degrees of shape anisotropy in their ground state including highly anisotropic fiber-like objects. All samples produced movement during the phase transition from the nematic to the isotropic phase of the liquid crystal. Depending on the direction of this shape change, we classified the samples in two groups. One type showed a contraction, while th…

1989

Some structurally disordered models for poly(tetrafluoroethylene) were elaborated and the corresponding calculated X-ray diffraction patterns compared with experimental patterns. This and differential scanning calorimetry results suggest some differences between the virgin powder and a sintered polycrystalline plate of poly(tetrafluoroethylene). These differences may be related to the crystal phase transitions that occur in a range of temperatures close to room temperature.

Optical control of vibrational coherence triggered by an ultrafast phase transition

2019

Femtosecond time-resolved x-ray diffraction is employed to study the dynamics of the periodic lattice distortion (PLD) associated with the charge-density-wave (CDW) in K0.3MoO3. Using a multi-pulse scheme we show the ability to extend the lifetime of coherent oscillations of the PLD about the undistorted structure through re-excitation of the electronic states. This suggests that it is possible to enter a regime where the symmetry of the potential energy landscape corresponds to the high symmetry phase but the scattering pathways that lead to the damping of coherent dynamics are still controllable by altering the electronic state population. The demonstrated control over the coherence time …

High-pressure theoretical and experimental study of HgWO4

2011

HgWO 4 at ambient pressure is characterized using a combination of ab initio calculations, X-ray diffraction and Raman scattering measurements. The effect of low pressure and temperature on the structural stability is analysed. Extending our ab initio study to the range of higher pressures, a sequence of stable phases up to 30GPa is proposed. © 2011 Taylor & Francis.

Evolution of structural and electronic properties of TiSe2 under high pressure

2021

A pressure-induced structural phase transition and its intimate link with the superconducting transition was studied for the first time in TiSe2 up to 40 GPa at room temperature using X-ray diffraction, transport measurement, and first-principles calculations. We demonstrate the occurrence of a first-order structural phase transition at 4 GPa from the standard trigonal structure (S.G.P3¯m1) to another trigonal structure (S-G-P3¯c1). Additionally, at 16 GPa, the P3¯c1 phase spontaneously transforms into a monoclinic C2/m phase, and above 24 GPa, the C2/m phase returns to the initial P3¯m1 phase. Electrical transport results show that metallization occurs above 6 GPa. The charge density wave …

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)

Structural study of α-Bi2O3 under pressure

2013

An experimental and theoretical study of the structural properties of monoclinic bismuth oxide (alpha-(BiO3)-O-2) under high pressures is here reported. Both synthetic and mineral bismite powder samples have been compressed up to 45 GPa and their equations of state have been determined with angle-dispersive x-ray diffraction measurements. Experimental results have been also compared with theoretical calculations which suggest the possibility of several phase transitions below 10 GPa. However, experiments reveal only a pressure-induced amorphization between 15 and 25 GPa, depending on sample quality and deviatoric stresses. The amorphous phase has been followed up to 45 GPa and its nature di…

Precursor effects of the Rhombohedral-to-Cubic Phase Transition in Indium Selenide

2002

We report on the observation of precursor effects of the rhombohedral-to-cubic phase transition in Indium Selenide (InSe) with several experimental techniques. The pressure at which these precursor defects are first observed depends on the sensitivity of the experimental technique. In transport measurements, which are very sensitive to low defect concentrations, precursor effects are observed 5 to 6 GPa below the phase transition pressure whereas in X-ray diffraction measurements precursor effects are only observed 2 GPa below the phase transition pressure. We report optical absorption measurements, in which the precursor effects are shown by the growth and propagation of dark linear defect…

Ferroelastic Fingerprints in Methylammonium Lead Iodide Perovskite

2016

Methylammonium lead iodide (MAPbI3) perovskite shows an outstanding performance in photovoltaic devices. However, certain material properties, especially the possible ferroic behavior, remain unclear. We observed distinct nanoscale periodic domains in the piezoresponse of MAPbI3(Cl) grains. The structure and the orientation of these striped domains indicate ferroelasticity as their origin. By correlating vertical and lateral piezoresponse force microscopy experiments performed at different sample orientations with X-ray diffraction, the preferred domain orientation is suggested to be the a1–a2-phase. The observation of these ferroelastic fingerprints appears to strongly depend on the film t…

Morphology and structures in double-, triple- and quadruple-chain phospholipid monolayers at the air/water interface

2007

The structure of double-, triple- and quadruple-chain phospholipid monolayers has been studied by Synchrotron x-ray diffraction. The double-chain mixed- linkage species exhibit an oblique structure at all pressures investigated. The triple-chain phospholipids show at lower lateral pressures a rectangular unit cell with a phase transition at higher pressures to a hexagonal packing of vertically arranged chains. The quadruple- chain lipid exhibits only the hexagonal phase structure. The position of the ether linkage and of the branched chain on the glycerol backbone has also a strong influence on the monolayer structures. Fluorescence microscopy shows different domain shapes for the different…