Search results for "Anisotropy"
showing 10 items of 912 documents
Electric properties of KTiOPO4and NaTiOPO4from temperature-dependent X-ray diffraction
1999
Single crystals of KTiOPO4(KTP) and NaTiOPO4(NaTP) show pronounced pyroelectric behaviour. In order to determine the origin of this property on an atomic scale, X-ray diffraction measurements have been carried out at several temperatures between 100 and 600 K. Modelling of the electron density and the evolution of the structure as a function of temperature has enabled the determination of values for the spontaneous polarization of the compounds and the pyroelectric coefficient of KTP, principally due to the alkaline-ion displacements with a value of 2.0 nC cm−2 K−1. Structure modifications, compared with NaTiOPO4, and the calculation of the electrostatic potential explain the anisotropic be…
The Instability of the NbTe2 Surface Structure
2002
Low energy electron diffraction from clean NbTe 2 surfaces shows very diffuse reflections. The effect is attributed to an anisotropic heating of the irradiated Te-Nb-Te surface layer. Diffraction patterns for electron energies below 90 eV correspond to an overlapped contribution from numerous domains, belonging to three orientational variants. Electrons of higher energies stabilize the parent high-temperature CdI 2 structure. A similar effect is observed during scanning tunneling microscopy, where the expected surface corrugation is usually lost for tunneling currents of a few ten nA.
Transversal thermovoltages of (1 1 9) Bi2Sr2CaCu2O8+δ thin films on vicinal (1 1 0) SrTiO3 substrates
1997
Abstract Biaxial textured (1 1 9) oriented Bi 2 Sr 2 CaCu 2 O 8+δ thin films were fabricated by DC-Magnetron sputtering on vicinal (1 1 0) SrTiO 3 substrates. The crystal orientation and stochiometry of the films were obtained from precise X-ray diffraction measurements in four-circle geometry. According to anisotropic transport measurements, the superconducting transition temperature is approximately 47 K and the normal state resistivities along two perpendicular paths differ by a factor of 13.5. Transversal thermoelectric effects were investigated by measuring thermovoltages transverse to temperature gradients parallel to the surface normal induced by pulsed laser irradiation. At room tem…
<title>Mechanisms of holographic recording in amorphous semiconductor films</title>
1998
The mechanisms of holographic recording in (mainly chalcogenide) amorphous semiconductor films are reviewed including photoinduced structural changes, relaxational structural changes, recharging of localized states in the band gap, and photoinduced anisotropy. The holographic properties in these cases are compared. Different effects of hologram self-enhancement are also considered.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
Holographic recording of surface relief gratings in tolyle-based azobenzene oligomers
2008
Abstract Holographic recording of surface relief gratings (SRG) in tolyle-based azobenzene oligomer films have been carried out at 514.5 nm. It was experimentally studied by spectroscopic and AFM methods. The maximum surface modulation amplitude was 35 nm. SRG formation is explained by the mean-field model and by the photodegradation of chromophores enabling the directional mass transport as the result of trans–cis transformations. The conclusion is made that SRG are stable but the diffraction efficiency (DE) changes are due to the complementary decaying amplitude-phase gratings. Strong polarization dependences of transmitted and reflected DE are found. They are explained by the diffraction…
Finite element analysis of laser shock peening of 2050-T8 aluminum alloy
2015
Laser shock processing is a recently developed surface treatment designed to improve the mechanical properties and fatigue performance of materials, by inducing a deep compressive residual stress field. The purpose of this work is to investigate the residual stress distribution induced by laser shock processing in a 2050-T8 aeronautical aluminium alloy with both X-ray diffraction measurements and 3D finite element simulation. The method of X-ray diffraction is extensively used to characterize the crystallographic texture and the residual stress crystalline materials at different scales (macroscopic, mesoscopic and microscopic).Shock loading and materials’ dynamic response are experimentally…
Structural and Dielectric Properties of Na0.99Li0.01NbO3Ceramics
2006
Na0.99Li 0,01 NbO3 ceramics were prepared by solid-state reaction method. The samples were characterized by X-ray diffraction, microstructure and dielectric permittivity measurements. The X-ray diffraction analysis showed that samples have a single phase of perovskite structure with orthorhombic symmetry. Microstructure investigations revealed crystalline structures in grains. It was found that dielectric properties of Na99Li0.01NbO3 ceramic are sensitive to the applied axial pressure. This includes the shift of phase transformation, diffuseness of the ϵ (T) profile and reduction of the thermal hysteresis. These effects can be caused by change in domain structure and anisotropy of crystalli…
Dense Post-Barite-type Polymorph of PbSO4 Anglesite at High Pressures
2019
Synchrotron X-ray diffraction measurements on lead sulfate have been performed up to 67 GPa using He as pressure transmitting medium. Experiments reveal the existence of a reversible pressure-induced phase transition from the initial Pnma barite-type to the P212121 post-barite-type structure at pressures above 27 GPa. This phase transition involves a volume collapse of 2.4% and requires a considerable pressure overshoot (large pressure range with coexistence of phases) to overcome the large kinetic barrier of the transition. DFT calculations confirm the experimental observations and support the hypothesis that post-barite-type phase is the thermodynamically stable high-pressure structure fo…
Experimental evidence for pressure-driven isostructural and symmetry-breaking phase transitions on Bi14CrO24
2014
Abstract We performed in situ X-ray diffraction experiments on Bi 14 CrO 24 under pressure up to 17.4 GPa. We discovered two reversible phase transitions that take place at 7.3 and 12.1 GPa. The first transition is isostructural and the second-transition involves a tetragonal-monoclinic symmetry reduction. Both transitions involve a small volume collapse indicating that they have a first-order character. For the three observed phases we determined a P – V equation of state. All of them are highly compressible having bulk moduli that range from 64 to 70 GPa. We also determined the axial compressibilities for difference phases, being the response of the structures to pressure anisotropic. Thi…
High-pressure phase transformations in NdVO4 under hydrostatic, conditions: a structural powder x-ray diffraction study
2019
Room temperature angle dispersive powder x-ray diffraction experiments on zircon-type NdVO4 were performed for the first time under quasi-hydrostatic conditions up to 24.5 GPa. The sample undergoes two phase transitions at 6.4 and 19.9 GPa. Our results show that the first transition is a zircon-to-scheelite-type phase transition, which has not been reported before, and contradicts previous non-hydrostatic experiments. In the second transition, NdVO4 transforms into a fergusonite-type structure, which is a monoclinic distortion of scheelite-type. The compressibility and axial anisotropy of the different polymorphs of NdVO4 are reported. A direct comparison of our results with former experime…