Search results for "neutron diffraction"
showing 10 items of 102 documents
Photoconversion and dynamic hole recycling process in anion vacancies in neutron-irradiated MgO crystals
1999
Optical spectroscopy and theory demonstrate that photon excitation of the positively charged anion vacancies (F{sup +} centers) at 5.0 eV in neutron-irradiated MgO crystals releases holes that are subsequently trapped at {ital V}-type centers, which are cation vacancies charge compensated by impurities, such as Al{sup 3+}, F{sup {minus}}, and OH{sup {minus}} ions. The concentration of trapped-hole centers was found to exceed that of available anion vacancies. The disproportionately large amount of holes produced is attributed to a dynamic recycling process, by which the F{sup +} center serves to release a hole to the {ital V}-type centers and subsequently trap a hole from an Fe{sup 3+} ion.…
Amorphous Ge15Te85: density functional, high-energy x-ray and neutron diffraction study
2011
The structure and electronic properties of amorphous Ge15Te85 have been studied by combining density functional (DF) simulations with high-energy x-ray and neutron diffraction measurements. Three models with 560 atoms have been constructed using reverse Monte Carlo methods constrained to (1) agree with the experimental structure factors S(Q), and have (2) energies close to the DF minimum and (3) a semiconducting band structure. The best structure is based on the melt-quenched DF structure and has a small number of Ge–Ge bonds. It shows interlocking networks of Te and GeTe with a significant fraction (22–24%) of voids (cavities). Ge occurs with both tetrahedral and 3 + 3 defective octahedral…
Crystal structure and magnetism of the double perovskites A2FeReO6 (A=Ca, Sr, Ba)
2004
Abstract We synthesized a series of double perovskites A 2 FeReO 6 (A=Ca, Sr, Ba) with Curie temperatures above room-temperature. Neutron and X-ray diffraction analysis have been performed in order to determine the structural and (local) magnetic properties of these materials. While Ba 2 FeReO 6 stays cubic over the whole temperature range we examined, the Sr-compound shows a tetragonal distortion of the perovskite structure which does not completely vanish up to about 520 K far above T C . Ca 2 FeReO 6 has a monoclinic unit cell at high temperatures. Below 400 K a phase separation in two monoclinic phases with identical cell volume is observed in neutron scattering.
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…
Structural evolution of LiOH: evidence of a solid–solid transformation toward Li2O close to the melting temperature
1998
Abstract The structural evolution of LiOH has been studied between 10 K and 1670 K using a combination of neutron and X-ray diffraction and calorimetric measurements. The room temperature tetragonal phase of LiOH has been observed down to 10 K. Above the room temperature a dehydration of solid LiOH into solid Li2O is observed at a temperature and speed which strongly changes with the thermal history and the partial pressure of water vapour. Depending on these conditions the transformation of LiOH in to Li2O before the fusion temperature can be complete, partial or suppressed. In this latter case, as previously reported in the literature, a first order structural phase transition of LiOH is …
NUMERICAL-EXPERIMENTAL METHOD FOR THE ANALYSIS OF RESIDUAL STRESSES IN COLD-EXPANDED HOLES
2012
Hole cold expansion is a technique widely used to improve the fatigue life of components with holes, e.g. bolted or riveted joints. As it has been demonstrated in literature by analytical, numerical and experimental analyses carried out by several authors, the compressive residual stresses introduced by the hole cold expansion have a beneficial influence on both the static and the fatigue strength of the treated component, because they reduce significantly the typical stress peaks around the hole due to stress concentration. In the literature, various analyses of the residual stresses introduced by the hole cold expansion have been performed by using several methods such as X-ray diffractio…
Measuring Inaccessible Residual Stresses Using Multiple Methods and Superposition
2010
The traditional contour method maps a single component of residual stress by cutting a body carefully in two and measuring the contour of the cut surface. The cut also exposes previously inaccessible regions of the body to residual stress measurement using a variety of other techniques, but the stresses have been changed by the relaxation after cutting. In this paper, it is shown that superposition of stresses measured post-cutting with results from the contour method analysis can determine the original (pre-cut) residual stresses. The general superposition theory using Bueckner’s principle is developed and limitations are discussed. The procedure is experimentally demonstrated by determini…
Boson peak and hybridization of acoustic modes with vibrations of nanometric heterogeneities in glasses
2007
9 pages; International audience; The low-frequency dynamics in glasses is compared with that in icosahedral quasicrystals. For both arrangements of matter, the existence of nanometric heterogeneities, implying the existence of a nanometric inhomogeneous elastic network, is expected to play a crucial role. Thanks to this comparison, mostly based on inelastic x-ray (neutron) scattering data, it is proposed that the excess of vibrational density of states observed in both materials is due to the hybridization of longitudinal and transverse acoustic modes with modes localized around the heterogeneities.
The influence of strain rate and presence of dispersed second phases on the deformation behaviour of polycrystalline D2O ice
2018
ABSTRACTThis contribution discusses results obtained from 3-D neutron diffraction and 2-D fabric analyser in situ deformation experiments on laboratory-prepared polycrystalline deuterated ice and ice containing a second phase. The two-phase samples used in the experiments are composed of an ice matrix with (1) air bubbles, (2) rigid, rhombohedral-shaped calcite and (3) rheologically soft, platy graphite. Samples were tested at 10°C below the melting point of deuterated ice at ambient pressures, and two strain rates of 1 × 10−5 s−1 (fast) and 2.5 × 10−6 s−1 (medium). Nature and distribution of the second phase controlled the rheological behaviour of the ice by pinning grain boundary migratio…
The effect of water on protein dynamics
2004
Neutron diffraction and spectroscopy were applied to describe the hydration and dynamics of a soluble protein and a natural membrane from extreme halophilic Archaea. The quantitative dependence of protein motions on water activity was clearly illustrated, and it was established that a minimum hydration shell is required for the systems to access their functional resilience, i.e. a dynamics state that allows biological activity.