Search results for "Crystallographic"

Optically active oxygen-deficiency-related centers in amorphous silicon dioxide

1998

Abstract The spectroscopic properties, structure and interconversions of optically active oxygen-deficiency-related point defects in vitreous silica are reviewed. These defects, the E′-centers (oxygen vacancies with a trapped hole or 3-fold-coordinated silicons), different variants of diamagnetic `ODCs' (oxygen-deficiency centers), and their Ge-related analogs play a key role in the fiber-optic Bragg grating writing processes. The controversy surrounding the structural models for the Si- and Ge-related ODCs is discussed and the similarity between the bulk and surface point defects in silica is emphasized. The possible interconversion mechanisms between 2-fold-coordinated Si, neutral oxygen …

Variation of lattice constant and cluster formation in GaAsBi

2013

We investigate the structural properties of GaAsBi layers grown by molecular beam epitaxy on GaAs at substrate temperatures between 220–315 C. Irrespective of the growth temperature, the structures exhibited similar Bi compositions, and good overall crystal quality as deduced from X-Ray diffraction measurements. After thermal annealing at temperatures as low as 500 C, the GaAsBi layers grown at the lowest temperatures exhibited a significant reduction of the lattice constant. The lattice variation was significantly larger for Bi-containing samples than for Bi-free low-temperature GaAs samples grown as a reference. Rutherford backscattering spectrometry gave no evidence of Bi diffusing out o…

The kinetics of F-center aggregation under irradiation: many-particle effects in ionic solids

1994

The accumulation kinetics of primary Frenkel defects created in solids under permanent irradiation is calculated using the microscopic formalism of many-particle densities. It is based on the Kirkwood superposition approximation for three-particle densities as described in our previous paper p. N. Kuzovkov and E. A. Kotomin, Physica Scripta 47, 585 (1993)l. This formalism is generalized in this paper by incorporating the elastic attraction between similar defects (called in ionic solids F-centers) which causes their efficient aggregation. It is shown that the aggregation process starts only if the dose rate and elastic attraction energy exceed certain critical values; it also happpens in th…

A mesoscopic approach to point-defect clustering in solids during irradiation

1993

Accumulation of point defects in solids during irradiation is often accompanied by self-organization processes which lead to point-defect clustering and thus to the formation of a spatially inhomogeneous defect structure. Within the framework of a mesoscopic phenomenological approach, the conditions for clustering of mobile point defects caused by their elastic interactions are studied. It is shown that differences between the elastic interaction of similar and that of dissimilar defects may lead to such clustering. Further, it is shown that the presence of impurities acting as traps for interstitials may promote the clustering process. The conditions for spatial clustering are studied for …

The Deformation of Single, Nanometer-Sized Metal Crystals in Graphitic Shells

2005

Basic properties of the F-type centers in halides, oxides and perovskites

2010

We present a short survey of the optical properties of primary radiation-induced point defects in alkali halides, simple oxides and some ABO3 perovskites. We discuss in details the optical properties of single electron F and F + centers in rock-salt (f.c.c.) alkali halides and oxides and show that the Mollwo–Ivey law well-known for the F-type centers in alkali halides may be extended for other rock-salt structure insulators. We also discuss the major differences in point defect production mechanisms in halides and oxides. We show that the Rabin–Klick diagram may be generalized for a whole family of alkali halides. The F-type center migration and aggregation into metal colloids in alkali hal…

Oxygen-excess-related point defects in glassy/amorphous SiO2 and related materials

2012

Abstract An insight is given into recent experimental advances in the spectroscopic studies of oxygen-excess intrinsic defects, in glassy SiO 2 and α-quartz. By controlling excess oxygen in a-SiO 2 , and the conditions of F 2 -laser irradiation, SiO 2 glass samples can be obtained with optical absorption almost exclusively dominated by single defect, oxygen dangling bonds (“non-bridging oxygen hole centers” or NBOHCs), without the presence of complementary Si dangling bonds (generic “E′-centers”). This allows for a more accurate determination of the spectral shape of NBOHC optical absorption in UV and vacuum UV spectral regions. The temperature dependence of NBOHC electron paramagnetic reso…

Theoretical and experimental study of primary radiation defects in KNbO3 perovskite crystals

2000

Abstract The results of large-scale computer simulations of point defects – F-type electron centers and hole polarons bound to a potassium vacancy – in perovskite KNbO3 crystals are presented. One-site polarons and two-site (molecular) polarons are expected to coexist, both are characterized by close absorption energies around 1 eV. The transient absorption spectra and decay kinetics observed after ns-pulsed electron beam irradiation have been measured for different KNbO3 crystals. The relaxation kinetics of the optical density vary considerably for different impurity concentrations and sample stoichiometries. Experimental data are discussed in the light of the presented calculations.

Influence of <formula formulatype="inline"><tex Notation="TeX">${\hbox{O}}_2$</tex></formula>-Loading Pretreatment on the Rad…

2014

We investigated the impact of an oxygen preloading on pure-silica-core or fluorine-doped-core fiber responses to high irradiation doses (up to 1 MGy (SiO 2 )). Oxygen enrichment was achieved through a diffusion-based technique, and the long-term presence of O 2 molecules was confirmed by micro-Raman experiments. Online radiation induced attenuation (RIA) experiments were carried out in both the pristine and the O 2 -loaded optical fibers to investigate the differences induced by this pretreatment in the UV and visible ranges. Contrary to results recently published on the positive impact of O 2 on infrared RIA, our results reveal a RIA increase with O 2 presence. Data are analyzed in order t…

Ab initio modelling of titanium impurities in α-Fe lattice

2020

Abstract Reduced activation ferritic-martensitic (RAFM) as well as ferritic steels strengthened by yttrium oxide are considered as candidate materials for future fusion and advanced fission reactors. Addition of Ti during the manufacturing of the oxide dispersed strengthened (ODS) leads to the formation of yttrium titanium oxide particles, which size is smaller compared to yttrium oxide particles. This improves the mechanical properties and radiation resistance of the ODS steels. DFT calculations of Ti impurities have been performed to determine the factors contributing to the formation of the nanoparticles in α-Fe (bcc-Fe) based steels. The interaction energies between TiFe-OFe, TiFe-Ooct,…