Search results for "Vacancy defect"
showing 10 items of 178 documents
Positron studies of hydrogen-defect interactions in proton irradiated molybdenum
1985
Molybdenum single crystals are irradiated at 20 K with 6 MeV protons. The radiation damage and lattice defect annealing is studied by positron lifetime spectroscopy in the temperature range from 15 to 720 K. Loss of vacancies due to recombination with mobile interstitials is observed at 40 K (Stage I) in agreement with resistivity measurements. This is the first time Stage I is observed by positrons below 77 K. The implanted hydrogen decorates the vacancies around 100 K, which is consistent with a hydrogen migration energy in molybdenum:E H = 0.3–0.4 eV. Clustering of spatially correlated vacancies takes place in a wide temperature region below the usual vacancy clustering stage (Stage III)…
Positron lifetime measurements on neutron‐irradiated InP crystals
1996
Neutron‐irradiated InP single crystals have been investigated by positron‐lifetime measurements. The samples were irradiated with thermal neutrons at different fluences yielding concentrations for Sn‐transmuted atoms between 2×1015 and 2×1018 cm−3. The lifetime spectra have been analyzed into one exponential decay component. The mean lifetimes show a monotonous increase with the irradiation dose from 246 to 282 ps. The increase in the lifetime has been associated to a defect containing an Indium vacancy. Thermal annealing at 550 °C reduces the lifetime until values closed to those obtained for the as‐grown and conventionally doped InP crystals. navarrof@evalvx.ific.uv.es ; Jose.Ferrero@uv.es
Temperature- and illumination-induced charge-state change in divacancies of GaTe
2010
Temperature-dependent positron annihilation lifetime spectroscopy measurements have been performed in GaTe samples, with and without illumination. The average lifetime shows a monotonous temperature evolution but the lifetime decomposition shows a rich behavior. It is produced by two types of vacancy defects. The vacancy-type defects characterized by their shorter lifetime change their charge state below 100 K and when illuminating with light of an energy of 0.8 eV.
Reducing the irreducible: Dispersed metal atoms facilitate reduction of irreducible oxides.
2021
Oxide reducibility is a central concept quantifying the role of the support in catalysis. While reducible oxides are often considered catalytically active, irreducible oxides are seen as inert supports. Enhancing the reducibility of irreducible oxides has, however, emerged as an effective way to increase their catalytic activity while retaining their inherent thermal stability. In this work, we focus on the prospect of using single metal atoms to increase the reducibility of a prototypical irreducible oxide, zirconia. Based on extensive self-consistent DFT+U calculations, we demonstrate that single metal atoms significantly improve and tune the surface reducibility of zirconia. Detailed ana…
Surface termination effects on the oxygen reduction reaction rate at fuel cell cathodes
2018
This research was partly funded by the Latvian project IMIS2 with the computer resources provided by the High Performance Computing Centre Stuttgart (HLRS) (Project DEFTD 12939). The authors thank D. Gryaznov for fruitful discussions and M. Sokolov for technical assistance. MMK is grateful to the Office of the Director of National Science Foundation for support under the Independent Research and Development program. The ndings, conclusions, and recommendations expressed in this material are those of the authors and do not necessarily reect the views of NSF and other funding agencies.
First Principles Modeling of Pd-doped (La,Sr)(Co,Fe)O3Complex Perovskites
2016
(La,Sr)(Co,Fe)O3 (LSCF) perovskites are well known promising materials for cathodes of solid oxide fuel cells. In order to reduce cathode operational temperature, doping on B-sublattice with different metals was suggested. Indeed, as it was shown recently experimentally, doping with low Pd content increases oxygen vacancy concentration which is one of factors controlling oxygen transport in fuel cells. In this Communication, we modeled this material using first principles DFT calculations combined with supercell model. The charge density redistribution, density of states, and local lattice distortion around palladium ions are analyzed and reduction of the vacancy formation energy confirmed.
A dilatometric study of the Lao.8Sr0.2MnO3 sintering behaviour
1997
Abstract The sintering behaviour of La0.8Sr0.2MnO3 has been studied by dilatometry between 1100 and 1800 K in various oxygen potentials [pure oxygen, air and nitrogen (PO2 = 1 × 10−5 bar)]. The starting material was prepared by spray pyrolysis of aqueous solutions of nitrates. Two classical sinter stages, neck formation and grain growth respectively, were evidenced from dilatometric curves and this result was corroborated by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses. The rate of densification was found to be directly dependent on oxygen partial pressure, the best conditions being in nitrogen. These results are discussed in terms of vacancy diffusion and oxygen …
Comprehensive defect suppression in perovskite nanocrystals for high-efficiency light-emitting diodes
2021
Electroluminescence efficiencies of metal halide perovskite nanocrystals (PNCs) are limited by a lack of material strategies that can both suppress the formation of defects and enhance the charge carrier confinement. Here we report a one-dopant alloying strategy that generates smaller, monodisperse colloidal particles (confining electrons and holes, and boosting radiative recombination) with fewer surface defects (reducing non-radiative recombination). Doping of guanidinium into formamidinium lead bromide PNCs yields limited bulk solubility while creating an entropy-stabilized phase in the PNCs and leading to smaller PNCs with more carrier confinement. The extra guanidinium segregates to th…
<title>F-type centers in LiBaF<formula><inf><roman>3</roman></inf></formula> crystals</title>
2003
A comparative study of optical properties of thermochemically reduced undoped LiBaF3 crystals is reported. In LiBaF3 crystals obtained or treated in a reducing atmosphere an absorption band at 240 nm and a corresponding luminescence band at 505 nm are observed at 85 K. The main constituent of the center may be an anion vacancy with a trapped electron (an F-type center in LiBaF3 crystals).© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
Optically detected magnetic resonances of nitrogen-vacancy ensembles inC13-enriched diamond
2016
We present an experimental and theoretical study of the optically detected magnetic resonance signals for ensembles of negatively charged nitrogen-vacancy (NV) centers in a $^{13}\mathrm{C}$ isotopically enriched single-crystal diamond. We observe four broad transition peaks with superimposed sharp features at zero magnetic field and study their dependence on an applied magnetic field. A theoretical model that reproduces all qualitative features of these spectra is developed. Understanding the magnetic-resonance spectra of NV centers in an isotopically enriched diamond is important for emerging applications in nuclear magnetic resonance.