Search results for "vacancy"
showing 10 items of 201 documents
Investigation of Oxygen-Related Luminescence Centres in AlN Ceramics
2000
The structure of oxygen-related luminescence centres in nominally undoped and Y 2 O 3 doped AlN ceramics was investigated by electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and optically-detected EPR. The photoluminescence-detected EPR lines having g-values of 1.990 and 2.008 were assigned to a recombination between neighbouring donor and acceptor pairs. The two EPR lines at g = 1.987 and 2.003 detected via the recombination luminescence in the afterglow are thought to be due to a recombination between the same, but distant donor and acceptor pairs. The donor was previously speculated to be an electron trapped on an oxygen impurity which substitutes for a ni…
First-principles simulations on the aggregation of F centers in BaF2: R centers
2011
Abstract F center (an electron trapped in the fluorine vacancy) and R center (a defect composed of three F centers) in BaF 2 crystal, have been studied by using density functional theory (DFT) with hybrid exchange potentials, namely DFT-B3PW. Our calculations show that the F -center transfer barrier is equal to 1.83 eV. During the F -center transfer, the trapped electron is more delocalized than that in the static F -center case, and the gap between defect leveland CB in the α-spin state decreases obviously. The association energy calculations on R centers indicate stable aggregations of isolated F centers. During F -center aggregation, a considerable covalency between two neighbor fluorine…
The VN2 negatively charged defect in diamond. A quantum mechanical investigation of the EPR response
2020
Abstract The VN 2 − defect in diamond consists of a vacancy surrounded by two substitutional nitrogen atoms, which lower the local symmetry from Td to C2v. Calculations of the doublet ground state geometry, electronic structure, EPR parameters, and IR spectra of this defect are reported along with a preliminary investigation of the observed optical transition. For the most part our results were obtained using a uniform charge compensated supercell approach together with the B3LYP functional and all-electron Gaussian basis sets designed for the properties studied. In particular, the computed hyperfine and quadrupolar EPR parameters for the carbon and nitrogen atoms adjacent to the vacancy ag…
Neutron irradiation defects in gallium sulfide: Optical absorption measurements
1997
Gallium sulfide single crystals have been irradiated with different thermal neutron doses. Defects introduced by neutron irradiation turn out to be optically active, giving rise to absorption bands with energies ranging from 1.2 to 3.2 eV. Bands lying in the band-gap exhibit Gaussian shape. Their energies and widths are independent of the irradiation dose, but their intensities are proportional to it. Thermal annealing is completed in two stages, ending at around 500 and 720 K, respectively. Centers responsible for the absorption bands are proposed to be gallium-vacancy-galliuminterstitial complexes in which the distance between the vacancy (acceptor) and the interstitial (donor) determines…
Ab initio simulations on AgCl(111) surface and AgCl(111)/α-Al2O3(0001) interface
2005
The defect chemistry and ionic transport properties of the AgCl(111)/α-Al 2 O 3 (0001) interface were consid by using ab initio slab calculations. These calculations were performed in the framework of plane-wave basis combined with the density functional theory (DFT), as implemented into the VASP computer code, and Gaus basis set combined with the Hartree-Fock method (CRYSTAL-98 code). We analyze the electron density distribu on the interface and the electrostatic potential distribution near the AgCl surface. The size of the silver ion is great to enter the corundum surface layer and to create excess silver ions in this way. This is in agreement the experiments on heterogeneous doping of Ag…
Computer modelling of point defects in ABO3 perovskites and MgO
2004
We present results for basic intrinsic defects: F-type electron centers (O vacancy which trapped one or two electrons) and hole polarons bound to Mg or K vacancy in ionic MgO and partly covalent KNbO3 perovskite, respectively. We demonstrate that a considerable covalency of the perovskite chemical bonding makes the F-type centers therein much more similar to defects in partly-covalent quartz-type oxides rather than the conventional F centers in alkali halides and ionic MgO. Both one-site (atomic) and two-site (molecular) polarons are expected to coexist in KNbO3 characterized by close absorption energies. Our calculations confirm existence of the self-trapped electron polarons in KNbO3, KTa…
A comparative study of the atomic and electronic structure of F centers in ferroelectric KNbO3: Ab initio and semi-empirical calculations
1998
Abstract The linear muffin-tin-orbital method combined with density functional theory (in a local density approximation) and the semi-empirical method of the intermediate neglect of the differential overlap (INDO) based on the Hartree-Fock formalism are used for the supercell study of the F centers (O vacancy with two electrons) in cubic and orthorhombic ferroelectric KNbO3 crystals. The two electrons are found to be considerably delocalized even in the ground state of the defect. Their wave functions extend over the two Nb atoms closest to the O vacancy and over other nearby atoms. Thus, the F center in KNbO3 resembles much more electron defects in the partly covalent SiO2 crystal (the so-…
Atomic and electronic structure of perfect and defective PbZrO3 perovskite: Hybrid DFT calculations of cubic and orthorhombic phases
2007
Abstract The structural and electronic properties of pure cubic and low-temperature orthorhombic PbZrO3 (antiferroelectric phase), as well as cubic PbZrO3 containing single F-centers (neutral oxygen vacancies) have been simulated by means of ab initio hybrid density functional calculations. We observed a substantial increase of the Pb–O bond covalency in ideal orthorhombic PbZrO3 with respect to its cubic phase. Relatively large displacement of four Pb atoms nearest to the F-center (0.25 A towards the defect) could affect the PbZrO3 ferroelectric properties. An O vacancy in the bulk PbZrO3 attracts ≈0.7 e, and the remaining electron density from the missing O2− is localized mostly on four n…
Imaging the local charge environment of nitrogen-vacancy centers in diamond
2018
Characterizing the local internal environment surrounding solid-state spin defects is crucial to harnessing them as nanoscale sensors of external fields. This is especially germane to the case of defect ensembles which can exhibit a complex interplay between interactions, internal fields and lattice strain. Working with the nitrogen-vacancy (NV) center in diamond, we demonstrate that local electric fields dominate the magnetic resonance behavior of NV ensembles at low magnetic field. We introduce a simple microscopic model that quantitatively captures the observed spectra for samples with NV concentrations spanning over two orders of magnitude. Motivated by this understanding, we propose an…
The catalytic properties of DNA G-quadruplexes rely on their structural integrity
2021
International audience; The influence of the G-quartet structural integrity on the catalytic activity of the G-quadruplex (G4) was investigated by comparing the G4-DNAzyme performances of a series of G4s with a G-vacancy site and a G-triplex (G-tri). The results presented herein not only confirm that the structural integrity of the 3’-end G-quartet is necessary for G4s to be catalytically competent but also show how to remediate G-vacancy-mediated catalytic activity losses via the addition of guanine surrogates in an approach referred to as G-vacancy complementation strategy that is applicable to parallel G4s only. Furthermore, this study demonstrates that the terminal G-quartet could act a…