Search results for "Vacancy defect"
showing 10 items of 178 documents
<title>Thermostimulated electronic and ionic processes in irradiated sapphire</title>
2003
Electronic and ionic thermostimulated (TS) relaxation (TSR) processes in nominally pure sapphire (α-Al2O3 grown with oxygen deficiency) have been investigated at 290 - 650 K by means of the TS current (TSC), ionic depolarization current (TSDC) and electron emission (TSEE) techniques. After thermal (ionic) polarization of the reduced sapphire wide (approximately 75 K) and asymmetric ionic dipolar TSDC peak at 590 K (disorientation of the anion vacancy-related dipoles) was detected. Above 450 - 500 K the anion vacancy hopping (migration) starts and their interaction with defects take place. This can lead to lattice dynamic disordering and anion vacancy diffusion-controlled processes in sapphi…
Hydration entropy of BaZrO3 from first principles phonon calculations
2015
The impact of phonons on the hydration and defect thermodynamics of undoped and acceptor (Sc, In, Y and Gd) doped BaZrO3 is addressed by means of first principles supercell calculations. In contrast to previous, similar investigations, we evaluate contributions from all phonon modes, and also pressure/volume effects on the phonon properties. The calculations are performed at the GGA-level with the PBE and RPBE functionals, both of which predict for BaZrO3 a stable cubic perovskite structure. For all dopants, the vibrational formation entropy of the doubly positively charged oxygen vacancy is significantly lower than that of the protonic defect , which therefore also is the dominant contribu…
The electronic properties of an oxygen vacancy at ZrO2-terminated (001) surfaces of a cubic PbZrO3: computer simulations from the first principles
2008
Combining B3PW hybrid exchange-correlation functional within the density functional theory (DFT) and a supercell model, we calculated from the first principles the electronic structure of both ideal PbZrO(3) (001) surface (with ZrO(2)- and PbO-terminations) and a neutral oxygen vacancy also called the F center. The atomic relaxation and electronic density redistributions are discussed. Thermodynamic analysis of pure surfaces indicates that ZrO(2) termination is energetically more favorable than PbO-termination. The O vacancy on the ZrO(2)-surface attracts approximately 0.3 e (0.7 e in the bulk PbZrO(3)), while the remaining electron density from the missing O(2-) ion is localized mostly on …
Effect of Vacancies on Positron Annihilation and Hyperfine Interactions in Fe-Al Alloys - <i>Ab Initio</i> Study
2012
Vacancies occupying different sub-lattices of D03 structure of Fe3Al alloy were investigated by the full potential linearized augmented plane wave method. The calculations basing on the super-cell approach have been performed for the vacancy concentrations of 1.6 and 3 at.%. For both concentrations the sub-lattice preference for vacancy location was determined. The dependence of vacancy formation energy on magnetic state of structure has been found. The positron lifetimes for the annihilation in the bulk (Fe3Al) and in vacancies have been investigated basing on the ab initio results for the electron density. The effect of vacancies on spin magnetic moment and hy-perfine parameters of Fe ato…
Influence of Compensating Defect Formation on the Doping Efficiency and Thermoelectric Properties of Cu2-ySe1–xBrx
2015
The superionic conductor Cu_(2−δ)Se has been shown to be a promising thermoelectric at higher temperatures because of very low lattice thermal conductivities, attributed to the liquid-like mobility of copper ions in the superionic phase. In this work, we present the potential of copper selenide to achieve a high figure of merit at room temperature, if the intrinsically high hole carrier concentration can be reduced. Using bromine as a dopant, we show that reducing the charge carrier concentration in Cu_(2−δ)Se is in fact possible. Furthermore, we provide profound insight into the complex defect chemistry of bromine doped Cu_(2−δ)Se via various analytical methods and investigate the conseque…
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…