scholarly journals Atomic, electronic and magnetic structure of an oxygen interstitial in neutron-irradiated Al2O3 single crystals

2020 ◽  
Vol 10 (1) ◽  
Author(s):  
V. Seeman ◽  
A. Lushchik ◽  
E. Shablonin ◽  
G. Prieditis ◽  
D. Gryaznov ◽  
...  
Keyword(s):  
Dft Calculations ◽  
Single Crystals ◽  
Density Functional ◽  
Magnetic Moments ◽  
Atomic Charges ◽  
Interstitial Oxygen ◽  
Absorption Bands ◽  
Magnetic Field Rotation ◽  
Formal Charge ◽  
Radiation Induced

Abstract A single radiation-induced superoxide ion $$O_{2}^{ - }$$ O 2 - has been observed for the first time in metal oxides. This structural defect has been revealed in fast-neutron-irradiated (6.9×1018n/cm2) corundum (α-Al2O3) single crystals using the EPR method. Based on the angular dependence of the EPR lines at the magnetic field rotation in different planes and the determined g tensor components, it is shown that this hole-type $$O_{2}^{ - }$$ O 2 - center (i) incorporates one regular and one interstitial oxygen atoms being stabilized by a trapped hole (S = 1/2), (ii) occupies one oxygen site in the (0001) plane being oriented along the a axis, and (iii) does not contain any other imperfection/defect in its immediate vicinity. The thermal stepwise annealing (observed via the EPR signal and corresponding optical absorption bands) of the $$O_{2}^{ - }$$ O 2 - centers, caused by their destruction with release of a mobile ion (tentatively the oxygen ion with the formal charge −1), occurs at 500–750 K, simultaneously with the partial decay of single F-type centers (mostly with the EPR-active F+ centers). The obtained experimental results are in line with the superoxide defect configurations obtained via density functional theory (DFT) calculations employing the hybrid B3PW exchange-correlation functional. In particular, the DFT calculations confirm the $$O_{2}^{ - }$$ O 2 - center spin S = 1/2, its orientation along the a axis. The $$O_{2}^{ - }$$ O 2 - center is characterized by a short O–O bond length of 1.34 Å and different atomic charges and magnetic moments of the two oxygens. We emphasize the important role of atomic charges and magnetic moments analysis in order to identify the ground state configuration.

scholarly journals Evidence for the formation of two types of oxygen interstitials in neutron-irradiated α-Al2O3 single crystals

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
A. Lushchik ◽  
V. N. Kuzovkov ◽  
E. A. Kotomin ◽  
G. Prieditis ◽  
V. Seeman ◽  
...  
Keyword(s):  
Single Crystals ◽  
Structural Defects ◽  
Fast Neutrons ◽  
Absorption Bands ◽  
Diffusion Parameters ◽  
Frenkel Defects ◽  
Secondary Defect ◽  
Radiation Induced ◽  
Α Al2o3 ◽  
Kinetics Of

AbstractDue to unique optical/mechanical properties and significant resistance to harsh radiation environments, corundum (α-Al2O3) is considered as a promising candidate material for windows and diagnostics in forthcoming fusion reactors. However, its properties are affected by radiation-induced (predominantly, by fast neutrons) structural defects. In this paper, we analyze thermal stability and recombination kinetics of primary Frenkel defects in anion sublattice − the F-type electronic centers and complementary oxygen interstitials in fast-neutron-irradiated corundum single crystals. Combining precisely measured thermal annealing kinetics for four types of primary radiation defects (neutral and charged Frenkel pairs) and the advanced model of chemical reactions, we have demonstrated for the first time a co-existence of the two types of interstitial defects – neutral O atoms and negatively charged O- ions (with attributed optical absorption bands peaked at energies of 6.5 eV and 5.6 eV, respectively). From detailed analysis of interrelated kinetics of four oxygen-related defects, we extracted their diffusion parameters (interstitials serve as mobile recombination partners) required for the future prediction of secondary defect-induced reactions and, eventually, material radiation tolerance.

Band engineering of borophene superlattice based on zigzag nanoribbons: A DFT study

2020 ◽  
Vol 34 (32) ◽  
pp. 2050359
Author(s):  
Yi Zhang ◽  
Weiwei Ju ◽  
Tongwei Li ◽  
Haisheng Li
Keyword(s):  
Density Functional Theory ◽  
Dft Calculations ◽  
Electronic Properties ◽  
Energy Band ◽  
Density Functional ◽  
Magnetic Moments ◽  
Dft Study ◽  
Functional Theory ◽  
Magnetic Systems ◽  
Band Engineering

By performing density functional theory (DFT) calculations, we demonstrate that periodically repeating heterostructures of zigzag borophene nanoribbons (BNR) of different widths can form stable borophene superlattice (BSL). The energy band structures of BSL can be modulated through modifying the width and length of the segments. A metal-semiconductor transition can be obtained when the length of each segment is lengthened, whereas, the magnetism of BSL is influenced by the width of the segments. In those magnetic systems, the magnetic moments are mainly localized on protruding B atoms located at the edge, while no magnetic moments occur in the center B atoms. The hydrogenated BNR and BSL are further investigated. The hydrogenation can modify the electronic properties of BNR and BSL as well as quench the magnetism. All hydrogenated BNR and BSL are non-magnetic. Our results indicate that great potential exists in these systems for borophene utilization in nanoelectronics and spintronics.

scholarly journals Element-specific contributions to improved magnetic heating of theranostic CoFe2O4 nanoparticles decorated with Pd

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
S. Fatemeh Shams ◽  
Detlef Schmitz ◽  
Alevtina Smekhova ◽  
Mohammad Reza Ghazanfari ◽  
Margret Giesen ◽  
...  
Keyword(s):  
Dft Calculations ◽  
Density Functional ◽  
Magnetic Circular Dichroism ◽  
Magnetic Moments ◽  
Remarkable Change ◽  
Spin Canting ◽  
Low Field ◽  
Magnetic Heating ◽  
Pd Clusters ◽  
X Ray Absorption

AbstractDecoration with Pd clusters increases the magnetic heating ability of cobalt ferrite (CFO) nanoparticles by a factor of two. The origin of this previous finding is unraveled by element-specific X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) combined with atomic multiplet simulations and density functional theory (DFT) calculations. While the comparison of XAS spectra with atomic multiplet simulations show that the inversion degree is not affected by Pd decoration and, thus, can be excluded as a reason for the improved heating performance, XMCD reveals two interrelated responsible sources: significantly larger Fe and Co magnetic moments verify an increased total magnetization which enhances the magnetic heating ability. This is accompanied by a remarkable change in the field-dependent magnetization particularly for Co ions which exhibit an increased low-field susceptibility and a reduced spin canting behavior in higher magnetic fields. Using DFT calculations, these findings are explained by reduced superexchange between ions on octahedral lattice sites via oxygen in close vicinity of Pd, which reinforces the dominating antiparallel superexchange interaction between ions on octahedral and tetrahedral lattice sites and thus reduces spin canting. The influence of the delocalized nature of Pd 4d electrons on the neighboring ions is discussed and the conclusions are illustrated with spin density isosurfaces of the involved ions. The presented results pave the way to design nanohybrids with tailored electronic structure and magnetic properties.

DFT Calculation of Rhodium-Doped Silver Bromide Crystals: Defect Structures, Photoelectric Property and EPR

2018 ◽  
Vol 74 (1) ◽  
pp. 59-69
Author(s):  
Li-Na Wu ◽  
Shao-Yi Wu ◽  
Li-Juan Zhang ◽  
Xu-Sheng Liu ◽  
Gao-Jun Zhang
Keyword(s):  
Density Functional ◽  
Near Infrared ◽  
Nearest Neighbor ◽  
Magnetic Moments ◽  
Density Functional Theory Calculations ◽  
Spectroscopic Properties ◽  
Mulliken Charge ◽  
Defect Structures ◽  
Absorption Bands ◽  
Paramagnetic Resonance

AbstractAgBr crystals containing impurity Rh2+ with and without a next nearest neighbor silver vacancy (VAg) in the [001] axis are theoretically studied. The defect structures, bandgap, Mulliken charge, density of states, ultraviolet-visible (UV-Vis) and electron paramagnetic resonance spectra are analysed for two distinct (i.e. tetragonally elongated and compressed) centres by using density functional theory calculations. The results demonstrate that the structural, photoelectric and spectroscopic properties of AgBr are significantly changed after Rh substitution. The covalency of Rh-Br bonds in the two centres is stronger than that of pure AgBr according to the larger magnitudes of the overlap population. Compared to pure AgBr, Rh doping also results in significantly enhanced red shifts of absorption bands in the UV-Vis region and some new d-d transitions in the visible and near-infrared regions. Meanwhile, the bandgap of Rh-doped AgBr is about 10 % narrower than that of pure AgBr. As a result, the visible light activity may be enhanced in AgBr:Rh systems. Rh dopants can induce the magnetic moments of about 1.73 μB for both centres. Therefore, transition-metal Rh dopants may effectively modulate the electronic, optical and magnetic properties of AgBr.

scholarly journals Polarization properties and crystal structures of ferroelectric (Ba,Ca)TiO3 single crystals

2014 ◽  
Vol 04 (01) ◽  
pp. 1450003 ◽  
Author(s):  
Ryota Imura ◽  
Yuuki Kitanaka ◽  
Takeshi Oguchi ◽  
Yuji Noguchi ◽  
Masaru Miyayama
Keyword(s):  
Density Functional Theory ◽  
Dft Calculations ◽  
Single Crystals ◽  
Crystal Structures ◽  
Spontaneous Polarization ◽  
Density Functional ◽  
Solution Growth ◽  
Functional Theory ◽  
Polarization Measurements ◽  
Top Seeded Solution Growth

We have investigated the spontaneous polarization (P s ) of Ba 1-x Ca x TiO 3 (BCT) by polarization hysteresis measurements using single crystals and by density functional theory (DFT) calculations. Single crystals of BCT (x = 0.07) were grown by a top-seeded solution growth (TSSG) method. The polarization measurements show that the crystals (x = 0.07) have a P s of 26.0 μC/cm2, which is slightly small compared with BaTiO 3 (27.3 μC/cm2). Our DFT calculations based on a supercell approach show that Ca atoms are markedly displaced cooperatively with the adjacent Ti atoms along the P s direction. It is suggested that the CaTiO 3-like octahedral rotation is constructed in the BCT supercell around the Ca atoms, which is the origin of the smaller P s observed for the BCT crystals.

Computationally Assisted Design of High Signal-to-Noise Photoinduced Electron Transfer-Based Voltage-Sensitive Dyes

2020 ◽  
Author(s):  
Rishikesh Kulkarni ◽  
Anneliese Gest ◽  
Chun Kei Lam ◽  
Benjamin Raliski ◽  
Feroz James ◽  
...  
Keyword(s):  
Electron Transfer ◽  
Dft Calculations ◽  
Photoinduced Electron Transfer ◽  
Density Functional ◽  
Embryonic Stem ◽  
High Sensitivity ◽  
Md Simulations ◽  
High Signal ◽  
Signal To Noise ◽  
Green Fluorescent

<p>High signal-to-noise optical voltage indicators will enable simultaneous interrogation of membrane potential in large ensembles of neurons. However, design principles for voltage sensors with high sensitivity and brightness remain elusive, limiting the applicability of voltage imaging. In this paper, we use molecular dynamics (MD) simulations and density functional theory (DFT) calculations to guide the design of a bright and sensitive green-fluorescent voltage-sensitive fluorophore, or VoltageFluor (VF dye), that uses photoinduced electron transfer (PeT) as a voltage-sensing mechanism. MD simulations predict an 11% increase in sensitivity due to membrane orientation, while DFT calculations predict an increase in fluorescence quantum yield, but a decrease in sensitivity due to a decrease in rate of PeT. We confirm these predictions by synthesizing a new VF dye and demonstrating that it displays the expected improvements by doubling the brightness and retaining similar sensitivity to prior VF dyes. Combining theoretical predictions and experimental validation has resulted in the synthesis of the highest signal-to-noise green VF dye to date. We use this new voltage indicator to monitor the electrophysiological maturation of human embryonic stem cell-derived medium spiny neurons. </p>

Magnetic Volume Effect of Hydrogen Diffusion in Palladium

2020 ◽  
Vol 310 ◽  
pp. 29-33
Author(s):  
Sarantuya Nasantogtokh ◽  
Xin Cui ◽  
Zhi Ping Wang
Keyword(s):  
Transition Metal ◽  
Density Of States ◽  
Density Functional ◽  
Hydrogen Diffusion ◽  
Magnetic Moments ◽  
Interstitial Site ◽  
Volume Effect ◽  
Face Centered Cubic ◽  
Hydrogen Atoms ◽  
Octahedral Interstitial Site

The electronic and magnetic properties of palladium hydrogen are investigated using first-principles spin-polarized density functional theory. By studying the magnetic moments and electronic structures of hydrogen atoms diffusing in face-centered cubic structure of transition metal Pd, we found that the results of magnetic moments are exactly the same in the two direct octahedral interstitial site-octahedral interstitial site diffusion paths-i.e. the magnetic moments are the largest in the octahedral interstitial site, and the magnetic moments are the lowest in saddle point positions. We also studied on the density of states of some special points, with the result that the density of states near the Fermi level is mainly contributed by 4d electrons of Pd and the change of magnetic moments with the cell volume in the unit cell of transition metal Pd with a hydrogen atom.

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