Psi Quantum Materials LLC

Laiwu, China

Psi Quantum Materials LLC

Laiwu, China
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Wang V.,Xi'an University of Technology | Kawazoe Y.,Tohoku University | Kawazoe Y.,RAS Kutateladze Institute of Thermophysics | Geng W.T.,University of Science and Technology Beijing | Geng W.T.,Psi Quantum Materials LLC
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

Using hybrid density functional theory combined with a semiempirical van der Waals dispersion correction, we have investigated the structural and electronic properties of vacancies and self-interstitials in defective few-layer phosphorene. We find that both a vacancy and a self-interstitial defect are more stable in the outer layer than in the inner layer. The formation energy and transition energy of both a vacancy and a self-interstitial P defect decrease with increasing film thickness, mainly due to the upward shift of the host valence band maximum in reference to the vacuum level. Consequently, both vacancies and self-interstitials could act as shallow acceptors, and this well explains the experimentally observed p-type conductivity in few-layer phosphorene. On the other hand, since these native point defects have moderate formation energies and are stable in negatively charged states, they could also serve as electron compensating centers in n-type few-layer phosphorene. © 2015 American Physical Society


Niu J.G.,University of Science and Technology Beijing | Niu J.G.,Hebei University | Ping D.H.,Japan National Institute of Materials Science | Ohno T.,Japan National Institute of Materials Science | And 3 more authors.
Modelling and Simulation in Materials Science and Engineering | Year: 2014

Oxygen has long been known to retard the formation of the hexagonal ω phase in body-centered-cubic β-Ti based alloys, but a quantum mechanical evaluation of the thermodynamics associated with the β to ω transformation remains unexplored. Our first-principles density functional theory calculations on a model Ti3Nb alloy containing 2 at% O reveal that O prefers an octahedral interstitial in both the β and ω phases and increases the ω energy relative to β (0.19 eV/O), thus making the ω phase thermodynamically less favorable. Interestingly, we find that O atoms in β can be categorized into two sets which have a strikingly distinct effect. The O atoms in collapsing {1 1 1} planes increase the energy barrier (29 meV/atom) in the β to ω transformation remarkably; whereas the O atoms in the uncollapsing {1 1 1} planes exert a much smaller retarding effect (5 meV/atom). This is because the former set of O atoms has to pass through high-lying intermediate tetragonal sites. The suppression effect of O can be understood with electronic structure analyses. © 2014 IOP Publishing Ltd.


Niu J.G.,University of Science and Technology Beijing | Niu J.G.,Hebei University | Zhan Q.,University of Science and Technology Beijing | Geng W.T.,University of Science and Technology Beijing | Geng W.T.,Psi Quantum Materials LLC
AIP Advances | Year: 2014

Despite well documented first-principles theoretical determination of the low migration energy (0.06 eV) of a single He in tungsten, fully quantum mechanical calculations on the migration of a He pair still present a challenge due to the complexity of its trajectory. By identifying the six most stable configurations of the He pair in W and decomposing its motion into rotational, translational, and rotational-translational routines, we are able to determine its migration barrier and trajectory. Our density functional theory calculations demonstrate a He pair has three modes of motion: a close or open circular two-dimensional motion in (100) plane with an energy barrier of 0.30 eV, a snaking motion along [001] direction with a barrier of 0.30 eV, and a twisted-ladder motion along [010] direction with the two He swinging in the plane (100) and a barrier of 0.31 eV. The graceful associative movements of a He pair are related to the chemical-bonding-like He-He interaction being much stronger than its migration barrier in W. The excellent agreement with available experimental measurements (0.24-0.32 eV) on He migration makes our first-principles result a solid input to obtain accurate He-W interatomic potentials in molecular dynamics simulations. © 2014 Author(s).


Niu J.G.,University of Science and Technology Beijing | Niu J.G.,Hebei University | Geng W.T.,University of Science and Technology Beijing | Geng W.T.,Psi Quantum Materials LLC
Acta Materialia | Year: 2014

We report a first-principles density functional theory study on the interaction of interstitial O atoms in body-centered cubic β-Ti3Nb alloy, the lattice distortion induced by O and its effect on the β to α″ transformation. Our calculations demonstrate strong repulsion between O atoms in general, but also reveal a weak attraction at an O-O separation of around 5.63 Å (3a), which corresponds to a Ti6Nb2O structure. At low concentration (Ti96Nb32O), O induces a local α″-like structure in β-Ti3Nb lattice, albeit to the extent of the nearest- and next-nearest neighbors, and exerts either a remarkable (6-8 meV atom-1) or a negligible (0-1 meV atom-1) energy barrier to the β to α″ transformation, dependent on the orientation of its elastic dipole with respect to the external stress. At high concentration (Ti6Nb2O), interstitial O atoms give rise to the shuffling of neighboring {1 1 0} planes along the 〈11¯0〉 direction and generate a semi-α″ structure, which is believed to be the nanodomain structure observed in experiment by Miyazaki and co-workers. In both cases, O brings up suppression effect on the β to α″ transformation. © 2014 Elsevier Ltd. All rights reserved.


He B.L.,University of Science and Technology Beijing | Ping D.H.,Japan National Institute of Materials Science | Geng W.T.,Japan National Institute of Materials Science | Geng W.T.,Psi Quantum Materials LLC
Journal of Nuclear Materials | Year: 2014

We report a first-principles density functional theory study of helium distribution in cementite Fe3C. The solution energy of interstitial He is similar to that in bcc Fe; by contrast, the substitutional He (replacing Fe) is remarkably (0.74 eV) more stable than in the latter, due to the easiness of Fe vacancy formation in Fe3C. Therefore, He is predicted to be significantly more soluble in cementite than in Fe matrix. We find the binding potencies of both a substitutional-interstitial He pair (0.21 eV) and a substitutional-substitutional He pair (0.22 eV) are noticeably weaker in cementite than in bcc Fe, indicating a less powerful self-trapping. As a consequence, small size cementite in ferritic steels might serve as scattered trapping centers for He, mitigate helium bubble growth, and make the steel more swelling resistant while under neutron irradiation, just as dispersed oxide particles do. © 2013 Published by Elsevier B.V.

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