87 research outputs found
The 5f localization/delocalization in square and hexagonal americium monolayers: A FP-LAPW electronic structure study
The electronic and geometrical properties of bulk americium and square and
hexagonal americium monolayers have been studied with the full-potential
linearized augmented plane wave (FP-LAPW) method. The effects of several common
approximations are examined: (1) non-spin polarization (NSP) vs. spin
polarization (SP); (2) scalar-relativity (no spin-orbit coupling (NSO)) vs.
full-relativity (i.e., with spin-orbit (SO) coupling included); (3)
local-density approximation (LDA) vs. generalized-gradient approximation (GGA).
Our results indicate that both spin polarization and spin orbit coupling play
important roles in determining the geometrical and electronic properties of
americium bulk and monolayers. A compression of both americium square and
hexagonal monolayers compared to the americium bulk is also observed. In
general, the LDA is found to underestimate the equilibrium lattice constant and
give a larger total energy compared to the GGA calculations. While spin orbit
coupling shows a similar effect on both square and hexagonal monolayer
calculations regardless of the model, GGA versus LDA, an unusual spin
polarization effect on both square and hexagonal monolayers is found in the LDA
results as compared with the GGA results. The 5f delocalization transition of
americium is employed to explain our observed unusual spin polarization effect.
In addition, our results at the LDA level of theory indicate a possible 5f
delocalization could happen in the americium surface within the same Am II (fcc
crystal structure) phase, unlike the usually reported americium 5f
delocalization which is associated with crystal structure change. The
similarities and dissimilarities between the properties of an Am monolayer and
a Pu monolayer are discussed in detail.Comment: 22 pages, 8 figure
A Density Functional Study of Atomic Hydrogen and Oxygen Chemisorption on the Relaxed (0001) Surface of Double Hexagonal Close Packed Americium
Ab initio total energy calculations within the framework of density
functional theory have been performed for atomic hydrogen and oxygen
chemisorption on the (0001) surface of double hexagonal packed americium using
a full-potential all-electron linearized augmented plane wave plus local
orbitals method. Chemisorption energies were optimized with respect to the
distance of the adatom from the relaxed surface for three adsorption sites,
namely top, bridge, and hollow hcp sites, the adlayer structure corresponding
to coverage of a 0.25 monolayer in all cases. Chemisorption energies were
computed at the scalar-relativistic level (no spin-orbit coupling NSOC) and at
the fully relativistic level (with spin-orbit coupling SOC). The two-fold
bridge adsorption site was found to be the most stable site for O at both the
NSOC and SOC theoretical levels with chemisorption energies of 8.204 eV and
8.368 eV respectively, while the three-fold hollow hcp adsorption site was
found to be the most stable site for H with chemisorption energies of 3.136 eV
at the NSOC level and 3.217 eV at the SOC level. The respective distances of
the H and O adatoms from the surface were found to be 1.196 Ang. and 1.164 Ang.
Overall our calculations indicate that chemisorption energies in cases with SOC
are slightly more stable than the cases with NSOC in the 0.049-0.238 eV range.
The work functions and net magnetic moments respectively increased and
decreased in all cases compared with the corresponding quantities of bare dhcp
Am (0001) surface. The partial charges inside the muffin-tins, difference
charge density distributions, and the local density of states have been used to
analyze the Am-adatom bond interactions in detail. The implications of
chemisorption on Am 5f electron localization-delocalization are also discussed.Comment: 9 Tables, 5 figure
Lattice anisotropy in uranium ternary compounds: UTX
Several U-based intermetallic compounds (UCoGe, UNiGe with the TiNiSi structure type and UNiAl with the ZrNiAl structure type) and their hydrides were studied from the point of view of compressibility and thermal expansion. Confronted with existing data for the compounds with the ZrNiAl structure type a common pattern emerges. The direction of the U-U bonds with participation of the 5f states is distinctly the "soft" crystallographic direction, exhibiting also the highest coefficient of linear thermal expansion. The finding leads to an apparent paradox: the closer the U atoms are together in a particular direction the better they can be additionally compressed together by applied hydrostatic pressure. (C) 2012 Elsevier B. V. All rights reserved
Probing the 5f Electrons in Am-I by Hybrid Density Functional Theory
The ground states of the actinides and their compounds continue to be matters
of considerable controversies. Experimentally, Americium-I (Am-I) is a
non-magnetic dhcp metal whereas theoretically an anti-ferromagnetic ground
state is predicted. We show that hybrid density functional theory, which
admixes a fraction of exact Hartree-Fock (HF) exchange with approximate DFT
exchange, can correctly reproduce the ground state properties of Am. In
particular, for a 0.40 fraction of HF exchange we obtain a non-magnetic ground
state with equilibrium atomic volume, bulk modulus, 5f electron population, and
the density of electronic states all in good agreement with experimental data.
We argue that the exact HF exchange corrects the overestimation of the
approximate DFT exchange interaction.Comment: 1 table, 4 figures. Chemical Physics Letters, in press (2009
Adsorption and dissociation of molecular oxygen on the (0001) surface of double hexagonal close packed americium
In our continuing attempts to understand theoretically various surface
properties such as corrosion and potential catalytic activity of actinide
surfaces in the presence of environmental gases, we report here the first ab
initio study of molecular adsorption on the double hexagonal packed (dhcp)
americium (0001) surface. Dissociative adsorption is found to be energetically
more favorable compared to molecular adsorption. The most stable configuration
corresponds to a horizontal approach molecular dissociation with the oxygen
atoms occupying neighboring h3 sites, with chemisorption energies at the NSOC
and SOC theoretical levels being 9.395 eV and 9.886 eV, respectively. The
corresponding distances of the oxygen molecule from the surface and
oxygen-oxygen distance were found to be 0.953 Ang. and 3.731 Ang.,
respectively. Overall our calculations indicate that chemisorption energies in
cases with SOC are slightly more stable than the cases with NSOC in the
0.089-0.493 eV range. The work functions and net magnetic moments respectively
increased and decreased in all cases compared with the corresponding quantities
of the bare dhcp Am (0001) surface. The adsorbate-substrate interactions have
been analyzed in detail using the partial charges inside the muffin-tin
spheres, difference charge density distributions, and the local density of
states. The effects, if any, of chemisorption on the Am 5f electron
localization-delocalization characteristics in the vicinity of the Fermi level
are also discussed.Comment: 6 tables, 10 figure
Microscopic theory of quadrupolar ordering in TmTe
We have calculated the crystal electric field of TmTe (T>T_Q) and have
obtained that the ground state of a Tm 4f hole is the doublet in
agreement with Mossbauer experiments. We study the quadrupole interactions
arising from quantum transitions of 4f holes of Tm. An effective attraction is
found at the L point of the Brillouin zone, . Assuming that the
quadrupolar condensation involves a single arm of we show that
there are two variants for quadrupole ordering which are described by the space
groups C2/c and C2/m. The Landau free energy is derived in mean-field theory.
The phase transition is of second order. The corresponding quadrupole order
parameters are combinations of and components. The obtained
domain structure is in agreement with observations from neutron diffraction
studies for TmTe. Calculated lattice distortions are found to be different for
the two variants of quadrupole ordering. We suggest to measure lattice
displacements in order to discriminate between those two structures.Comment: 10 pages, 2 figures, 5 tables; accepted by PR
On the Convergence of the Electronic Structure Properties of the FCC Americium (001) Surface
Electronic and magnetic properties of the fcc Americium (001) surface have
been investigated via full-potential all-electron density-functional electronic
structure calculations at both scalar and fully relativistic levels. Effects of
various theoretical approximations on the fcc Am (001) surface properties have
been thoroughly examined. The ground state of fcc Am (001) surface is found to
be anti-ferromagnetic with spin-orbit coupling included (AFM-SO). At the ground
state, the magnetic moment of fcc Am (001) surface is predicted to be zero. Our
current study predicts the semi-infinite surface energy and the work function
for fcc Am (001) surface at the ground state to be approximately 0.82 J/m2 and
2.93 eV respectively. In addition, the quantum size effects of surface energy
and work function on the fcc Am (001) surface have been examined up to 7 layers
at various theoretical levels. Results indicate that a three layer film surface
model may be sufficient for future atomic and molecular adsorption studies on
the fcc Am (001) surface, if the primary quantity of interest is the
chemisorption energy.Comment: 34 pages, 9 figure
Mixing theory for culture and harvest in bioreactors of human mesenchymal stem cells on microcarriers
The use of human mesenchymal stem cells (hMSCs) in regenerative medicine is a potential major advance for the treatment of many medical conditions, especially with the use of allogeneic therapies where the cells from a single donor can be used to treat ailments in many patients. Such cells must be grown attached to surfaces and for large scale production, it is shown that stirred bioreactors containing ~200 μm particles (microcarriers) can provide such a surface. It is also shown that the just suspended condition, agitator speed NJS, provides a satisfactory condition for cell growth by minimizing the specific energy dissipation rate, εT, in the bioreactor whilst still meeting the oxygen demand of the cells. For the cells to be used for therapeutic purposes, they must be detached from the microcarriers before being cryopreserved. A strategy based on a short period (~7 min) of very high εT, based on theories of secondary nucleation, is effective at removing >99% cells. Once removed, the cells are smaller than the Kolmogorov scale of turbulence and hence not damaged. This approach is shown to be successful for culture and detachment in 4 types of stirred bioreactors from 15 mL to 5 L
Pharmacokinetic modeling of R and S-Methadone and their metabolites to study the effects of various covariates in post-operative children
Methadone is a synthetic opioid used as an analgesic and for the treatment of opioid abuse disorder. The analgesic dose in the pediatric population is not well-defined. The pharmacokinetics (PKs) of methadone is highly variable due to the variability in alpha-1 acid glycoprotein (AAG) and genotypic differences in drug-metabolizing enzymes. Additionally, the R and S enantiomers of methadone have unique PK and pharmacodynamic properties. This study aims to describe the PKs of R and S methadone and its metabolite 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) in pediatric surgical patients and to identify sources of inter- and intra-individual variability. Children aged 8-17.9 years undergoing orthopedic surgeries received intravenous methadone 0.1 mg/kg intra-operatively followed by oral methadone 0.1 mg/kg postoperatively every 12 h. Pharmacokinetics of R and S methadone and EDDP were determined using liquid chromatography tandem mass spectrometry assays and the data were modeled using nonlinear mixed-effects modeling in NONMEM. R and S methadone PKs were well-described by two-compartment disposition models with first-order absorption and elimination. EDDP metabolites were described by one compartment disposition models with first order elimination. Clearance of both R and S methadone were allometrically scaled by bodyweight. CYP2B6 phenotype was a determinant of the clearance of both the enantiomers in an additive gene model. The intronic CYP3A4 single-nucleotide polymorphism (SNP) rs2246709 was associated with decreased clearance of R and S methadone. Concentrations of AAG and the SNP of AAG rs17650 independently increased the volume of distribution of both the enantiomers. The knowledge of these important covariates will aid in the optimal dosing of methadone in children
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