352 research outputs found
Calculation of The Band Gap Energy and Study of Cross Luminescence in Alkaline-Earth Dihalide Crystals
The band gap energy as well as the possibility of cross luminescence
processes in alkaline-earth dihalide crystals have been calculated using the ab
initio Perturbed-Ion (PI) model. The gap is calculated in several ways: as a
difference between one-electron energy eigenvalues and as a difference between
total energies of appropriate electronic states of the crystal, both at the HF
level and with inclusion of Coulomb correlation effects. In order to study the
possibility of ocurrence of cross luminescence in these materials, the energy
difference between the valence band and the upmost core band for some
representative crystals has been calculated. Both calculated band gap energies
and cross luminescence predictions compare very well with the available
experimental results.Comment: LaTeX file containing 8 pages plus 1 postscript figure. Final version
accepted for publication in The Journal of the Physical Society of Japan. It
contains a more complete list of references, as well as a more detailed
comparison with previous theoretical investigations on the subjec
Weak ferromagnetism with very large canting in a chiral lattice: (pyrimidine)2FeCl2
The transition metal coordination compound (pyrimidine)2FeCl2 crystallizes in
a chiral lattice, space group I 4_1 2 2 (or I4_3 2 2). Combined magnetization,
Mossbauer spectroscopy and powder neutron diffraction studies reveal that it is
a canted antiferromagnet below T_N = 6.4 K with an unusually large canting of
the magnetic moments of 14 deg. from their general antiferromagnetic alignment,
one of the largest reported to date. This results in weak ferromagnetism with a
ferromagnetic component of 1 mu_B. The large canting is due to the interplay
between the antiferromagnetic exchange interaction and the local single-ion
anisotropy in the chiral lattice. The magnetically ordered structure of
(pyrimidine)2FeCl2, however, is not chiral. The implications of these findings
for the search of molecule based materials exhibiting chiral magnetic ordering
is discussed.Comment: 6 pages, 5 figure
Correlation effects in ionic crystals: I. The cohesive energy of MgO
High-level quantum-chemical calculations, using the coupled-cluster approach
and extended one-particle basis sets, have been performed for (Mg2+)n (O2-)m
clusters embedded in a Madelung potential. The results of these calculations
are used for setting up an incremental expansion for the correlation energy of
bulk MgO. This way, 96% of the experimental cohesive energy of the MgO crystal
is recovered. It is shown that only 60% of the correlation contribution to the
cohesive energy is of intra-ionic origin, the remaining part being caused by
van der Waals-like inter-ionic excitations.Comment: LaTeX, 20 pages, no figure
Correlation effects in MgO and CaO: Cohesive energies and lattice constants
A recently proposed computational scheme based on local increments has been
applied to the calculation of correlation contributions to the cohesive energy
of the CaO crystal. Using ab-initio quantum chemical methods for evaluating
individual increments, we obtain 80% of the difference between the experimental
and Hartree-Fock cohesive energies. Lattice constants corrected for correlation
effects deviate by less than 1% from experimental values, in the case of MgO
and CaO.Comment: LaTeX, 4 figure
Zinc and silica are active components to efficiently treat in vitro simulated eroded dentin.
Objectives: Biomaterials for treating dentin hypersensitivity and dentin wear were
evaluated, to efficiently occlude the dentinal tubules and to increase dentin resistance to
abrasion. Materials and Methods: 24 dentin surfaces were treated with EDTA to expose
dentinal tubules, and were: 1) non-brushed, 2) brushed with distilled water, or with pastes
containing 3) Monetite, 4) Brushite, 5) Zn-Monetite, 6) Zn-Brushite, 7) Silica-Brushite
and 8) NovaMin®. Topography, nanomechanical and chemical analysis were assessed on
dentin surfaces (n=3) after artificial saliva immersion for 24 h, and after citric acid
challenge. 21 further dentin specimens were created to evaluate dentin permeability after
brushing, saliva storage and acid application (n=3). ANOVA, Student-Newman-Keuls
(p<0.05) and Student t-test (p<0.001) were used. Results: Particles containing major
proportion of silica attained intratubular occlusion by carbonate crystals (Raman
carbonate peak heights: 15.17 and 19.24 au; complex modulus: 110 and 140 GPa, at
intratubular dentin). When brushing with pastes containing higher proportion of silica or
zinc, phosphate calcium compounds were encountered into tubules and over dentin
surfaces (Raman intratubular phosphate peak heights: 49 to 70 au, and at the intertubular
dentin: 78 to 92). The formed carbonated apatite and calcium phosphate layer were
resistant to citric acid application. Zinc compounds drastically increased tubule occlusion,
decreased dentin permeability (up to 30%) and augmented mechanical properties at the
intertubular dentin (90-130 GPa), it was maintained after acid challenging. Conclusions:
Zinc-containing pastes occluded dentinal tubules and improved dentin mechanical
properties. Clinical Relevance: Using zinc as an active component to treat eroded dentin
is encouraged.Projects RTC-2014-1731-1 and MAT2014-52036-P
supported by the Ministry of Economy and Competitiveness and European Regional
Development Fund
A Small-Molecule Inhibitor of T. gondii Motility Induces the Posttranslational Modification of Myosin Light Chain-1 and Inhibits Myosin Motor Activity
Toxoplasma gondii is an obligate intracellular parasite that enters cells by a process of active penetration. Host cell penetration and parasite motility are driven by a myosin motor complex consisting of four known proteins: TgMyoA, an unconventional Class XIV myosin; TgMLC1, a myosin light chain; and two membrane-associated proteins, TgGAP45 and TgGAP50. Little is known about how the activity of the myosin motor complex is regulated. Here, we show that treatment of parasites with a recently identified small-molecule inhibitor of invasion and motility results in a rapid and irreversible change in the electrophoretic mobility of TgMLC1. While the precise nature of the TgMLC1 modification has not yet been established, it was mapped to the peptide Val46-Arg59. To determine if the TgMLC1 modification is responsible for the motility defect observed in parasites after compound treatment, the activity of myosin motor complexes from control and compound-treated parasites was compared in an in vitro motility assay. TgMyoA motor complexes containing the modified TgMLC1 showed significantly decreased motor activity compared to control complexes. This change in motor activity likely accounts for the motility defects seen in the parasites after compound treatment and provides the first evidence, in any species, that the mechanical activity of Class XIV myosins can be modulated by posttranslational modifications to their associated light chains
Polymorphism and magnetic properties of Li2MSiO4 (M 5 Fe, Mn) cathode materials
Transition metal-based lithium orthosilicates (Li2MSiO4,M=Fe, Ni, Co, Mn) are gaining a wide interest as
cathode materials for lithium-ion batteries. These materials present a very complex polymorphism that
could affect their physical properties. In this work, we synthesized the Li2FeSiO4 and Li2MnSiO4 compounds
by a sol-gel method at different temperatures. The samples were investigated by XRPD, TEM, 7Li MAS
NMR, and magnetization measurements, in order to characterize the relationships between crystal structure
and magnetic properties. High-quality 7Li MAS NMR spectra were used to determine the silicate structure,
which can otherwise be hard to study due to possible mixtures of different polymorphs. The magnetization
study revealed that the Neel temperature does not depend on the polymorph structure for both iron and
manganese lithium orthosilicates
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