135 research outputs found
Experimental and first-principles studies of magnetism and magnetoelectric effect in Co4Nb2O9 and Co4Ta2O9
We report results of joint experimental and theoretical studies on
magnetoelectric (ME) compounds Co4Nb2O9 and Co4Ta2O9. On the experimental side,
we present results of the magnetization and dielectric permittivity
measurements in the magnetic field. On the theoretical side, we construct the
low-energy Hubbard-type model for the magnetically active Co 3d bands in the
Wannier basis, using the input of first-principles electronic structure
calculations, solve this model in the mean-field Hartree-Fock approximation,
and evaluate the electric polarization in terms of the Berry phase theory. Both
experimental and theoretical results suggest that Co4Ta2O9 is magnetically
softer than Co4Nb2O9. Therefore, it is reasonable to expect that the
antiferromagnetic structure of Co4Ta2O9 can be easier deformed by the external
magnetic field, yielding larger polarization. This trend is indeed reproduced
by our theoretical calculations, but does not seem to be consistent with the
experimental behavior of the polarization and dielectric permittivity. Thus, we
suggest that there should be a hidden mechanism controlling the ME coupling in
these compounds, probably related to the magnetic striction or a spontaneous
change of the magnetic structure, which breaks the inversion symmetry.
Furthermore, we argue that unlike in other ME systems (e.g. Cr2O3), in Co4Nb2O9
and Co4Ta2O9 there are two crystallographic sublattices, which contribute to
the ME effect. These contributions are found to be of the opposite sign and
tend to compensate each other. The latter mechanism can be also used to control
and reverse the electric polarization in these compounds.Comment: 22 pages, 15 figure
Doping and temperature-dependent optical properties of oxygen-reduced BaTiO3-d
We report on optical properties of reduced BaTiO3-d at different doping
levels including insulating and metallic samples. In all the samples, including
metallic one, we observe structural phase transitions from the changes in the
infrared-active phonon modes. Metallic ground state is confirmed by the
Drude-type lowfrequency optical reflectance. Similar to SrTiO3-d we find that
the midinfrared-absorption band in BaTiO3-d appears and grows with an increase
in the oxygen-vacancy concentration. Upon decrease in temperature from 300 K,
the midinfrared band shifts slightly to higher frequency and evolves into two
bands: the existing band and a new and smaller band at lower frequency. The
appearance of the new and smaller band seems to be correlated with the
structural phase transitionsComment: 8 pages, 7 figure
Spin resonance in EuTiO3 probed by time-domain GHz ellipsometry
We show an example of a purely magnetic spin resonance in EuTiO3 and the
resulting new record high Faraday rotation of 590 deg/mm at 1.6 T for 1 cm
wavelengths probed by a novel technique of magneto-optical GHz time-domain
ellipsometry. From our transmission measurements of linear polarized light we
map out the complex index of refraction in the GHz to THz range. We observe a
strong resonant absorption by magnetic dipole transitions involving the Zeeman
split S=7/2 magnetic energy levels of the Eu 2+ ions, which causes a very large
dichroism for circular polarized radiation.Comment: 4 pages, 4 figure
Spin singlet small bipolarons in Nb-doped BaTiO3
The magnetic susceptibility and electrical resistivity of n-type
BaTi{1-x}Nb{x}O3 have been measured over a wide temperature range. It is found
that, for 0 < x < 0.2, dopant electrons form immobile spin singlet small
bipolarons with binding energy around 110 meV. For x = 0.2, a maximum in the
electrical resistivity around 15 K indicates a crossover from band to hopping
transport of the charge carriers, a phenomenon expected but rarely observed in
real polaronic systems.Comment: 5 pages, 4 figure
Hybrid suspension/solution precursor plasma spraying of a complex Ba(Mg1/3Ta2/3)O3 perovskite: Effects of processing parameters and precursor chemistry on phase formation and decomposition
Abstract: Ba(Mg1/3Ta2/3)O3 (BMT) has a high melting point and is envisioned as a thermal barrier coating material. In this study, a hybrid suspension/solution precursor plasma spray process with a radio frequency thermal plasma torch is designed to deposit BMT nanostructured coatings. Six combinations of chemical reagents are investigated as coating precursors: one BMT powder suspension and five Ta2O5 suspensions in nitrate- or acetate-based solutions. X-ray photoelectron spectroscopy is used to evaluate the element evaporation during plasma spraying, while a thermogravimetric/differential thermal analysis is applied to investigate the BMT formation. Parameters such as precursor chemistry, plasma power, spraying distance and substrate preheating are studied with regard to the coating phase structure. Twice the Mg stoichiometric amount with a power of 50 kW shows the best results when using nanocrystallized Ta2O5 as a tantalum precursor. When choosing nitrates as Ba and Mg precursors, crystallized BMT is obtained at lower plasma power (45 kW) when compared to acetates (50 kW). BaTa2O6, Ba3Ta5O15, Ba4Ta2O9, Mg4Ta2O9 are the main secondary phases observed during the BMT coatings deposition. Because of the complicated acetate decomposition process, the coating deposition rate from nitrate precursors is 1.56 times higher than that from acetate precursors
The total synthesis of (-)-cyanthiwigin F by means of double catalytic enantioselective alkylation
Double catalytic enantioselective transformations are powerful synthetic methods that can facilitate the construction of stereochemically complex molecules in a single operation. In addition to generating two or more stereocentres in a single reaction, multiple asymmetric reactions also impart increased enantiomeric excess to the final product in comparison with the analogous single transformation. Furthermore, multiple asymmetric operations have the potential to independently construct several stereocentres at remote points within the same molecular scaffold, rather than relying on pre-existing chiral centres that are proximal to the reactive site. Despite the inherent benefits of multiple catalytic enantioselective reactions, their application to natural product total synthesis remains largely underutilized. Here we report the use of a double stereoablative enantioselective alkylation reaction in a concise synthesis of the marine diterpenoid (-)-cyanthiwigin F (ref. 8). By employing a technique for independent, selective formation of two stereocentres in a single stereoconvergent operation, we demonstrate that a complicated mixture of racemic and meso diastereomers may be smoothly converted to a synthetically useful intermediate with exceptional enantiomeric excess. The stereochemical information generated by means of this catalytic transformation facilitates the easy and rapid completion of the total synthesis of this marine natural product
Biocatalytic production of bicyclic β-lactams with three contiguous chiral centres using engineered crotonases
YesThere is a need to develop asymmetric routes to functionalised β-lactams, which remain the
most important group of antibacterials. Here we describe biocatalytic and protein engineering
studies concerning carbapenem biosynthesis enzymes, aiming to enable stereoselective
production of functionalised carbapenams with three contiguous chiral centres. Structurallyguided substitutions of wildtype carboxymethylproline synthases enable tuning of their C-N
and C-C bond forming capacity to produce 5-carboxymethylproline derivatives substituted at
C-4 and C-6, from amino acid aldehyde and malonyl-CoA derivatives. Use of tandem enzyme
incubations comprising an engineered carboxymethylproline synthase and an alkylmalonylCoA forming enzyme (i.e. malonyl-CoA synthetase or crotonyl-CoA carboxylase reductase)
can improve stereocontrol and expand the product range. Some of the prepared 4,6-disubstituted-5-carboxymethylproline derivatives are converted to bicyclic β-lactams by carbapenam synthetase catalysis. The results illustrate the utility of tandem enzyme systems
involving engineered crotonases for asymmetric bicyclic β-lactam synthesis
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