13,997 research outputs found
Theoretical determination of the Raman spectra of MgSiO3 perovskite and post-perovskite at high pressure
We use the density functional perturbation theory to determine for the first
time the pressure evolution of the Raman intensities for a mineral, the two
high-pressure structures of MgSiO3 perovskite and post-perovskite. At high
pressures, the Raman powder spectra reveals three main peaks for the perovskite
structure and one main peak for the post-perovskite structure. Due to the large
differences in the spectra of the two phases Raman spectroscopy can be used as
a good experimental indication of the phase transition.Comment: 16 pages, submitted to Geophysical Research Letter
A Multiferroic Ceramic with Perovskite Structure: La0.5Bi0.5Mn0.5Fe0.5O3.09
ABO3 perovskite multiferroic La0.5Bi0.5Mn0.5Fe0.5O3.09 where the B-site
cations is responsible for the magnetic properties and the A-site cation with
lone pair electron is responsible for the ferroelectric properties was
synthesized at normal conditions. This oxide exhibits a ferromagnetic
transition around 240 K with a well defined hysteresis loop, and a significant
reversible remnant polarization below 67K similar to ferroelectric behavior.
The magnetic interaction is interpreted by the ferromagnetic Fe3+-O-Mn3+ and
antiferromagnetic Fe3+(Mn3+)-O-Fe3+(Mn3+) interactions competed each other,
whereas the ferroelectricity is predominantly due to the polar nature
introduced by the 6s2 lone pair of Bi3+ cationsComment: Submitted to Applied Physics Letters, 7 pages, 3 figure
Non- Mn-driven ferroelectricity in antiferromagnetic BaMnO
Using first-principles density functional theory we predict a ferroelectric
ground state -- driven by off-centering of the magnetic Mn ion -- in
perovskite-structure BaMnO.
Our finding is surprising, since the competition between energy-lowering
covalent bond formation, and energy-raising
Coulombic repulsions usually only favors off-centering on the perovskite
-site for non-magnetic ions.
We explain this tendency for ferroelectric off-centering by analyzing the
changes in electronic structure between the centrosymmetric and polar states,
and by calculating the Born effective charges; we find anomalously large values
for Mn and O consistent with our calculated polarization of 12.8 C/cm.
Finally, we suggest possible routes by which the perovskite phase may be
stabilized over the usual hexagonal phase, to enable a practical realization of
a single-phase multiferroic.Comment: 6 pages, 3 figure
Evaluation of Thermal Testing and X-ray Diffraction of Ka0,5Na0,5NbO3
The structure of perovskite-based material contained in the niobate and titanate. The fabric was a perovskite crystal structure having the formula ABO3. Ferroelectric and piezoelectric materials had a perovskite structure. This material could store an electric charge, which was good because of the polarization resulting in a material that was a dielectric. Unleaded piezoelectric material, K0,5Na0,5NbO3 (KNN), was synthesized using reliable state methods. Synthesis was done by first setting up K2CO3, Na2CO3, and Nb2O5 as a base KNN system. Studies cover X-ray diffraction, thermal analysis TGA-DTA and lattice parameter analysis. From the TGA-DTA analysis obtained for KNN calcination temperature at 7000C for 2 hours can produce a single-phase ABO3 where A = (K, Na) and B = (Nb). Orthorhombic perovskite structure KNN material owned by P4mm space group with lattice parameters a = 3,572 Ã…; b = 3,570 Ã…; and c = 3,565 Ã…
Crystallography and Chemistry of Perovskites
Despite the simplicity of the original perovskite crystal structure, this
family of compounds shows an enormous variety of structural modifications and
variants. In the following, we will describe several examples of perovskites,
their structural variants and discuss the implications of distortions and
non-stoichiometry on their electronic and magnetic properties.Comment: 11 pages, 8 figures, further information http://www.peter-lemmens.d
Methane dry reforming over nickel perovsikite catalysts
In recent years dry reforming of methane (DRM) has received considerable attention as a promising alternative to steam reforming for synthesis gas (H2 and CO) production. This process could be industrially advantageous, yielding a syngas with a H2/CO ratio close to 1, suitable for Fischer-Tropsch synthesis to liquid hydrocarbons and for production of valuable oxygenated chemicals. The major drawback of the process is the endothermicity of the reaction that implies the use of a suitable catalyst to work at relatively low temperatures (923-1,023 K). Higher temperatures would make the process unaffordable for an industrial development and would increase the risk of undesirable side reactions, such as coke formation, that are the main causes of catalyst deactivation. In this work the activity of nickel perovskite catalysts were studied and the results were compared with rhodium perovskite. It is well known that rhodium is very active and stable for dry reforming but its high cost makes its utilization limited. The Ni, due to its low cost, is a promising substitute even if it is more susceptible to coking. The perovskite structure allows a high dispersion of the metal into the catalyst increasing the catalytic activity. In this work the Ni perovskite was obtained with two methods (auto-combustion and modified citrate methods). The results pointed out that the Ni perovskite obtained with the auto-combustion method is a promising route for the use of Ni in this process. The experimental tests show that with Ni catalyst very good activity can be achieved from temperature of 973 K
Designing Optimal Perovskite Structure for High Ionic Conduction.
Solid-oxide fuel/electrolyzer cells are limited by a dearth of electrolyte materials with low ohmic loss and an incomplete understanding of the structure-property relationships that would enable the rational design of better materials. Here, using epitaxial thin-film growth, synchrotron radiation, impedance spectroscopy, and density-functional theory, the impact of structural parameters (i.e., unit-cell volume and octahedral rotations) on ionic conductivity is delineated in La0.9 Sr0.1 Ga0.95 Mg0.05 O3- δ . As compared to the zero-strain state, compressive strain reduces the unit-cell volume while maintaining large octahedral rotations, resulting in a strong reduction of ionic conductivity, while tensile strain increases the unit-cell volume while quenching octahedral rotations, resulting in a negligible effect on the ionic conductivity. Calculations reveal that larger unit-cell volumes and octahedral rotations decrease migration barriers and create low-energy migration pathways, respectively. The desired combination of large unit-cell volume and octahedral rotations is normally contraindicated, but through the creation of superlattice structures both expanded unit-cell volume and large octahedral rotations are experimentally realized, which result in an enhancement of the ionic conductivity. All told, the potential to tune ionic conductivity with structure alone by a factor of ≈2.5 at around 600 °C is observed, which sheds new light on the rational design of ion-conducting perovskite electrolytes
Development by Mechanochemistry of La0.8Sr0.2Ga0.8Mg0.2O2.8 Electrolyte for SOFCs
In this work, a mechanochemical process using high-energy milling conditions was employed to synthesize La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) powders from the corresponding stoichiometric amounts of La2O3, SrO, Ga2O3, and MgO in a short time. After 60 min of milling, the desired final product was obtained without the need for any subsequent annealing treatment. A half solid oxide fuel cell (SOFC) was then developed using LSGM as an electrolyte and La0.8Sr0.2MnO3 (LSM) as an electrode, both obtained by mechanochemistry. The characterization by X-ray diffraction of as-prepared powders showed that LSGM and LSM present a perovskite structure and pseudo-cubic symmetry. The thermal and chemical stability between the electrolyte (LSGM) and the electrode (LSM) were analyzed by dynamic X-ray diffraction as a function of temperature. The electrolyte (LSGM) is thermally stable up to 800 and from 900 °C, where the secondary phases of LaSrGa3O7 and LaSrGaO4 appear. The best sintering temperature for the electrolyte is 1400 °C, since at this temperature, LaSrGaO4 disappears and the percentage of LaSrGa3O7 is minimized. The electrolyte is chemically compatible with the electrode up to 800 °C. The powder sample of the electrolyte (LSGM) at 1400 °C observed by HRTEM indicates that the cubic symmetry Pm-3m is preserved. The SOFC was constructed using the brush-painting technique; the electrode-electrolyte interface characterized by SEM presented good adhesion at 800 °C. The electrical properties of the electrolyte and the half-cell were analyzed by complex impedance spectroscopy. It was found that LSGM is a good candidate to be used as an electrolyte in SOFC, with an Ea value of 0.9 eV, and the LSM sample is a good candidate to be used as cathode
Pressure Effects in Manganites with Layered Perovskite Structure
Pressure effects on the charge and spin dynamics in the bilayer manganite
compounds are studied theoretically by taking into
account the orbital degrees of freedom. The orbital degrees are active in the
layered crystal structure, and applied hydrostatic pressure stabilizes the
orbital in comparison with . The change of the
orbital states weakens the interlayer charge and spin couplings, and suppresses
the three dimensional ferromagnetic transition. Numerical results, based on an
effective Hamiltonian which includes the energy level difference of the
orbitals, show that the applied pressure controls the dimensionality of the
spin and charge dynamics through changes of the orbital states.Comment: 5 pages, 2 figure
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