1,721,110 research outputs found
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Solid oxide fuel cell studies based on Sr- and Mg-doped LaGaO₃ electrolyte
No full texttextFuel cells constitute an attractive power-generating technology that converts
chemical energy directly and with high efficiency into electricity while causing little
pollution. Conventional solid oxide fuel cells (SOFC) are operated at 1000o
C or more in
order to attain reasonable power density. It is desirable to operate a fuel cell at reduced
temperatures (700-800o
C), while still maintaining the power densities achieved at high
temperatures. The widely-used yttria-stablized zirconia electrolyte possess a relatively
low ionic conductivity at 800o
C. Strontium and magnesium doped lanthanum gallate
(LSGM) was identified as a superior oxide-ion conductors with good chemical stability.
To enhance the power density of a SOFC, a thinner electrolyte, better electrode materials,
surface catalyst, and reduced interface reactions were the means to achieve the goal. The
alternative anode material was also investigated in order to utilize logistic fuels other than
H2.Chemistr
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A study of bond-length fluctuations in transition metal oxides
Get PDFtextBond-length fluctuations with different origins have been investigated by
thermal conductivity measurement performed on La1.60-xNd0.40SrxCuO4, RCoO3,
and RVO3 single crystals grown by floating zone method. Thermal conductivity
has been proved to be a sensitive probe to bond-length fluctuations in stronglycorrelated
transition-metal oxides.
Superconductivity in cuprates occurs at a crossover from localized to
itinerant electronic behavior. The segregation of localized spins and delocalized
holes into hole-poor and hole-rich regions in La2-xSrxCuO4 induces bond-length
fluctuations via a strong electron-lattice coupling. This bond-length fluctuation
suppresses in-plane thermal conductivity due to charge fluctuations in this quasi-
2D system. In the La1.60-xNd0.40SrxCuO4 system, the low-temperature orthorhombic
(LTO) phase transforms into a low-temperature-tetragonal (LTT) phase with
decreasing temperature. The hole-rich regions order into static stripes in the LTT
phase of La2-x-yNdySrxCuO4; this charge order revives the phonon contribution to
the thermal conductivity. The phonon thermal conductivity in the normal state of
LTT phase and the LTO phase of some underdoped compositions of LSCO calls
for reconsideration of the role of bond-length fluctuations on superconducting
pairing in different structures. Suppression of the phonon thermal conductivity in
the Mott-Hubbard insulator RCoO3 is interpreted to be caused by the spin-state
transition from the low-spin t6
e
0
ground state to a higher spin-state, either
intermediate-spin t5
e
1
or high-spin t4
e
2
, with increasing temperature. RVO3 offers
us a unique chance to study the bond-length fluctuations caused by strong spinorbital-lattice
coupling. An unusually strong orbital-lattice and spin-lattice
coupling has been clearly demonstrated.Materials Science and Engineerin
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Electronic transitions and multiferroicity in transition metal oxides
Get PDFFour systems have been studied for the localized-itinerant electronic transition
in transition-metal oxides: (i) In CaV1-xTixO3, substitution of Ti(IV) introduces
Anderson-localized states below a mobility edge µ c that increases with x, crossing
F ε in the range 0.2 < x< 0.4 and also transforms the strong-correlation fluctuations
to localized V(IV): t1e0 configurations for x ≥ 0.1. (ii) The properties of LaTiO3+δ
reveal that a hole-poor, strongly correlated electronic phase coexists with a hole-rich, itinerant-electron phase. With δ ≤ 0.03, the hole-rich phase exists as a
minority phase of isolated, mobile itinerant-electron clusters embedded in the hole-poor phase. With δ ≥ 0.08, isolated hole-poor clusters are embedded in an
itinerant-electron matrix. As δ > 0.08 increases, the hole-poor clusters become smaller and more isolated until they are reduced to super-paramagnetic strong-correlation fluctuations by δ = 0.12. (iii) The data of Y1-xLaxTiO3 appears to
distinguish an itinerant-electron antiferromagnetic phase in the La-rich samples
from a localized-electron ferromagnetic phase with a cooperative Jahn-Teller
distortion in the Y-rich phase. (iv) The transition at Tt in Mg[Ti2]O4 is a
semiconductor-semiconductor transition associated with Ti-Ti dimerization
instabilities. The dimerization is caused by lattice instabilities resulting from a
double-well Ti-Ti bond potential at a crossover from localized to itinerant
electronic behavior.
RMn1-xGaxO3 (R = Ho, Y) and Ho1-xYxMnO3 have been studied for the
multiferroicity of RMnO3. Ga doping raises the ferrielectric Curie temperature TC
and the Mn-spin reorientation temperature TSR while lowering TN of the Mn spins
and the Ho magnetic ordering temperature T2. The data show an important
coupling between the Mn3+-ion and Ho3+-ion spins as well as a TSR that is driven
by a cooperative MnO5 site rotation and R3+-ion displacements that modify the c
lattice parameter. The data also support an enhanced spin-lattice interaction in the
geometrically frustrated (GF) Mn-spin system. Y doping enhances the temperature
region for the P6’3cm’ magnetic phase and thereby increases TSR for Ho1-
xYxMnO3.
The studies of several oxygen non-stoichiometric Fe4+/Fe3+ oxoperovskite
show that two mechanisms, the formation of Fe3+-O-Fe4+ pair and the
disproportionation reaction 2Fe(IV)O6/2 = Fe3+ + Fe(V)O6, dominate the electronic
behavior. The properties of DyBaCo2O5.5 reveal a spin-state transition from the
low-spin t
6
e
0
ground state to higher spin-state at octahedral-site Co3+, which is also
accounted for the metamagnetism in the sample.Physic
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Characterization of manganese-oxide perovskites, exhibiting a colossal magnetoresistance
Get PDFThe complex phase diagrams of the La1-xSrxMnO3 ( 0 ≤ x ≤ 0.35), La
1-
xCaxMnO3 ( 0 ≤ x ≤ 0.40 ), and R0.7A0.3MnO3 ( 0.949 ≤ t ≤ 0.997 ) systems, in
which R is one or more the trivalent rare earth ions, A is one or two divalent
alkaline earth, and t is tolerance factor, are presented; they were mapped out with
measurements of resistivity ρ(T), thermoelectric power α(T), specific-heat Cp(T),
and magnetization M(T) on a series of melt-grown single-crystal samples. A
transition from localized to itinerant behavior of electrons of e-orbital parentage
in the presence of a localized t3 configuration with spin S = 3/2 was shown to be
characterized by an electronic phase segregation into orbitally ordered, electron-rich antiferromagnetic regions and orbitally disordered, conductive ferromagnetic
regions that grow in an applied magnetic field. This two-phase electronic
character in the crossover compositions gives rise to spin-glass behavior and a colossal magnetoresistance. Particular emphasis was placed on the evolution of
the specific-heat data Cp(T) on crossing the transition from localized to itinerant
electrons and the fact that Cp(T) is suppressed at a spin-glass freezing
temperature.Electrical and Computer Engineerin
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Investigation of electronic transport and magnetic properties in Ca₂-₂xLa₂xRu₂-xCoxO₆
No full textCa2-2xLa2xRu2-xCoxO6 ( 0.0 ≤ x ≤ 1.0 ) is prepared by a "Polymerizable-Complex Method." Electronic transport and magnetic properties are measured to show that the system becomes semiconducting at x ≈ 0.1 and goes through long-range antiferromagnetic ordering at x ≈ 0.4. The proximity in Néel temperature TN of La2RuCoO6 to that of La2TiCoO6 previously reported suggests that the magnetic moment of Ru4+ becomes quenched as temperature decreases. This suppression is explained by a theory that due to (1) the strong spin-orbit coupling ( large spin-orbit coupling constant [Greek small letter lambda] ) present in Ru4+ ions and (2) the sign inversion of [Greek small letter lambda] suggested by Kanamori, the Ruions favors the antiparallel alignment of L and S leading to J = 0. Based on (1) the similarity in the inverse magnetic susceptibility 1/[Greek small letter chi] (T) between CaRuO3 and La2RuZnO6 and (2) the presence of features in CaRuO3 commonly associated with phase fluctuations, the suppression of magnetic moment in Ru4+ is proposed to be responsible for the absence of long-range magnetic ordering in CaRuO3 as well. Finally, the appearance of long-range ferromagnetic ordering in CaRu1-xMxO3 is explained by (1) the degree of hybridization of the Ru4+/Ru5+ redox couple with empty d orbitals and (2) the formation of ferrimagnetic clusters from t2g 3-O- t2g 3 interactions between dopants and localized Ru5+: t2g 3eg 0 configurations.Electrical and Computer Engineerin
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Physical properties of transition metal oxides synthesized by floating zone method and spark plasma sintering
Get PDFTransition metal oxides have attracted growing attention over the last few decades because of rich physical properties they exhibit. Perovskite structure transition metal oxides AMO₃ are of particular interest to the design of functional materials in modern techniques, since a variety of ways can be used to tune the physical properties of AMO₃. Single crystals of Y₁₋ₓLaₓTiO₃ are grown by floating zone method to study the magnetic transition from ferromagnetic in YTiO₃ to G-type antiferromagnetic in LaTiO₃. Y₁₋ₓ LaₓTiO₃ shows similar magnetic phase diagram with RTiO₃ family, and the magnetism and the transition temperature can be finely tuned by varying the La doping x. By measuring the change of magnetic transition temperatures on single crystal samples under uniaxial stress, the correlation between the lattice distortions and the cooperative orbital ordering can be distinguished. Double perovskite CaMnTi₂O₆ is the first columnar A-site ordered perovskite exhibiting ferroelectric property. Spark plasma sintering (SPS) is used to successfully synthesize gram-level Ca₂₋ₓMnₓTi₂O₆, which has the same crystal structure and similar high-T [subscript c] ferroelectric property. Through neutron diffraction, the detailed information of the structure is obtained, and the driving force for ferroelectricity is identified. Inspired by the successful synthesis of double perovskite Ca₂₋ₓMnₓTi₂O₆, perovskites La₁₋ₓPrₓRuO₃ are obtained by SPS as well. The substitution of La by smaller rare earth ion Pr gives rise to the crossover from itinerant to localized electronic behavior. A systematical study of physical properties is made and an unusual second-order metal insulator transition is found in La₁₋ₓPrₓRuO₃. The A²⁺V₂O₄ spinels have the smallest gap caused by electron-electron correlations in the single-valent spinels, and the V-V bond length in these spinels decreases as the A-site cation is replaced by cations in the order of A = Cd, Mn, Fe, Mg, Zn, Co. The density functional theory (DFT) calculation and transport properties of CoV₂O₄ under pressure indicate that CoV₂O₄ might be at the crossover between localized electron and itinerant electronic behavior. In order to clarify this, the series of AV₂O₄ spinels (A = Cd, Mn, Fe, Mg, Zn, Co) are studied with in situ high-pressure x-ray and neutron diffraction at different temperatures.Materials Science and Engineerin
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The high pressure synthesis, crystal growth and physical properties of transition metal perovskites
Get PDFtextThe perovskite structure has an incredible versatility that results in myriad compounds with varied and eccentric behaviors. Perovskite oxides have been extensively studied and used for over 60 years. In order to expand on our already thorough knowledge of these compounds, it is necessary to use modern and creative experimental techniques. High-pressure synthesis and high oxygen-gas pressure annealing techniques are used to synthesize oxygen stoichiometric RNiO₃ (R = lanthanide). The particularly rich phase diagram of this compound allows for the study of the crossover from localized to itinerant electronic behavior and from an enhanced Pauli to a Curie-Weiss law paramagnetism. Single crystals of RFeO₃ are grown in order to analyze the spin canting in these antiferromagnetic samples. The size of the rare earth-cation is used to tune the magnitude of octahedral-tilt distortions. This tuning allows distinguishing between the two possible drivers for spin canting and weak ferromagnetism in these compounds, the octahedral-tilt-dependent single-ion anisotropy and the octahedral-tilt-independent Dzyaloshinskii-Moriya interaction. Although it is a fluoride compound, KCuF₃ has been used as an analogue to transition-metal oxide perovskites such as LaMnO₃ because of the similarity of their orbital ordering. Through the use of high-temperature neutron diffraction, it is shown that the orbital ordering and Jahn-Teller distortion in this compound are not lifted at the predicted temperature. Another mechanism for orbital ordering is identified. La₂[subscript-x] Sr [subscript x] CuO₄ has long been of interest as the progenitor system of the highTc superconductors. Despite having an exceedingly well-studied phase diagram in the over-doped region of its superconducting dome, little is known about this system in the region x > 0.3 because of the difficulty of synthesizing fully oxygen-stoichiometric samples. With high-oxygen-gas-pressure annealing and high-pressure synthesis, the completion of the phase diagram up to x = 1.0 is attempted. Finally, like many iridates, post-Perovskite CaIrO₃ exhibits a very strong spinorbit coupling of its 5d electrons. Because its magnetism is very weak, traditional methods to measure the magnitude of its orbital moment and spin-orbit coupling, such as neutron powder diffraction, are not viable. In order to address this issue, direct measurement of the orbital moments was conducted by using x-ray absorption spectroscopy and x-ray magnetic circular dichroism techniques.Materials Science and Engineerin
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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