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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Phase equilibria, crystal structure and properties of complex oxides in the Nd{sub 2}O{sub 3}–SrO–CoO system

The phase equilibria in the ½Nd{sub 2}O{sub 3}–SrO–CoO system were systematically studied at 1373 K in air. The intermediate phases formed in the ½Nd{sub 2}O{sub 3}–SrO–CoO system at 1373 K in air are: Nd{sub 1-x}Sr{sub x}CoO{sub 3-δ} (0.0≤x≤0.5 with orthorhombic structure, sp. gr. Pbnm and 0.6≤x≤0.95 whose structure was detected as cubic according to XRD sp. gr. Pm3m, but shown to be tetragonal by TEM due to the oxygen vacancy ordering), Nd{sub 2-y}Sr{sub y}CoO{sub 4-δ} (0.6≤y≤1.1 with tetragonal K{sub 2}NiF{sub 4}-type structure, sp. gr. I4/mmm) and Nd{sub 2-z}Sr{sub z}O{sub 3} (0.0≤z≤0.15 with hexagonal structure, sp. gr. P-3m1). The unit cell parameters for the single phase samples were refined by the Rietveld analysis. The changes of oxygen content in Nd{sub 1-x}Sr{sub x}CoO{sub 3-δ} (0.6≤x≤0.95) and Ruddlesden-Popper oxide Nd{sub 2-y}Sr{sub y}CoO{sub 4-δ} were examined by TGA. All were found to be oxygen deficient phases. High-temperature dilatometry allows calculating the thermal expansion coefficient and evaluating the chemical expansion coefficient at high temperature. The projection of isothermal-isobaric phase diagram for the Nd–Sr–Co–O system at 1373 K in air to the compositional triangle of metallic components has been constructed. The phase equilibria in the studied Nd–Sr–Co–O system were compared to La–Sr–Co–O and Nd–M–Co–O (M=Ca and Ba). - Graphical abstract: Crystal structure of vacancy ordered supercell for Nd{sub 0.2}Sr{sub 0.8}CoO{sub 3-δ} and projection of phase diagram for the Nd–Sr–Co–O system onto the triangle edge of metallic components at 1373 K in air. - Highlights: • The diagram for the Nd–Sr–Co–O system at 1373 K in air has been constructed. • The crystal structure of Nd{sub 1-x}Sr{sub x}CoO{sub 3-δ} and Nd{sub 2-y}Sr{sub y}CoO{sub 4±δ} was refined. • The formation of superstructure due to the oxygen vacancy ordering was proved. • The changes of oxygen content in Nd{sub 1-x}Sr{sub x}CoO{sub 3-δ} and Nd{sub 2-y}Sr{sub y}CoO{sub 4±δ} were examined. • Thermal expansion and chemical expansion for Nd{sub 1-x}Sr{sub x}CoO{sub 3-δ} were evaluated.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Reactive flash sintering of the complex oxide Li 0.5 La 0.5 TiO 3 starting from an amorphous precursor powder

We report the transformation of an amorphous mixture of Li 0.5 La 0.5 TiO 3 (LLTO) directly into a single phase dense polycrystal, all within a few seconds at about 800°C by the flash method. The starting material is a chemically-prepared precursor powder that experiences concurrent crystallization and densification to cubic LLTO during the flash event, as shown by in-situ X-ray measurements. The experiments were conducted at a constant heating rate with fields from 80 to 120 V cm –1 and a current limit of 60 mA mm –2 . Here, the resulting polycrystal yielded a bulk conductivity of 0.5 mS cm –1 .

36 MATERIALS SCIENCE↗
DOE OSTI2016

Growth Mode Transition in Complex Oxide Heteroepitaxy: Atomically Resolved Studies

Here we performed investigations of the atomic-scale surface structure of epitaxial La 5/8 Ca 3/8 MnO 3 thin films as a model system dependent on growth conditions in pulsed laser deposition with emphasis on film growth kinetics. Postdeposition in situ scanning tunneling microscopy was combined with in operando reflective high-energy electron diffraction to monitor the film growth and ex situ X-ray diffraction for structural analysis. We find a correlation between the out-of-plane lattice parameter and both adspecies mobility and height of the Ehrlich–Schwoebel barrier, with mobility of adatoms greater over the cationically stoichiometric terminations. We find that the data suggest that the out-of-plane lattice parameter is dependent on the mechanism of epitaxial strain relaxation, which is controlled by the oxidative power of the deposition environment.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Deep Data Analysis of Conductive Phenomena on Complex Oxide Interfaces: Physics from Data Mining

Spatial variability of electronic transport in BiFeO3-CoFe2O4 (BFO-CFO) self-assembled heterostructures is explored using spatially resolved first order reversal curve (FORC) current voltage (IV) mapping. Multivariate statistical analysis of FORC-IV data classifies statistically significant behaviors and maps characteristic responses spatially. In particular, regions of grain, matrix, and grain boundary responses are clearly identified. K-means and Bayesian demixing analysis suggests the characteristic response be separated into four components, with hysteretic type behavior localized at the BFO-CFO tubular interfaces. The conditions under which Bayesian components allow direct physical interpretation are explored, and transport mechanisms at the grain boundaries and individual phases are analyzed. This approach conjoins multivariate statistical analysis with physics-based interpretation, actualizing a robust, universal, data driven approach to problem solving, which can be applied to exploration of local transport and other functional phenomena in other spatially inhomogeneous systems.

big data↗
DOE OSTI2015

Polarization screening-induced magnetic phase gradients at complex oxide interfaces

Thin-film oxide heterostructures show great potential for use in spintronic memories, where electronic charge and spin are coupled to transport information. Here we use a La 0.7 Sr 0.3 MnO 3 (LSMO)/PbZr 0.2 Ti 0.8 O 3 (PZT) model system to explore how local variations in electronic and magnetic phases mediate this coupling. We present direct, local measurements of valence, ferroelectric polarization and magnetization, from which we map the phases at the LSMO/PZT interface. We combine these experimental results with electronic structure calculations to elucidate the microscopic interactions governing the interfacial response of this system. We observe a magnetic asymmetry at the LSMO/PZT interface that depends on the local PZT polarization and gives rise to gradients in local magnetic moments; this is associated with a metal–insulator transition at the interface, which results in significantly different charge-transfer screening lengths. This work establishes a framework to understand the fundamental asymmetries of magnetoelectric coupling in oxide heterostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Valence-state reflectometry of complex oxide heterointerfaces

Emergent phenomena in transition-metal-oxide heterostructures such as interface superconductivity and magnetism have been attributed to electronic reconstruction, which, however, is difficult to detect and characterise. Here we overcome the associated difficulties to simultaneously address the electronic degrees of freedom and distinguish interface from bulk effects by implementing a novel approach to resonant X-ray reflectivity (RXR). Our RXR study of the chemical and valance profiles along the polar (001) direction of a LaCoO 3 film on NdGaO 3 reveals a pronounced valence-state reconstruction from Co 3+ in the bulk to Co 2+ at the surface, with an areal density close to 0.5 Co 2+ ions per unit cell. An identical film capped with polar (001) LaAlO 3 maintains the Co 3+ valence over its entire thickness. As a result, we interpret this as evidence for electronic reconstruction in the uncapped film, involving the transfer of 0.5e – per unit cell to the subsurface CoO 2 layer at its LaO-terminated polar surface.

36 MATERIALS SCIENCE↗
DOE OSTI2019

A termination-insensitive and robust electron gas at the heterointerface of two complex oxides

The single-crystal SrTiO 3 (001) has two different surface terminations, TiO 2 and SrO. One most remarkable observation in previous studies is that only the heterointerfaces with TiO 2 -terminated SrTiO 3 , which usually combines with polar oxides such as LaAlO 3 , host an electron gas. Here we show that a robust electron gas can be generated between a non-polar oxide, CaHfO 3 , and SrTiO 3 (001) with either termination. Unlike the well-known electron gas of LaAlO 3 /SrTiO 3 , the present one of CaHfO 3 /SrTiO 3 essentially has no critical thickness of CaHfO 3 , can survive a long-time oxygen annealing at high temperature, and its transport properties are stable under exposure to water and other polar solvents. By electrostatic gating through CaHfO 3 , field-effect devices are demonstrated using CaHfO 3 /SrTiO 3 heterointerfaces with both terminations. These results show that the electron gas reported in the present study is unique and promising for applications in oxide electronics.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Interface-induced magnetic polar metal phase in complex oxides

Abstract Polar metals are commonly defined as metals with polar structural distortions. Strict symmetry restrictions make them an extremely rare breed as the structural constraints favor insulating over metallic phase. Moreover, no polar metals are known to be magnetic. Here we report on the realization of a magnetic polar metal phase in a BaTiO 3 /SrRuO 3 /BaTiO 3 heterostructure. Electron microscopy reveals polar lattice distortions in three-unit-cells thick SrRuO 3 between BaTiO 3 layers. Electrical transport and magnetization measurements reveal that this heterostructure possesses a metallic phase with high conductivity and ferromagnetic ordering with high saturation moment. The high conductivity in the SrRuO 3 layer can be attributed to the effect of electrostatic carrier accumulation induced by the BaTiO 3 layers. Density-functional-theory calculations provide insights into the origin of the observed properties of the thin SrRuO 3 film. The present results pave a way to design materials with desired functionalities at oxide interfaces.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Polarity compensation in ultra-thin films of complex oxides: The case of a perovskite nickelate

We address the fundamental issue of growth of perovskite ultra-thin films under the condition of a strong polar mismatch at the heterointerface exemplified by the growth of a correlated metal LaNiO 3 on the band insulator SrTiO 3 along the pseudo cubic [111] direction. While in general the metallic LaNiO 3 film can effectively screen this polarity mismatch, we establish that in the ultra-thin limit, films are insulating in nature and require additional chemical and structural reconstruction to compensate for such mismatch. A combination of in-situ reflection high-energy electron diffraction recorded during the growth, X-ray diffraction, and synchrotron based resonant X-ray spectroscopy reveal the formation of a chemical phase La 2 Ni 2 O 5 (Ni 2+ ) for a few unit-cell thick films. First-principles layer-resolved calculations of the potential energy across the nominal LaNiO 3 /SrTiO 3 interface confirm that the oxygen vacancies can efficiently reduce the electric field at the interface.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effect of doping and chemical ordering on the optoelectronic properties of complex oxides: Fe 2 O 3 –V 2 O 3 solid solutions and hetero-structures

The electronic and optical properties of a-(Fe1xVx)2O3 at low (x = 0.04) and high (x = 0.5) doping levels are investigated using a combination of periodic and embedded cluster approaches, and time dependent density functional theory. At low V concentrations the onset of the optical absorption is B0.5 eV (i.e., nearly 1.6 eV lower than that in pure a-Fe2O3) and corresponds to the electron transitions from V 3d to Fe 3d* orbitals. At high V concentrations, optical absorption energies and intensities are sensitive to specific arrangements of Fe and V atoms and their spin configuration that determine Fe–V hybridization. The onset of the lowest inter-vanadium absorption band in the case of Fe2O3/V2O3 hetero-structures is as low as B0.3 eV and the corresponding peak is at B0.7 eV. In contrast, in the case of solid solutions this peak has lower intensity and is shifted to higher energy (B1.2 eV). Analysis of the orbital character of electronic excitation suggests that Fe2O3/V2O3 hetero-structures absorb light much more effectively than random alloys, thus promoting efficient photo-induced carrier generation. These predictions can be tested in a-(Fe1xVx)2O3 thin films synthesized with well-controlled spatial distribution of Fe and V species.

Environmental Molecular Sciences Laboratory↗