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Results for “PYROCHLORE”

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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Structural Changes of (KGd)2Ta2O7 Pyrochlore at High Pressure

The structure of K-bearing tantalate pyrochlore (K{sub 2-x}Gd{sub x})Ta{sub 2}O{sub 6+x}(x {approx} 0.4) was studied at high pressures using in situ X-ray diffraction and Raman scattering methods. Experimental results indicated that (K{sub 2-x}Gd{sub x})Ta{sub 2}O{sub 6+x}(x {approx} 0.4) retains the pyrochlore structure up to 40 GPa, but partial amorphization occurred at pressures above 23 GPa. The amorphous phase was also confirmed in the quenched sample by means of transmission electron microscopy. The tantalate pyrochlore lattice is more stable than pyrochlore compounds in other systems, such as rare earth titanates, zirconates and stannates. The structural stability of pyrochlore tantalate may be mainly related to the size ratio of cations on the 16d and 16c sites in the lattice.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Atomic disorder in Gd 2 Zr 2 O 7 pyrochlore

Gd 2 Zr 2 O 7 pyrochlore with different degrees of cation disorder were synthesized by isothermal annealing at various temperatures (1100–1550 °C), and the related changes in the structure were investigated by ambient and high pressure x-ray diffraction (XRD) measurements. Unit cell parameters increase almost linearly with increasing treatment temperature. The degree of cation order in pyrochlore also increases with the increase of temperature, but saturates at ~60%. The compressibility of the pyrochlore structures decreases when the degree of cation order increases. Lastly, high pressure XRD measurements also indicate that the phase stability of Gd 2 Zr 2 O 7 is not very sensitive to the degree of atomic disorder in the pyrochlore structure.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Enhanced Electronic Conductivity by Controlled Self-Doping in Pyrochlores

Most 5d transition-metal (TM) pyrochlores exhibit metallic behavior, but 3d and 4d TM pyrochlores are generally electronic semiconductors or insulators. Here, we report a semiconductor–metal transition induced by introducing excess Ti metal as interstitials into Y2Ti2O7. These Ti interstitials prefer anion vacant 8a sites or bridge sites between two neighboring cations along the h010i direction. Density functional theory calculations suggest that an increased electronic conductivity originates from the interplay between the extra Ti and its neighboring cations. These findings suggest a means for achieving metallic behavior in semiconducting pyrochlore oxides and tuning the electronic conduction in pyrochlores for their electrochemical applications in solid oxide fuel cells.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2022

Bond valence sum analysis of pyrochlore oxides including the novel dielectric Te 6+ pyrochlores: A Bi M TeO 7-y ( A = Cd, Ca; M = Cr, Ga, Sc, In, Fe)

Bond valence sum analysis is a powerful tool used in evaluating and validating crystal structures; especially when those structures are complex in nature. The pyrochlore structure type is versatile in not only the unique bonding that it exhibits, but also in the properties that results from the structure. Here this paper aims to center the discussion of evaluating the pyrochlore structure using the bond valence sum method. In this study, novel quaternary pyrochlores with a general stoichiometry of ABiMTeO 7 (A = Cd, Ca; M = Cr, Ga, Sc, In, Fe) were synthesized and characterized for their structural, magnetic, and dielectric properties. Two representative compounds within this series of pyrochlores, BiCaFeTeO7 and BiCdFeTeO 7 , were structurally characterized utilizing a combination of high-resolution synchrotron X-ray diffraction and neutron diffraction revealing oxygen deficient pyrochlore systems which were off from the expected stoichiometry with respect to the M site. The A site of both pyrochlores were found to be moved off-center from the expected 16d site to the 96h displaced position at a magnitude of 0.25 Å and 0.22 Å for the Bi/Ca and Bi/Cd systems, respectively. These structures were evaluated using the bond valence sum method and compared with trends in the literature. The properties are also reported for the Bi/Ca system for the first time, showing relatively high dielectric constants with a low dielectric loss which are primarily independent of frequency and temperature. The magnetic measurements for the Bi/Ca system for the magnetic substitutions reveal a paramagnet and antiferromagnetic properties for the Fe and Cr analogs, respectively. The novel BiCaMTeO 7 quaternary pyrochlore system shows great promise as an emerging dielectric material.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Epitaxial crystals of Bi{sub 2}Pt{sub 2}O{sub 7} pyrochlore through the transformation of δ–Bi{sub 2}O{sub 3} fluorite

Bi{sub 2}Pt{sub 2}O{sub 7} pyrochlore is thought to be one of the most promising oxide catalysts for application in fuel cell technology. Unfortunately, direct film growth of Bi{sub 2}Pt{sub 2}O{sub 7} has not yet been achieved, owing to the difficulty of oxidizing platinum metal in the precursor material to Pt{sup 4+}. In this work, in order to induce oxidation of the platinum, we annealed pulsed laser deposited films consisting of epitaxial δ–Bi{sub 2}O{sub 3} and co-deposited, comparatively disordered platinum. We present synchrotron x-ray diffraction results that show the nonuniform annealed films contain the first epitaxial crystals of Bi{sub 2}Pt{sub 2}O{sub 7}. We also visualized the pyrochlore structure by scanning transmission electron microscopy, and observed ordered cation vacancies in the epitaxial crystals formed in a bismuth-rich film but not in those formed in a platinum-rich film. The similarity between the δ–Bi{sub 2}O{sub 3} and Bi{sub 2}Pt{sub 2}O{sub 7} structures appears to facilitate the pyrochlore formation. These results provide the only route to date for the formation of epitaxial Bi{sub 2}Pt{sub 2}O{sub 7}.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Strain engineered pyrochlore at high pressure

Strain engineering is a promising method for next-generation materials processing techniques. Here, we use mechanical milling and annealing followed by compression in diamond anvil cell to tailor the intrinsic and extrinsic strain in pyrochlore, Dy 2 Ti 2 O 7 and Dy 2 Zr 2 O 7 . Raman spectroscopy, X-ray pair distribution function analysis, and X-ray diffraction were used to characterize atomic order over short-, medium-, and long-range spatial scales, respectively, under ambient conditions. Raman spectroscopy and X-ray diffraction were further employed to interrogate the material in situ at high pressure. High-pressure behavior is found to depend on the species and concentration of defects in the sample at ambient conditions. Overall, we show that defects can be engineered to lower the phase transformation onset pressure by ~50% in the ordered pyrochlore Dy 2 Zr 2 O 7 , and lower the phase transformation completion pressure by ~20% in the disordered pyrochlore Dy 2 Zr 2 O 7 . These improvements are achieved without significantly sacrificing mechanical integrity, as characterized by bulk modulus.

36 MATERIALS SCIENCE↗
DOE OSTI2017

High-pressure behavior of A 2 B 2 O 7 pyrochlore (A=Eu, Dy; B=Ti, Zr)

In situ high-pressure X-ray diffraction and Raman spectroscopy were used to determine the influence of composition on the high-pressure behavior of A 2 B 2 O 7 pyrochlore (A=Eu, Dy; B=Ti, Zr) up to ~50GPa. Based on X-ray diffraction results, all compositions transformed to the high-pressure cotunnite structure. The B-site cation species had a larger effect on the transition pressure than the A-site cation species, with the onset of the phase transformation occurring at ~41 GPa for B=Ti and ~16 GPa B=Zr. However, the A-site cation affected the kinetics of the phase transformation, with the transformation for compositions with the smaller ionic radii, i.e., A=Dy, proceeding faster than those with a larger ionic radii, i.e., A=Eu. These results were consistent with previous work in which the radius-ratio of the A- and B-site cations determined the energetics of disordering, and compositions with more similarly sized A- and B-site cations had a lower defect formation energy. Raman spectra revealed differences in the degree of short-range order of the different compositions. Due to the large phase fraction of cotunnite at high pressure for B=Zr compositions, Raman modes for cotunnite could be observed, with more modes recorded for A=Eu than A=Dy. These additional modes are attributed to increased short-to-medium range ordering in the initially pyrochlore structured Eu 2 Zr 2 O 7 as compared with the initially defect-fluorite structured Dy 2 Zr 2 O 7 .

36 MATERIALS SCIENCE↗
DOE OSTI2017

High-Performance Pyrochlore-Type Yttrium Ruthenate Electrocatalyst for Oxygen Evolution Reaction in Acidic Media

Development of acid-stable electrocatalysts with low overpotential for oxygen evolution reaction (OER) is a major challenge for the production of hydrogen directly from water. We report in this paper a pyrochlore yttrium ruthenate (Y2Ru2O7-δ) electrocatalyst that has significantly enhanced performance towards OER in acid media over the best-known catalysts, with an onset overpotential of 190 mV and high stability in 0.1-M perchloric acid solution. X-ray absorption near-edge structure (XANES) indicates Y2Ru2O7-δ electrocatalyst had a low valence state that favors the high OER activity. Density functional theory (DFT) calculation shows this pyrochlore has lower band center energy for the overlap between Ru 4d and O 2p orbitals and therefore more stable Ru-O bond than RuO2, highlighting the effect of yttrium on the enhancement in stability. The Y2Ru2O7-δ pyrochlore is also free of expensive iridium metal, thus a cost-effective candidate for practical applications.

Kim, Jaemin↗
DOE OSTI2019

Synthesis, crystal growth and characterization of the pyrochlore Er 2 Ti 2 O 7

Pyrochlore erbium titanate samples in the form of powders (with nominal compositions of Er 2 Ti 2+ x O 7 , -0.08 ≤ x ≤ 0.08) and bulk single crystals were prepared to elucidate the effect of synthesis and growth conditions on their composition, structure, and transition temperature. All the samples were characterized using X-ray diffraction and specific heat measurements. Larger lattice parameters were measured for Ti-deficient stuffed powders, Er 2 (Ti 2- x Er x )O 7- δ , while Ti-rich anti-stuffed powders, (Er 2- x Ti x )Ti 2 O 7+ δ , showed a decrease in the lattice parameter. The lattice parameter for the stoichiometrically synthesized powder, Er 2 Ti 2 O 7 , was measured to be a = 10.07522(9) Å. Single crystals grown by the conventional floating zone (FZ) technique were Ti deficient (stuffed) and darker in color due to oxygen vacancies. Using the traveling solvent floating zone (TSFZ) method, a high structural quality, transparent and stoichiometric Er 2 Ti 2 O 7 single crystal was grown at a lower temperature using the TiO 2 solvent. The heat capacity measurements of the TSFZ grown Er 2 Ti 2 O 7 crystal and stoichiometric powder exhibited a very close magnetic transition temperature T c of ~1.23 K that was suppressed in off-stoichiometric samples, demonstrating correlations between the stoichiometry and ground state magnetism in Er 2 Ti 2 O 7 .

optics, thermal conductivity, defects, charge tran↗