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10 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Small-moment antiferromagnetic ordering in single-crystalline La 2 Ni 7

Single crystals of La 2 Ni 7 have been grown out of a binary, La-Ni melt. Temperature dependent, zero magnetic field, specific heat, electrical resistivity, and low field magnetization measurements indicate that there is a series of antiferromagnetic phase transitions at T 1 = 61.0 ± 0.2 K, T 2 = 56.5 ± 0.2 K and T 3 = 42.2 ± 0.2 K. The three specific heat anomalies found at these temperatures qualitatively have very small entropy changes associated with them and the anisotropic M(H) data saturate at ~ 0.12 μ B /Ni; both observations strongly suggesting the AFM order is associated with very small, itinerant, moments. Anisotropic, H ||c and H ⟂c , ρ(H) and M(H) isotherms as well as constant field, ρ(T) and M(T) sweeps manifest signatures of multiple phase lines and result in HT phase diagrams that are clearly anisotropic. Analysis of M(T) and M(H) data allow for the identification of the two lower temperature magnetically ordered states as antiferromagnetically ordered, with the moments aligned along the crystallographic c-axis, and the higher temperature, T 2 < T < T 1 , state as having a finite ferromagnetic component. In addition, the metamagnetic transition at low temperatures, for H applied along the crystallographic c-axis (H ||c ) appears to be a near classic example of a spin-flop transition, resulting in a field stabilized antiferromagnetic state with the moments ordered perpendicular to the c-axis. Finally, although the small moment ordering, and existence of multiple phase transitions in field and temperature, suggesting an energetic proximity of these states, could foretell a degree of pressure sensitivity, our measurements of R(T) for applied pressures up to 2.0 GPa indicate that there is very little pressure dependence of T 1 , T 2 and T 3 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1992

Catalysts for ultrahigh current density oxygen cathodes for space fuel cell applications

The objective was to identify promising electrocatalyst/support systems for oxygen cathodes capable of operating at ultrahigh current densities in alkaline fuel cells. Such cells will require operation at relatively high temperatures and O2 pressures. A number of materials were prepared, including Pb-Ru and Pb-Ir pyrochlores, RuO2 and Pt-doped RuO2, lithiated NiO and La-Ni perovskites. Several of these materials were prepared using techniques that had not been previously used to prepare them. Particularly interesting was the use of the alkaline solution technique to prepare Pt-doped and Pb-Ru pyrochlores in high area form. Also interesting was the use of the fusion (melt) method for preparing the Pb-Ru pyrochlore. Several of the materials were also deposited with platinum. Well-crystallized Pb2Ru2O(7-y) was used to fabricate very high performance O2 cathodes with good stability in room temperature KOH. This material was also found to be stable over a useful potential range at approx. 140 C in concentrated KOH. For some of the samples, fabrication of the gas-fed electrodes could not be fully optimized during this project period. Future work may be directed at this problem. Pyrochlores that were not well-crystallized were found to be unstable in alkaline solution. Very good O2 reduction performance and stability were observed with Pb2RuO(7-y) in a carbon-based gas-fed electrode with an anion-conducting membrane placed on the electrolyte side of the electrode. The performance came within a factor of about two of that observed without carbon. High area platinum and gold supported on several conductive metal oxide supports were examined. Only small improvements in O2 reduction performance at room temperature were observed for Pb2Ru2O(7-y) as a support because of the high intrinsic activity of the pyrochlore. In contrast, a large improvement was observed for Li-doped NiO as a support for Pt. Very poor performance was observed for Au deposited on Li-NiO at approx. 150 C. Nearly reversible behavior was observed for the O2/OH(-) couple for Li-doped NiO at approx. 200 C. The temperature dependence for the O2 reduction was examined.

Tryk, Donald A.↗
DOE OSTI2020

Materials Data on La2Ni7 by Materials Project

La2Ni7 crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. there are two inequivalent La sites. In the first La site, La is bonded in a 6-coordinate geometry to eighteen Ni atoms. There are a spread of La–Ni bond distances ranging from 2.90–3.30 Å. In the second La site, La is bonded in a 12-coordinate geometry to twelve Ni atoms. There are a spread of La–Ni bond distances ranging from 2.88–3.24 Å. There are five inequivalent Ni sites. In the first Ni site, Ni is bonded to five La and seven Ni atoms to form a mixture of distorted corner, edge, and face-sharing NiLa5Ni7 cuboctahedra. There are a spread of Ni–Ni bond distances ranging from 2.46–2.59 Å. In the second Ni site, Ni is bonded to six equivalent La and six equivalent Ni atoms to form NiLa6Ni6 cuboctahedra that share corners with twelve equivalent NiLa5Ni7 cuboctahedra, edges with six equivalent NiLa6Ni6 cuboctahedra, and faces with eighteen equivalent NiLa5Ni7 cuboctahedra. In the third Ni site, Ni is bonded in a 12-coordinate geometry to three equivalent La and six Ni atoms. All Ni–Ni bond lengths are 2.47 Å. In the fourth Ni site, Ni is bonded in a 12-coordinate geometry to three equivalent La and six Ni atoms. All Ni–Ni bond lengths are 2.48 Å. In the fifth Ni site, Ni is bonded to four equivalent La and eight Ni atoms to form a mixture of corner, edge, and face-sharing NiLa4Ni8 cuboctahedra. There are two shorter (2.51 Å) and two longer (2.52 Å) Ni–Ni bond lengths.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on La2Ni3 by Materials Project

La2Ni3 crystallizes in the orthorhombic Cmce space group. The structure is three-dimensional. La is bonded in a 8-coordinate geometry to ten Ni atoms. There are a spread of La–Ni bond distances ranging from 2.95–3.53 Å. There are two inequivalent Ni sites. In the first Ni site, Ni is bonded to eight equivalent La and four equivalent Ni atoms to form a mixture of distorted corner, edge, and face-sharing NiLa8Ni4 cuboctahedra. All Ni–Ni bond lengths are 2.51 Å. In the second Ni site, Ni is bonded in a 10-coordinate geometry to six equivalent La and four Ni atoms. Both Ni–Ni bond lengths are 2.52 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on LaNi5 by Materials Project

LaNi5 crystallizes in the hexagonal P6/mmm space group. The structure is three-dimensional. La is bonded in a 6-coordinate geometry to eighteen Ni atoms. There are six shorter (2.89 Å) and twelve longer (3.19 Å) La–Ni bond lengths. There are two inequivalent Ni sites. In the first Ni site, Ni is bonded in a 12-coordinate geometry to three equivalent La and six equivalent Ni atoms. All Ni–Ni bond lengths are 2.45 Å. In the second Ni site, Ni is bonded to four equivalent La and eight Ni atoms to form a mixture of corner, edge, and face-sharing NiLa4Ni8 cuboctahedra. All Ni–Ni bond lengths are 2.50 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on LaNi2 by Materials Project

LaNi2 is Cubic Laves structured and crystallizes in the cubic Fd-3m space group. The structure is three-dimensional. La is bonded in a 12-coordinate geometry to twelve equivalent Ni atoms. All La–Ni bond lengths are 3.05 Å. Ni is bonded to six equivalent La and six equivalent Ni atoms to form a mixture of edge, face, and corner-sharing NiLa6Ni6 cuboctahedra. All Ni–Ni bond lengths are 2.60 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on LaNi3 by Materials Project

LaNi3 crystallizes in the trigonal R-3m space group. The structure is three-dimensional. there are two inequivalent La sites. In the first La site, La is bonded in a 12-coordinate geometry to twelve Ni atoms. There are a spread of La–Ni bond distances ranging from 2.90–3.21 Å. In the second La site, La is bonded in a distorted hexagonal planar geometry to eighteen Ni atoms. There are six shorter (2.91 Å) and twelve longer (3.23 Å) La–Ni bond lengths. There are three inequivalent Ni sites. In the first Ni site, Ni is bonded to five La and seven Ni atoms to form a mixture of distorted edge, face, and corner-sharing NiLa5Ni7 cuboctahedra. There are a spread of Ni–Ni bond distances ranging from 2.47–2.59 Å. In the second Ni site, Ni is bonded to six equivalent La and six equivalent Ni atoms to form NiLa6Ni6 cuboctahedra that share corners with twelve equivalent NiLa5Ni7 cuboctahedra, edges with six equivalent NiLa6Ni6 cuboctahedra, and faces with eighteen equivalent NiLa5Ni7 cuboctahedra. In the third Ni site, Ni is bonded in a 12-coordinate geometry to three equivalent La and six equivalent Ni atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on La7Ni3 by Materials Project

La7Ni3 crystallizes in the hexagonal P6_3mc space group. The structure is three-dimensional. there are three inequivalent La sites. In the first La site, La is bonded in a distorted water-like geometry to two equivalent Ni atoms. Both La–Ni bond lengths are 2.84 Å. In the second La site, La is bonded in a 3-coordinate geometry to three equivalent Ni atoms. All La–Ni bond lengths are 2.96 Å. In the third La site, La is bonded in a 3-coordinate geometry to three equivalent Ni atoms. There are two shorter (2.92 Å) and one longer (3.04 Å) La–Ni bond lengths. Ni is bonded in a 6-coordinate geometry to six La atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on La7Ni16 by Materials Project

La7Ni16 crystallizes in the tetragonal I-42m space group. The structure is three-dimensional. there are three inequivalent La sites. In the first La site, La is bonded in a 6-coordinate geometry to ten Ni atoms. There are a spread of La–Ni bond distances ranging from 2.82–3.34 Å. In the second La site, La is bonded in a 12-coordinate geometry to twelve Ni atoms. There are a spread of La–Ni bond distances ranging from 2.93–3.07 Å. In the third La site, La is bonded in a 12-coordinate geometry to twelve Ni atoms. There are four shorter (2.95 Å) and eight longer (2.98 Å) La–Ni bond lengths. There are three inequivalent Ni sites. In the first Ni site, Ni is bonded in a 12-coordinate geometry to four La and six Ni atoms. There are one shorter (2.45 Å) and five longer (2.52 Å) Ni–Ni bond lengths. In the second Ni site, Ni is bonded in a 11-coordinate geometry to five La and six Ni atoms. There are a spread of Ni–Ni bond distances ranging from 2.56–2.72 Å. In the third Ni site, Ni is bonded in a 11-coordinate geometry to five La and six Ni atoms. The Ni–Ni bond length is 2.90 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on LaNi by Materials Project

LaNi crystallizes in the orthorhombic Cmcm space group. The structure is three-dimensional. La is bonded in a 7-coordinate geometry to seven equivalent Ni atoms. There are a spread of La–Ni bond distances ranging from 2.96–3.15 Å. Ni is bonded in a 9-coordinate geometry to seven equivalent La and two equivalent Ni atoms. Both Ni–Ni bond lengths are 2.71 Å.

36 MATERIALS SCIENCE↗