Low temperature susceptibility and phase diagrams of the Nb-H and Ta-H systems

Köbler, U.; Welter, J. M.

doi:10.1016/0022-5088(82)90147-3

Cited by 68 publications

(17 citation statements)

References 14 publications

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“…This is in agreement with Brun et al [7] who observed only pure phases at concentrations c H < 0.85 but not a two-phase region, confirmed by theoretical contributions of Kajitani et al [26] and Vaks et al [27]. The phase transition observed by Köbler and Welter [6] at c H = 0.83, however, is not confirmed by the data nor is the transition at T ≈ 70 K. The transition above 100 K is confirmed by our diffraction measurements. This phase transition may be correlated with the one detected in [28] around 100 K at concentrations 0.82 < c H < 0.93.…”

Section: Comparison With the Results Of Previous Investigationssupporting

confidence: 92%

Neutron scattering studies on the vibrational excitations and the structure of ordered niobium hydrides: the phases

Hauer¹,

Hempelmann²,

Udovic³

et al. 2004

J. Phys.: Condens. Matter

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Neutron vibrational spectroscopy and neutron diffraction have been used to study the optical hydrogen vibrations in and the structure of ordered niobium hydrides NbH x and deuterides NbD x in the concentration range 0.73 < x < 1.0 and at temperatures between 13 and 250 K. The measurements performed at temperatures above 200 K confirm the established model of the β phase. Neutron diffraction data collected in the regime of the λ phases (T < 200 K, 0.73 < x < 1.0) show that the structure for x < 0.85 is made up of microdomains with β phase structure. Neutron vibrational spectra display up to three families of peaks that can be assigned to three different hydrogen sites: an undisturbed tetrahedral site within the microdomains and two strongly disturbed tetrahedral sites within two different kinds of microdomain boundaries. Incommensurately modulated phases probably exist in this concentration range. For hydrogen concentrations around x ≈ 0.85 there is evidence for a change of symmetry of the basic cell as well as a change of the modulation at even higher concentrations. In the range of the γ phase at x ≈ 0.9, strong isotopic effects occur. The results give new insights into the phase diagram.

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Section: Comparison With the Results Of Previous Investigationssupporting

confidence: 92%

Neutron scattering studies on the vibrational excitations and the structure of ordered niobium hydrides: the phases

Hauer¹,

Hempelmann²,

Udovic³

et al. 2004

J. Phys.: Condens. Matter

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“…Hydrogen entering Nb during chemical etching or mechanical polishing could actually be the source of the high defect density in the ''hot spot'' samples measured by EBSD. (iii) Hydrogen affects the magnetic behavior of Nb by lowering the magnetic susceptibility for increasing H concentration [38]. It is possible that hydrogen is the ''magnetic impurity'' invoked by the PCT results.…”

Section: B Surface-included Hydrogenmentioning

confidence: 96%

High fieldQslope and the baking effect: Review of recent experimental results and new data on Nb heat treatments

Ciovati

Myneni

Stevie

et al. 2010

Phys. Rev. ST Accel. Beams

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The performance of superconducting radio-frequency (SRF) cavities made of bulk Nb at high fields (peak surface magnetic field greater than about 90 mT) is characterized by exponentially increasing rf losses (high-field Q slope), in the absence of field emission, which are often mitigated by low-temperature (100-140 C, 12-48 h) baking. In this contribution, recent experimental results and phenomenological models to explain this effect will be briefly reviewed. New experimental results on the high-field Q slope will be presented for cavities that had been heat treated in a vacuum furnace at high temperature without subsequent chemical etching. These studies are aimed at understanding the role of hydrogen on the highfield Q slope and at the passivation of the Nb surface during heat treatment. Improvement of the cavity performances, particularly of the cavities' quality factor, have been obtained following the hightemperature heat treatments, while secondary ion mass spectroscopy surface analysis measurements on Nb samples treated with the cavities revealed significantly lower hydrogen concentration than for samples that followed standard cavity treatments.

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“…(1,4-'3C2)Succinic acid was synthesised I by known procedures from tetraethylammonium (13C)cyanide [14] and 1,2-dibromoethane via (1 ,4-'3C2)succinic dinitrile [15].…”

Section: Synthesesmentioning

confidence: 99%

On the mechanism of action of methylmalonyl-CoA mutase. Change of the steric course on isotope substitution

et al. 1986

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1. When (methyL2H 3)methylmalonyl-CoA was reacted with partially purified methylmalonyl-CoA mutase, 'H-NMR revealed that about 24% of the migrating deuterium was lost after 88% conversion.2. When [methyf-3H]methylmalonyl-CoA was incubated with highly purified methylmalonyl-CoA mutase, tritium exchange with the medium depended on added methylmalonyl-CoA epimerase. 3. With highly purified preparations of methylmalonyl-CoA mutase, effective tritium exchange from [5'-3H]adenosylcobalamin to water required the addition of methylmalonyl-CoA epimerase and of substrate (e.g. succinyl-CoA).4. By addition of ['4C]succinyl-CoA to a partially purified preparation of methylmalonyl-CoA mutase, it was shown that the mutase binds one substrate molecule very tightly.5. Coupling the mutase reaction with the transcarboxylase reaction and using variously labelled succinyl-CoA as substrate, revealed that only (2R)-and not (2S)-methylmalonyl-CoA will be formed by the mutase with a kinetic isotope effect of 3.5 using ('H4)succinyl-CoA.6 . When (I -I3C) propionyl-CoA was reacted with a mixture of highly purified methylmalonyl-CoA carboxylase, epimerase and mutase, 13C-NMR signals were obtained for the thioester carbonyl of succinyl-CoA (relative intensity 100%) and of methylmalonyl-CoA (5%) as well as for the carboxyl of free succinic acid (27%) and of succinyl-CoA (< 4.5%). Thus very little, if any, migration of the CoA from one carboxyl to the other appears to take place.(1 ,4-13C2)Succinic acid and (1 ,4-'3C2)succinyl-CoA were synthesised and their I3C-NMR chemical shifts were exactly determined. 7. Evidence is provided for a strict stereospecificity of the mutase toward the (2R)-epimer of methylmalonylCoA and for an incomplete stereospecificity toward the two diastereotopic 3-H atoms of succinyl-CoA. The latter, combined with a high intramolecular isotope discrimination, causes rapid washing-out of the migrating 2H and 3 H to water and slow washing-in from the medium. Whenever migration of protium from the sterically less preferred 3-pro(S)-position of succinyl-CoA occurs and simultaneously a heavy isotope is maneuvered from the migratable 3-pro(R)-position into the labile a-position of methylmalonyl-CoA, the substitution by the COSCoA group takes place with inversion of configuration. When the sterically preferred 3-pro(R)-hydrogen atom migrates, the previously reported stereochemical retention occurs.A mechanistic and stereochemical scheme is discussed that fully accounts for all observations.

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Low temperature susceptibility and phase diagrams of the Nb-H and Ta-H systems

Cited by 68 publications

References 14 publications

Neutron scattering studies on the vibrational excitations and the structure of ordered niobium hydrides: the phases

Neutron scattering studies on the vibrational excitations and the structure of ordered niobium hydrides: the phases

High fieldQslope and the baking effect: Review of recent experimental results and new data on Nb heat treatments

On the mechanism of action of methylmalonyl-CoA mutase. Change of the steric course on isotope substitution

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