The NbB2-phase revisited: Homogeneity range, defect structure, superconductivity

Carlos A. Nunes, Dariusz Kaczorowski, Peter Franz Rogl, Márcia Regina Baldissera, Paulo A. Suzuki, Gilberto C. Coelho, Andrij Grytsiv, Gilles André, Francoise Boureé, Shigeru Okada

Veröffentlichungen: Beitrag in FachzeitschriftArtikelPeer Reviewed

Abstract

The discovery of superconductivity below 40 K in MgB2 has motivated new investigations on similar compounds, especially on binary diborides. The great majority of these compounds represent the AlB 2-type structure (P6/mmm space group, number 191) and comprise line compounds. However, among those, NbB2 and TaB2 are reported to present a significant homogeneity region, a value of 12 at.% being reported for the case of NbB2. In this work we have evaluated the homogeneity range of the NbB2-phase through detailed microstructural characterization of as-cast, as-cast + heat-treated and solid state sintered Nb-B alloys. Neutron diffraction experiments were performed to assess the defect mechanism responsible for accommodating the non-ideal NbB2 stoichiometries (ideal = 66.7 at.% B). The results clearly showed that the width of the homogeneity range of this phase is nearly 5 at.%, extending from 65 at.% B (NbB1.86) up to 70 at.% B (NbB2.34). Rietveld refinement of the neutron intensity diffraction data indicated a random distribution of vacancies in the Nb-subnet for hyperstoichiometric NbB 2. The occurrence of a possible Nb-vacancy ordered supercell was evaluated; however, a simple AlB2-type is observed throughout the entire homogeneity range. The superconducting properties of selected alloys were checked via magnetic measurements. The Nb-deficient samples were found to contain traces of a superconducting phase with Tc ˜ 3.5 K. Œ 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
OriginalspracheEnglisch
Seiten (von - bis)3679-3687
Seitenumfang9
FachzeitschriftActa Materialia
Jahrgang53
Ausgabenummer13
DOIs
PublikationsstatusVeröffentlicht - 2005

ÖFOS 2012

  • 104017 Physikalische Chemie

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