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Publicly Available Published by De Gruyter (O) September 25, 2009

Preparation, phase stability and structure of the C36 Laves phase Nb1–xCo2+x

  • Daniel Grüner , Frank Stein , Martin Palm , Joachim Konrad , Alim Ormeci , Walter Schnelle , Yuri Grin and Guido Kreiner

Abstract

The C36 Laves phase Nb1–xCo2+x has been re-investigated in order to study phase stability and structure. Constitutional data have been obtained by investigating homogenized single- and two-phase samples and from diffusion couples. The C36 phase Nb1–xCo2+x crystallizes with hexagonal MgNi2 structure type (Z = 8, space group P63/mmc, a = 4.7414(4) Å and c = 15.458(1) Å at x = 0.265(4)). The homogeneity range extends from 24.6(2) to 25.3(5) at% Nb. The temperature range of the phase field is limited by a eutectoid (C36 Nb1–xCo2+x = Nb2Co7 + C15 NbCo2) and a peritectic (L + C15 NbCo2 = C36 Nb1–xCo2+x) reaction at ∼ 1050 °C and 1264 °C, respectively. In addition, the title phase is involved in the peritectoid reaction Co(Nb) + C36 Nb1–xCo2+x = Nb2Co7 at 1086 °C and in the eutectic reaction L = Co(Nb) + C36 Nb1–xCo2+x at 1239 °C. The C36 and C15 Laves phases of the Nb—Co system are separated by an extremely small two-phase field (<0.5 at%). The crystal structure exhibits pronounced deviations from ideal parameters obtained from a hard sphere model. The Co network displays a type of distortion known from many hexagonal Laves phases. Kagom, layers display an elongation of the Co—Co edges of the basal triangles of Co5 trigonal bipyramids and a contraction of Co—Co edges of the uncapped triangles. The Nb atoms are also displaced from their idealized sites. In the crystal structure of C36 Nb1–xCo2+x excess Co atoms randomly substitute Nb atoms — (Nb1–xCox)Co2. The excess Co atoms occupy preferentially the Nb2 site approximately twice as much as Nb1. These positions differ mainly in the conformation of the corresponding Nb6Nb2 fragments (Nb1—Nb1 eclipsed and Nb2–Nb2 staggered). In addition, Co atoms are displaced from the original Nb positions. The distortion of the Co and the Nb network is a consequence of the bonding situation of the defect-free crystal structure. The preferential site occupation of excess Co atoms is triggered by interactions with atoms beyond the first coordination shell. The C36 phase Nb1–xCo2+x exhibits Pauli-paramagnetic behavior (χP = +1.31 · 10–3 emu mol–1). The temperature dependent part of the electrical resistivity ρ(300 K) – ρ0 is only 17 μΩ cm whereas the residual resistivity is very large with ρ0 = 140 μΩ cm indicating strong structural disorder.

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Published Online: 2009-9-25
Published in Print: 2006-5-1

© by Oldenbourg Wissenschaftsverlag, München

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