Preparation, phase stability and structure of the C36 Laves phase Nb_1–xCo_2+x

Abstract

Abstract The C36 Laves phase Nb 1–x Co 2+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 Nb 1–x Co 2+x crystallizes with hexagonal MgNi 2 structure type ( Z = 8, space group P 6 3 / 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 Nb 1–x Co 2+x = Nb 2 Co 7 + C15 NbCo 2 ) and a peritectic (L + C15 NbCo 2 = C36 Nb 1–x Co 2+x ) reaction at ∼ 1050 °C and 1264 °C, respectively. In addition, the title phase is involved in the peritectoid reaction Co(Nb) + C36 Nb 1–x Co 2+x = Nb 2 Co 7 at 1086 °C and in the eutectic reaction L = Co(Nb) + C36 Nb 1–x Co 2+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 Co 5 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 Nb 1–x Co 2+x excess Co atoms randomly substitute Nb atoms — (Nb 1–x Co x )Co 2 . 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 Nb 6 Nb 2 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 Nb 1–x Co 2+x exhibits Pauli-paramagnetic behavior ( χ P = +1.31 · 10 –3 emu mol –1 ). The temperature dependent part of the electrical resistivity ρ (300 K) – ρ 0 is only 17 μΩ cm whereas the residual resistivity is very large with ρ 0 = 140 μΩ cm indicating strong structural disorder.