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

The C36 Laves phase Nb1-xCO2+x has been reinvestigated 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 P6(3)/mmc, a = 4.7414(4) angstrom and c = 15.458(1) angstrom 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 approximate to 1050 degrees C and 1264 degrees C, respectively. In addition, the title phase is involved in the peritectoid reaction Co(Nb) + C36 Nb1-xCo2+x = Nb2Co7 at 1086 degrees C and in the eutectic reaction L = Co(Nb) + C36 Nb1-xCo2+x at 1239 degrees 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. Kagome 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 Nb1-xCo2+x excess Co atoms randomly substitute Nb atoms - (Nb1-xCox)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 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 (chi(p) = +1.31 center dot 10(-3) emu mol(-1)). The temperature dependent part of the electrical resistivity rho(300 K) - rho(0) is only 17 mu Omega cm whereas the residual resistivity is very large with rho(0) = 140 mu Omega cm indicating strong structural disorder.