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

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) A and c = 15.458(1) A 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. kagome 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. (orig.)