STRUCTURE REFINEMENT ON SINGLE CRYSTALS OF Y(Ni1-xMx)2B2C (M=Co, Cu)
U. Jaenicke-Roessler 1,
G. Zahn 1, P. Paufler 1
and H. von Lips 2
1 Institut
für Kristallographie und Festkörperphysik, Technische
Universität Dresden, D-01062 Dresden, Germany
2 Institut für Festkörper-
und Werkstofforschung, Postfach 270016,
D-01171 Dresden, Germany
Keywords: single crystals; structure; rare earth;
quaternary borocarbide; YNi2B2C;
superconductivity
Single crystalline samples of Y(Ni1-xMx)2B2C (M=Co [x=0.1, 0.2, 0.3], Cu [x=0.1, 0.2]) were examined by monochromatized MoKa radiation on a four-circle-diffractometer at room temperature. Precise lattice parameters were determined by a profile-fitting method. Although they differ from the values reported for powder samples elsewhere [1], the general tendency, that the lattice parameters a and c decrease for increasing percentage of Co and increase with increasing percentage of Cu, is the same. The relative change of the c axis is much higher than that of the a axis. The unit cell volume VUC, as well as the axial ratio c/a, increase with x for Cu and decrease with x for Co.
The critical temperature of superconductivity TC is reported to decrease with x (for both Co and Cu) [1,2,3,4]. Structure refinements show a change of the bonding angle B-Ni-B in the NiB4 tetrahedron between 106.9° and 107.6°. This change of angle has been found to be accompanied by a change of the atomic coordinate zB of B.
Refinements of occupancy factors confirm that the dopant atoms are located at Ni sites. No further improvement of the refinements could be achieved by assuming Co or Cu atoms to take interstitial positions. The determined occupancy factors are in good agreement with the Co and Cu nominal compositions.
Agreement factors R of the least square refinements lie
between 1.28% and 2.46%.