Crystal structure, layer defects, and the origin of plastic deformability of Nb2Co7

The crystal structure of Nb2Co7 was identified on the basis of X-ray powder-diffraction data recorded by synchrotron radiation to be of the Zr2Ni7 type having monoclinic C2/m symmetry and Pearson symbol mC18 with lattice parameters a = 4.5874(2) Å, b = 8.1509(4) Å, c = 6.2223(3) Å and β = 107.181(3)°. Fractional coordinates were determined on the basis of Rietveld-refinement methods and geometrical considerations. The structure can be described by stacking of certain extended layer sandwiches parallel to (001). Each of these extended layer sandwiches consists of a central Kagomé layer composed of Co atoms only, which is sandwiched by two close-packed layers consisting of Co and Nb atoms. The occurrence of Warren peaks as well as of line broadening is attributed to a stacking disorder of adjacent layer extended sandwiches in the investigated specimen. The unexpected plastic deformability of the intermetallic compound Nb2Co7 can be related to specific features of the layered structure model, which provides a glide plane parallel to (001).