Vacancy ordering in AlB2-type RGe2-x compounds (R=Y,Nd,Sm,Gd-Lu)

The AlB2-type RGe2-x compounds (0.33<x<0.5; R=Y,Nd,Sm,Gd-Lu) have been synthesized at 1073 K and checked by X-ray powder diffraction. The study shows additional lines evidencing the ordering of the Ge vacancies. In all the investigated systems (except Yb-Ge), the superstructure lines may be indexed considering propagating vectors (qx, 0, qz) with respect to the ortho-hexagonal subcell of the AlB2 structure. The qx and qz values allow the characterization of 16 new ordered AlB2-type derivatives. The atomic coordinates of the Tm2Ge3 structure (qx=0.5 and qz=0.25) have been refined by the Rietveld method whereas structural hypotheses are proposed for the other compounds. The evolution of the wavevector components with the rare earth size and the electronic factor are discussed. (orig.)