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 s ynthesized at 1073 K and checked by X-ray powder diffraction. The study sho ws additional linea evidencing the ordering of the Gz vacancies. In all the investigated systems (except Yb-Ge), the superstructure lines may be index ed considering propagating vectors (q(x), 0, q(z)) with respect to the orth o-hexagonal subcell of the AlB2 structure. The q(x) and q(z) values allow t he characterization of 16 new ordered AlB2-type derivatives. The atomic coo rdinates of the Tm2Ge3 structure (q(x)=0.5 and q(z)=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 e arth size and the electronic factor are discussed. (C) 1999 Elsevier Scienc e B.V. All rights reserved.