Evolution of the non-stoichiometry in the Er(Ge1-xGax)(approximate to 2) compounds - Crystal structure of Er-4(Ge, Ga)(7), a new hexagonal AlB2 derivative

The Er-Ga-Ge system has been investigated around the Er(Ga,Ge)(2) compositi on. The AlB2-type solid solution is not continuous from ErGa2 to ErGe2-x. T he ternary compounds with weak gallium concentration display monoclinic dis tortion of the AlB2 subcell and the ordering of the (Ge, Ga) vacancies is c haracterized by (q(x), 0, q(z)) propagating vectors as observed in the bina ry RGe2-x compounds. The minimum q(x) values (related to the (Ge, Ga) conce ntration) are smaller than the minimum values measured in the binary ErGe2- x compounds thus indicating that the (Ge, Ga) concentration increases with the Ge substitution. For larger Ga contents (approximate to 10%), a new hex agonal ordered derivative characterized by a four-times larger cell is stab ilized. The idealized formula Er-4(Ga, Ge)(7) indicates a significant decre ase of the vacancy concentration. These results suggest that metric and ele ctronic criteria control the vacancy concentration. (C) 1999 Elsevier Scien ce S.A. All rights reserved.