Positional ordering in the light RE-Ga systems (RE=rare earth element)

Crystal data and results of structure refinement, using powder X-ray diffraction and Rietveld analysis, for (epsilon)-ordered structures near 75 at. % Ga are reported. The structures are based on the AlB2-type with deviation from stoichiometry due to pairwise substitution of rare earth (RE) atoms by gallium. Weak reflections which are forbidden in the AlB2-type structure were observed and indicate the existence of positional ordering of Ga2 pairs located in the basal planes preserving the hexagonal symmetry (S.G. P6/mmm). This ordering results in a new unit cell with parameters aʹ,cʹ, in which aʹ=a(SQRT)3 and cʹ=c. The experimental data of the cells parameters are: a=7.467(2)A, c=4.276(3)A for Nd1-xGa2-x, (x=0.18), and a=7.400(3)A, c=4.184(2)A for Gd1-xGa2+2x (x=0.165).