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

Crystal data and results of structure refinement,using powder X-ray diffrac tion and Rietveld analysis, for epsilon-ordered structures near 75 at. % Ga are reported. The structures are based on the AlB2-type with deviation fro m stoichiometry due to pain;vise substitution of rare earth (RE) atoms by g allium. Weak reflections which are forbidden in the AlB2-type structure wer e observed and indicate the existence of positional ordering of Ga-2 pairs located in the basal planes preserving the hexagonal symmetry (S.C. P6/mmm) .This ordering results in a new unit cell with parameters a',c', in which a ' = a root 3 and c' = c. The experimental data of the cells parameters are: a = 7.467(2) Angstrom, c = 4.276(3)Angstrom for Nd1-xGa2-x (x = 0.18), and a = 7.400(3)Angstrom, c = 4.184(2)Angstrom for Gd1-xGa2+2x (x = 0.165). (C ) 1999 International Centre for Diffraction Data. [S0885-7156(98)00204-8].