Order-disorder phase transition in the deuterated hexagonal (C14-type) Laves phase ZrCr2D3.8

We have performed a neutron-diffraction study of hydrogen-disordered and hy drogen-ordered phases in the hexagonal (C14-type) hydrogen solid solution Z rCr2D3.8. The disordered phase exists above 250 K. At room temperature, hyd rogen is located mainly in the 2Zr + 2Cr interstices as well as, about 6%, in the 1Zr + 3Cr ones. At low temperature, a superstructure with two propag ation vectors, k(1) = (1/3 1/3 1/3) and k(2) = (-2/3 -2/3 -2/3), forms in t he hydrogen sublattice, with only the 2Zr + 2Cr interstices being occupied. The superstructure belongs to space group R3 and has the stoichiometric co mposition ZrCr2D4. Unlike the known superstructure ZrCr2D4 in the cubic (C1 5-type) hydrogen solid solution, its formation is determined by only short- range hydrogen-hydrogen interaction. (C) 2000 Elsevier Science S.A. All rig hts reserved.