Crystal Structure Analysis of La2Ni6CoD(x) During Deuterium Absorption Process.

The crystal structures of La2Ni6CoD(x) (x = 5.2 and 9.6) were determined by in situ neutron diffraction along the P-C isotherm. La2Ni6CoD(5.2) (phase I) was found to be orthorhombic with lattice parameters a = 0.500670(2) nm, b = 0.867211(4) nm, and c = 2.99569(7) nm. The 10 deuterium sites were located in the MgZn2-type and CaCu5-type cells, with deuterium contents of 0.95 D/M and 0.39 D/M, respectively. The full deuteride La2Ni6CoD(9.6) (phase II) was monoclinic with lattice parameters a = 0.516407(3) nm, b = 0.894496(6) nm, c = 3.11206(1) nm, and β = 90.15(1)°. The phase II had 11 sites for deuterium occupation. The deuterium contents of the MgZn2-type and the CaCu5-type cell were 1.63 D/M and 0.78 D/M, respectively. The sequence of phase transformation of La2Ni6Co was hexagonal, followed by orthorhombic (phase I), and then monoclinic (phase II), for the first absorption process. The phase transformation resulted in lowered symmetry and the variation of deuterium atom occupation.