Crystal Structure of Pr₃MgNi₁₄D_x Studied by in Situ Neutron Diffraction

The crystal structure of Pr₃MgNi₁₄D₁₈ was determined by neutron diffraction. The determined structure of Pr₃MgNi₁₄D₁₈ consisted of 89.0% Gd₂Co₇-type structure and 11.0% PuNi₃-type structure. The lattice parameters of a and c of Gd₂Co₇-type structure were refined at 0.52903(7) nm and 3.90179(1) nm. The deuterium atoms were distributed among nine deuterium sites in both the CaCu₅-type and MgZn₂-type cells. The D2 occupancy in the Pr₂Ni₄ octahedral sites of the CaCu₅-type cell was the largest (0.75) when compared with the other deuterium sites (<0.49). The deuterium content of the CaCu₅-type cell showed 0.75 D/M, but the D/M value of the MgZn₂-type cell was 1.53. The volume expansions during deuteration of the CaCu₅-type and MgZn₂-type cells were nearly equal. The cyclic hydrogenation property of Pr₃MgNi₁₄ is comparable to that of LaNi₅. It is inferred that the similar expansion behavior of the CaCu₅-type and MgZn₂-type cells during deuteration is the origin of this cyclic stability.