Hydrogen-induced atomic rearrangement in MgPd3

The hydrogenation behavior of MgPd3 has been studied by in situ X-ray powder diffraction and by neutron powder diffraction. At room temperature and p ≈500 kPa hydrogen pressure its structure is capable of incorporating up to one hydrogen atom per formula unit (α-MgPd3H≈1), thereby retaining a tetragonal ZrAl3-type metal atom arrangement. Upon heating to 750 K in a hydrogen atmosphere of 610 kPa it transforms into a cubic modification with AuCu3-type metal atom arrangement (image-MgPd3H≈0.7). Neutron diffraction on the deuteride reveals an anion deficient anti-perovskite-type structure (image-MgPd3D0.67, image pm) in which octahedral sites surrounded exclusively by palladium atoms are occupied by deuterium. Complete removal of hydrogen (480 K, 1 Pa) stabilizes a new binary modification (image-MgPd3, image pm) crystallizing with a primitive cubic AuCu3-type structure. Mechanical treatment (grinding) transforms both image and image modifications of MgPd3 into a cubic face-centered solid solution Mg0.25Pd0.75 showing a random distribution of magnesium and palladium atoms.