In situ Neutron Powder Diffraction on Intermediate Hydrides of MgPd3 in a Novel Sapphire Gas Pressure Cell

Abstract

AbstractA novel gas pressure cell for in situ neutron powder diffraction has been developed. It is based on a single crystal sapphire tube as a sample holder, allows a 360° unobstructed access by the neutron beam and has little background contribution. This device was used to study the hydrogenation of α‐MgPd3, which undergoes a hydrogen driven rearrangement from a ZrAl3 to a AuCu3 type structure. Deuterium could be located and a strong preference of [Pd6] voids was found in α‐MgPd3D0.79 under 5 bar and in α‐MgPd3D0.94 under 20 bar deuterium pressure. The crystal structure may be described as a new defect superstructure variant of the NaCl type. In situ thermal analysis under 5 bar hydrogen pressure showed that both the hydrogen uptake of α‐MgPd3, which is complete at temperatures below 450 K, and the transformation to the hydride of cubic β‐MgPd3, starting around 550 K, are exothermic. This completion of the hydrogenation‐dehydrogenation series of MgPd3 suggests, that the rearrangement of the metal structure proceeds by a hydrogen assisted gliding mechanism with a shift vector of ${1 \over 2}$[110]. This is also supported by quantum chemical calculations, which show a decohesion of the intermetallic structure upon hydrogenation accompanied by the appearance of Pd–H bonding interactions.