Structural studies of hydrides of Zr2Cu, Zr2Pd and LaPtIn

Abstract

In this work hydrides formed from three different intermetallics compounds, Zr2Cu, Zr2Pd and LaPtIn, were investigated, with focus on hydrogen induced structural changes. The main methods were neutron (PND) and synchrotron powder diffraction (SR-PXD).

Zr2Cu absorbs hydrogen up to Zr2CuD4.5, and the saturated hydride structure differs from the tetragonal \cu structure. In this work saturated \cu hydride was synthesized and studied by PND and high resolution SR-PXD. Desorption of saturated Zr2Cu hydride was investigated in situ by SR-PXD. The low decomposition temperature of the system complicated investigation of possible intermediate hydride phases, but the observed changes in unit cell parameters below the decomposition temperature might have indicated the existence of such a phase. A sudden change in behaviour of the cell parameters was observed, consistent with release of hydrogen from Zr3Cu2 coordinated sites (D5). A Zr4Cu2 oxide not previously reported in this system was discovered in the intermetallic samples. The kinetics of the system was found to change on cycling.

Zr2Pd absorbs hydrogen up to Zr2PdD~3. Zr2PdD~2 is known to retain the tetragonal Zr2Pd structure, but for hydrogen contents both below and above this the structure was not clear. In this work three Zr2PdDx samples with x below 2, x~2 and x~3 were studied by PND. Saturated Zr2Pd hydride was also studied in situ under hydrogen pressure by PND. Desorption of saturated Zr2Pd hydride was investigated in situ by SR-PXD. Zr2PdDx, x<2 was determined to retain the tetragonal intermetallic type structure. For Zr2PdDx, x>2 the structure transforms. The saturated hydride type structure appeared to be close to the tetragonal intermetallics type structure.

From calculations LaPtIn had been predicted to form hydrides with H—H separations shorter than ever seen before. To test this LaPtIn was hydrogenated at pressures up to 182 bar and temperatures up to 400 oC. LaPtIn did not form hydrides with extraordinary short H--H separations under these conditions. At pressure 182 bar and temperature 400 oC LaPtInH0.53 was formed. In this hydride hydrogen was found to occupy the tetrahedral 4h sites, in such a way that voids sharing a common face is avoided. Refined parameters for LaPtInH0.53 was a=7.8229(2) Å, c=4.1647(2) Å and V=220.73(1) Å3. Pt was also attempted partially substituted with Ni, but homogeneous intermetallics samples could not be obtained.