In-situ X-ray diffraction study of the decomposition of NaAlH4

Dehydrogenation kinetics of pure and nickel-doped magnesium hydride investigated by in situ time-resolved powder X-ray diffraction

In situ X-ray diffraction measurements during stepwise hydrogenation of the intermetallic compound ZrNi confirmed the existence of a stable compound ZrNiH. Neutron diffraction measurements on ZrNiD allowed a determination of the site occupied by deuterium. This site is tetrahedrally coordinated by four zirconium atoms as predicted earlier from a model based solely on geometric considerations. Occupation of this site causes the lattice to distort from an orthorhombic to a triclinic structure. The trihydride is formed when two completely different types of sites are filled, to the exclusion of the sites occupied originally, causing reversion of the lattice to an orthorhombic structure. Other confirmed predictions of the geometric model are discussed.

Interstitial site occupation in ZrNiH

Thermodynamic analysis of the hydriding process of Mg-Ni alloys

v-ray powder diffraction patterns of the monoclinic low-temperature phase of Mg 2 NiH 4 were analysed in terms of a structure model which allowed for microtwinning. In contrast to previous studies, the intensity profiles of reflections that were broadened due to twinning and of reflections that had been attributed to a second, orthorhombic phase could be reproduced. A structure refinement based on this model gave a better fit ( R p = 0.12, R F = 0.04) and more accurate metal atom parameters than those reported previously. In samples which were vuenched from temperatures above 300 °C the average distance between twin planes was found to be a few lattice translation periods.

Low-temperature structure of Mg2NiH4: Evidence for microtwinning

The crystal symmetry of the reaction product of hydrogen with the intermetallic compound Mg2Ni has been determined to be tetragonal with unit cell dimensions: a=6.533 AA and c=7.499 AA, by in situ X-ray diffractometry at about 623K and under 4.6 MPa hydrogen pressure. The resultant hydride phase transforms at 503K to an orthorhombic structure with lattice dimensions: a=11.394 AA, b=11.196 AA and c=9.165 AA. This phase transition is of polymorphic type as is evident from the well defined exothermic (endothermic) peak in the pressure-differential scanning calorimetry curves obtained during cooling (heating) after the formation of the nearest stoichiometric hydride Mg2NiH4. The results are briefly compared with previously reported data.

https://iopscience.iop.org/article/10.1088/0305-4608/11/3/002/pdf

T Hirata

Papers

In situ X-ray diffractometry study of the hydride in the intermetallic compound Mg2Ni