Hydrogen induced site depopulation in the LaMgNi4-hydrogen system
Chotard, Jean-Noël ; Sheptyakov, Denis ; Yvon, Klaus
In: Zeitschrift für Kristallographie, 2008, vol. 223, no. 10, p. 690-696
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- The LaMgNi4-hydrogen system was investigated by in-situ neutron powder diffraction and pressure-composition isotherm measurements at 100 °C and hydrogen (deuterium) pressures of up 50 bar. The system displays three hydride phases that have distinctly different hydrogen plateau pressures and H atom distributions. The cubic α-LaMgNi4H0.75 phase forms below 0.01 bar hydrogen pressure and H atoms fill one type of tetrahedral Ni4 interstices. The orthorhombic distorted β-LaMgNiH3.7 phase forms at about 3 bar hydrogen pressure and H atoms fill both tetrahedral LaNi3 and triangular bi-pyramidal La2MgNi2 interstices. Interestingly, tetrahedral Ni4 interstices are no longer occupied. Finally, the most hydrogen rich γ-LaMgNi4H4.85 phase forms above 20 bar. It has again cubic symmetry and H atoms continue to occupy triangular bi-pyramidal La2MgNi2 interstices while filling a new type of tetrahedral Ni4 interstices that are neither occupied in the α- nor in the β-phase. The tetrahedral LaNi3 interstices occupied in the β-phase are empty. Hydrogen induced depopulations of interstitial sites in metal hydrides are relatively rare and consistent with, but not entirely due to, the onset of repulsive H-H interactions at increasing hydrogen concentrations