Hydrogen induced site depopulation in the LaMgNi4-hydrogen system

The LaMgNi(4)-hydrogen system was investigated by in-situ neutron powder diffraction and pressure-composition isotherm measurements at 100 degrees 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 alpha-LaMgNi(4)H(0.75) phase forms below 0.01 bar hydrogen pressure and H atoms fill one type of tetrahedral Ni(4) interstices. The orthorhombic distorted beta-LaMgNiH(3.7) phase forms at similar to 3 bar hydrogen pressure and H atoms fill both tetrahedral LaNi(3) and triangular bi-pyramidal La(2)MgNi(2) interstices. Interestingly, tetrahedral Ni(4) interstices are no longer occupied. Finally, the most hydrogen rich gamma-LaMgNi(4)H(4.85) phase forms above 20 bar. It has again cubic symmetry and H atoms continue to occupy triangular bipyramidal La(2)MgNi(2) interstices while filling a new type of tetrahedral Ni(4) interstices that are neither occupied in the alpha- nor in the beta-phase. The tetrahedral LaNi3 interstices occupied in the beta-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.