Abstract
This work focused on the high pressure PCT and in situ neutron powder diffraction studies of the LaMg2Ni9-H2 (D2) system at pressures up to 1,000 bar. LaMg2Ni9 alloy was prepared by a powder metallurgy route from the LaNi9 alloy precursor and Mg powder. Two La3−xMgxNi9 samples with slightly different La/Mg ratios were studied, La1.1Mg1.9Ni9 (sample 1) and La0.9Mg2.1Ni9 (sample 2). In situ neutron powder diffraction studies of the La1.09Mg1.91Ni9D9.5 (1) and La0.91Mg2.09Ni9D9.4 (2) deuterides were performed at 25 bar D2 (1) and 918 bar D2 (2). The hydrogenation properties of the (1) and (2) are dramatically different from those for LaNi3. The Mg-containing intermetallics reversibly form hydrides with DHdes = 24.0 kJ/molH2 and an equilibrium pressure of H2 desorption of 18 bar at 20 °C (La1.09Mg1.91Ni9). A pronounced hysteresis of H2 absorption and desorption, ~100 bar, is observed. The studies showed that LaNi5-assisted hydrogenation of MgNi2 in the LaMg2Ni9 hybrid structure takes place. In the La1.09Mg1.91Ni9D9.5 (1) and La0.91Mg2.09Ni9D9.4 (2) (a = 5.263/5.212; c = 25.803/25.71 Å) D atoms are accommodated in both Laves and CaCu5-type slabs. In the LaNi5 CaCu5-type layer, D atoms fill three types of interstices; a deformed octahedron [La2Ni4], and [La(Mg)2Ni2] and [Ni4] tetrahedra. The overall chemical compositions can be presented as LaNi5H5.6/5.0 + 2*MgNi2H1.95/2.2 showing that the hydrogenation of the MgNi2 slab proceeds at mild H2/D2 pressure of just 20 bar. A partial filling by D of the four types of the tetrahedral interstices in the MgNi2 slab takes place, including [MgNi3] and [Mg2Ni2] tetrahedra.
Highlights
Despite significant differences in chemistry between La and Mg, magnesium forms a very extensive solid solution in the LaNi3 intermetallic alloy, crystallizing with a PuNi3 type trigonal structure
67% of La atoms can be replaced by Mg to form a LaMg2Ni9 intermetallic compound
The LaNi3 crystal structure is formed by a stacking of the LaNi5 (Haucke CaCu5 type) and MgNi2 (Laves type) slabs along the trigonal 00z axis (LaNi5 + 2MgNi2 = LaMg2Ni9)
Summary
Despite significant differences in chemistry between La and Mg, magnesium forms a very extensive solid solution in the LaNi3 intermetallic alloy, crystallizing with a PuNi3 type trigonal structure. 67% of La atoms can be replaced by Mg to form a LaMg2Ni9 intermetallic compound. The LaNi3 crystal structure is formed by a stacking of the LaNi5 (Haucke CaCu5 type) and MgNi2 (Laves type) slabs along the trigonal 00z axis (LaNi5 + 2MgNi2 = LaMg2Ni9). The building blocks of LaMg2Ni9—LaNi5 and MgNi2—are well characterized individually as hydride-forming intermetallic compounds. The thermodynamics and structural features of their interaction with hydrogen are quite different. LaNi5 forms a saturated LaNi5H6.7 hydride and shows a reversible interaction with hydrogen at hydrogen pressures slightly exceeding atmospheric pressure. Hydrogen atoms fill tetrahedral La2Ni2, LaNi3 and Ni4 sites in the hydride crystal structure [3]
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