Hydrogenation Properties of LnAl2 (Ln = La, Eu, Yb), LaGa2, LaSi2 and the Crystal Structure of LaGa2H0.71(2)

Many Zintl phases take up hydrogen and form hydrides. Hydrogen atoms occupy interstitial sites formed by alkali or alkaline earth metals and / or bind covalently to the polyanions. The latter is the case for polyanionic hydrides like SrTr2H2 (Tr = Al, Ga) with slightly puckered honeycomb-like polyanions decorated with hydrogen atoms. This study addresses the hydrogenation behavior of LnTr2, where the lanthanide metals Ln introduce one additional valence electron. Hydrogenation reactions were performed in autoclaves and followed by thermal analysis up to 5.0 MPa hydrogen gas pressure. Products were analyzed by powder X-ray and neutron diffraction, transmission electron microscopy, and NMR spectroscopy. Phases LnAl2 (Ln = La, Eu, Yb) decompose into binary hydrides and aluminium-rich intermetallics upon hydrogenation, while LaGa2 forms a ternary hydride LaGa2H0.71(2). Hydrogen atoms are statistically distributed over two kinds of trigonal-bipyramidal La3Ga2 interstitials with 67% and 4% occupancy, respectively. Ga-H distances (2.4992(2) Å) are considerably longer than in polyanionic hydrides and not indicative of covalent bonding. 2H solid-state NMR spectroscopy and theoretical calculations on Density Functional Theory (DFT) level confirm that LaGa2H0.7 is a typical interstitial metallic hydride.

Hydrogenation Properties of Laves Phases LnMg2 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb)

The hydrogenation properties of Laves phases LnMg₂ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb) were investigated by thermal analysis, X-ray, synchrotron, and neutron powder diffraction. At 14.0 MPa hydrogen gas pressure and 393 K, PrMg₂ and NdMg₂ take up hydrogen and form the colorless, ternary hydrides PrMg₂H₇ (P4₁2₁2, a = 632.386(6) pm, c = 945.722(11) pm) and NdMg₂H₇ (P4₁2₁2, a = 630.354(9) pm, c = 943.018(16) pm). The crystal structures were refined by the Rietveld method from neutron powder diffraction data on the deuterides (PrMg₂D₇, P4₁2₁2, a = 630.56(2) pm, c = 943.27(3) pm; NdMg₂D₇, P4₁2₁2, a = 628.15(2) pm, c = 940.32(3) pm) and shown to be isotypic to LaMg₂D₇. The LaMg₂D₇ type of hydrides decompose at 695 K (La), 684 K (Ce), 684 K (Pr), 672 K (Nd), and 639 K (Sm) to lanthanide hydrides and magnesium. The Laves phase EuMg₂ forms a hydride EuMg₂H_x of black color. Its crystal structure (P2₁2₁2₁, a = 664.887(4) pm, b = 1136.993(7) pm, c = 1069.887(7) pm) is closely related to the hexagonal Laves phase (MgZn₂ type) of the hydrogen-free parent intermetallic. GdMg₂ and TbMg₂ form hydrides GdMg₂H_x with orthorhombic unit cells (a = 1282.7(4) pm, b = 572.5(2) pm, c = 881.7(2) pm) and TbMg₂H_x (a = 617.8(3) pm, b = 1045.8(8) pm, c = 997.1(5) pm), presumably also with a distorted MgZn₂ type of structure. CeMg₂H₇ and NdMg₂H₇ are paramagnetic with effective magnetic moments of 2.49(1) μ_B and 3.62(1) μ_B, respectively, in good agreement with the calculated magnetic moments of the free trivalent rare-earth cations (μ_calc(Ce³⁺) = 2.54 μ_B; μ_calc(Nd³⁺) = 3.62 μ_B).