Abstract
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.
Highlights
Zintl phases are polar intermetallic compounds formed by alkali or alkaline earth metals M and a main group element X of group 13–16
Zintl phases and related compounds have recently been investigated for potential application, for example as thermoelectric materials (e.g., Mg2 Si [7]), in magnetocaloric devices (e.g., Gd5 Si4 [8]) or in thin-film solar cells (e.g., BaSi2 [9])
Intermetallic compounds with lanthanum were synthesized from the elements in stoichiometric amounts (La: smart-elements (Vienna, Austria), ≥99.99%, surface cleaned mechanically, cut into small pieces; Si: abcr (Karlsruhe, Germany), 99.9999%; Al, Ga: Smart elements, 99.99%) by fusing the starting materials in a water-cooled copper crucible of an arc furnace under 80 kPa argon atmosphere
Summary
Zintl phases are polar intermetallic compounds formed by alkali or alkaline earth metals M and a main group element X of group 13–16. According to the Zintl–Klemm concept, M atoms formally transfer valence electrons to relatively electronegative X atoms, which form polyanions with the connectivity of an isoelectronic element [1,2,3,4,5,6]. K4 Si4 is an archetypal example with (Si4 )4− tetrahedra akin to P4 tetrahedra in white phosphorous [2]. Zintl phases and related compounds have recently been investigated for potential application, for example as thermoelectric materials (e.g., Mg2 Si [7]), in magnetocaloric devices (e.g., Gd5 Si4 [8]) or in thin-film solar cells (e.g., BaSi2 [9]).
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