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Keywords = Zintl phase hydride

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16 pages, 3284 KiB  
Article
Size Matters: New Zintl Phase Hydrides of REGa (RE = Y, La, Tm) and RESi (RE = Y, Er, Tm) with Large and Small Cations
by Anton Werwein, Thomas C. Hansen and Holger Kohlmann
Crystals 2019, 9(11), 600; https://doi.org/10.3390/cryst9110600 - 16 Nov 2019
Cited by 2 | Viewed by 3605
Abstract
Many Zintl phases exhibiting a CrB type structure form hydrides. Systematic studies of AeTtHx (Ae = Ca, Sr, Ba; Tt = Si, Ge, Sn), LnTtHx (Ln = La, Nd; Tt = Si, Ge, Sn), and LnGaH [...] Read more.
Many Zintl phases exhibiting a CrB type structure form hydrides. Systematic studies of AeTtHx (Ae = Ca, Sr, Ba; Tt = Si, Ge, Sn), LnTtHx (Ln = La, Nd; Tt = Si, Ge, Sn), and LnGaHx (Ln = Nd, Gd) showed the vast structural diversity of these systems. Hydrogenation reactions on REGa (RE = Y, La, Tm) and RESi (RE = Y, Er, Tm) were performed in steel autoclaves under hydrogen pressure up to 5 MPa and temperatures up to 773 K. The products were analyzed by X-ray and neutron powder diffraction. RESi (RE = Y, Er, Tm) form hydrides in the C-LaGeD type. LaGaD1.66 is isostructural to NdGaD1.66 and shows similar electronic features. Ga-D distances (1.987(13) Å and 2.396(9) Å) are considerably longer than in polyanionic hydrides and not indicative of covalent bonding. In TmGaD0.93(2) with a distorted CrB type structure deuterium atoms exclusively occupy tetrahedral voids. Theoretical calculations on density functional theory (DFT) level confirm experimental results and suggest metallic properties for the hydrides. Full article
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13 pages, 3719 KiB  
Article
Covalent Si–H Bonds in the Zintl Phase Hydride CaSiH1+x (x ≤ 1/3)
by Henry Auer, Fangshun Yang, Helen Y. Playford, Thomas C. Hansen, Alexandra Franz and Holger Kohlmann
Inorganics 2019, 7(9), 106; https://doi.org/10.3390/inorganics7090106 - 21 Aug 2019
Cited by 7 | Viewed by 3992
Abstract
The crystal structure of the Zintl phase hydride CaSiH≈4/3 was discussed controversially, especially with respect to the nature of the silicon-hydrogen interaction. We have applied X-ray and neutron powder diffraction as well as total neutron scattering on a deuterated sample, CaSiD1.1 [...] Read more.
The crystal structure of the Zintl phase hydride CaSiH≈4/3 was discussed controversially, especially with respect to the nature of the silicon-hydrogen interaction. We have applied X-ray and neutron powder diffraction as well as total neutron scattering on a deuterated sample, CaSiD1.1. Rietveld refinement (CaSiD1.1, Pnma, a = 14.579(4) Å, b = 3.8119(4) Å, c = 11.209(2) Å) and an analysis of the neutron pair distribution function show a silicon-deuterium bond length of 1.53 Å. The Si–H bond may thus be categorized as covalent and the main structural features described by a limiting ionic formula Ca2+H−(Si−)2/3(SiH−)1/3. Hydrogen atoms decorating the ribbon-like silicon polyanion made of three connected zigzag chains are under-occupied, resulting in a composition CaSiH1.1. Hydrogen-poor Zintl phase hydrides CaSiH<1 with hydride ions in Ca4 tetrahedra only were found in an in situ neutron diffraction experiment at elevated temperature. Hydrogen (deuterium) uptake and release in CaSiDx (0.05 ≤ x ≤ 0.17) is a very fast process and takes less than 1 min to complete, which is of importance for possible hydrogen storage applications. Full article
(This article belongs to the Special Issue Structure, Properties, and Bonding in Solid State Compounds)
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12 pages, 3793 KiB  
Article
Hydrogenation Properties of LnAl2 (Ln = La, Eu, Yb), LaGa2, LaSi2 and the Crystal Structure of LaGa2H0.71(2)
by Anton Werwein, Christopher Benndorf, Marko Bertmer, Alexandra Franz, Oliver Oeckler and Holger Kohlmann
Crystals 2019, 9(4), 193; https://doi.org/10.3390/cryst9040193 - 3 Apr 2019
Cited by 11 | Viewed by 4681
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 [...] Read more.
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. Full article
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