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Hydrogenous Zintl Phase Ba3Si4Hx (x=1-2): Transforming Si-4 Butterfly Anions into Tetrahedral Moieties
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
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2015 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 54, no 3, 756-764 p.Article in journal (Refereed) Published
Abstract [en]

The hydride Ba3Si4Hx (x = 1-2) was prepared by sintering the Zintl phase Ba3Si4, which contains Si-4(6-) butterfly-shaped polyanions, in a hydrogen atmosphere at pressures of 10-20 bar and temperatures of around 300 degrees C. Initial structural analysis using powder neutron and X-ray diffraction data suggested that Ba3Si4Hx adopts the Ba3Ge4C2 type [space group I4/mcm (No. 140), a approximate to 8.44 angstrom, c approximate to 11.95 angstrom, Z = 8] where Ba atoms form a three-dimensional array of corner-condensed octahedra, which are centered by H atoms. Tetrahedron-shaped Si-4 polyanions complete a perovskite-like arrangement. Thus, hydride formation is accompanied by oxidation of the butterfly polyanion, but the model with the composition Ba3Si4H is not charge-balanced. First-principles computations revealed an alternative structural scenario for Ba3Si4Hx, which is based on filling pyramidal Ba-5 interstices in Ba3Si4. The limiting composition is x = 2 [space group P4(2)/mmm (No. 136), a approximate to 8.4066 angstrom, c approximate to 12.9186 angstrom, Z = 8], and for x > 1, Si atoms also adopt tetrahedron-shaped polyanions. Transmission electron microscopy investigations showed that Ba3Si4Hx is heavily disordered in the c direction. Most plausible is to assume that Ba3Si4Hx has a variable H content (x = 1-2) and corresponds to a random intergrowth of P- and I-type structure blocks. In either form, Ba3Si4Hx is classified as an interstitial hydride. Polyanionic hydrides in which H is covalently attached to Si remain elusive.

Place, publisher, year, edition, pages
2015. Vol. 54, no 3, 756-764 p.
National Category
Chemical Sciences
Research subject
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:su:diva-115692DOI: 10.1021/ic501421uISI: 000348887400009PubMedID: 25247666OAI: oai:DiVA.org:su-115692DiVA: diva2:799309
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AuthorCount:7;

Available from: 2015-03-30 Created: 2015-03-27 Last updated: 2017-09-13Bibliographically approved
In thesis
1. Investigating Hydrogenous Behavior of Zintl Phases: Interstitial Hydrides, Polyanionic Hydrides, Complex Hydrides, Oxidative Decomposition
Open this publication in new window or tab >>Investigating Hydrogenous Behavior of Zintl Phases: Interstitial Hydrides, Polyanionic Hydrides, Complex Hydrides, Oxidative Decomposition
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is an investigation into the hydrogenous behavior of Zintl phases. Zintl phases are comprised of an active metal (i.e alkali, alkaline earth, and rare earth) and a p-block element. The discussion gives an overview of the influence hydrogen affects the electronic and geometric structure of Zintl phases and subsequent properties. Incorporation of hydrogen into a Zintl phase is categorized as either polyanionic or interstitial Zintl phase hydrides. In the former the hydrogen covalently bonds to the polyanion and in the latter the hydrogen behaves hydridic, coordinates exclusively with the active metal, leading to an oxidation of the polyanion. Synthesis of hydrogenous Zintl phases may be through either a direct hydrogenation of a Zintl phase precursor or by combining active metal hydrides and p-block elements. The latter strategy typically leads to thermodynamically stable hydrides, whereas the former supports the formation of kinetically controlled products. 

Polyanionic hydrides are exemplified by SrAlGeH and BaAlGeH. The underlying Zintl phases SrAlGe and BaAlGe have a structure that relates to the AlB2 structure type. These Zintl phases possess 9 valence electrons for bonding and, thus, are charge imbalanced species. Connected to the charge imbalance are superconductive properties (the Tc of SrAlGe and BaAlGe is 6.7 and 6.3 °C, respectively). In the polyanionic hydrides the hydrogen is covalently bonded as a terminating ligand to the Al atoms. The Al and Ge atoms in the anionic substructure [AlGeH]2- form corrugated hexagon layers. Thus, with respect to the underlying Zintl phases there is only a minimal change to the arrangement of metal atoms. However, the electronic properties are drastically changed since the Zintl phase hydrides are semiconductors. 

Interstitial hydrides are exemplified by Ba3Si4Hx (1 < x < 2) which was obtained from the hydrogenation of the Zintl phase Ba3Si4. Ba3Si4 contains a Si46- “butterfly” polyanion. Hydrogenation resulted in a disordered hydride in which blocks of two competing tetragonal structures are intergrown. In the first structure the hydrogen is located inside Ba6 octahedra (I-Ba3Si4H), and in the second structure the hydrogen is located inside Ba5 square pyramids (P-Ba3Si4H2). In both scenarios the “butterfly anions appear oxidized and form Si44- tetrahedra.

Hydrogenation may also be used as a synthesis technique to produce p-block element rich Zintl phases, such as silicide clathrates. During hydrogenation active metal is removed from the Zintl phase precursor as metal hydride. This process, called oxidative decomposition, was demonstrated with RbSi, KSi and NaSi. Hydrogenation yielded clathrate I at 300 °C and 500 °C for RbSi and KSi, respectively. Whereas a mixture of both clathrate I and II resulted at 500 °C for NaSi. 

Low temperature hydrogenations of KSi and RbSi resulted in the formation of the silanides KSiH3 and RbSiH3. These silanides do not represent Zintl phase hydrides but are complex hydrides with discrete SiH3- complex species. KSiH3 and RbSiH3 occur dimorphic, with a disordered α-phase (room temperature; SG Fm-3m) and an ordered β-phase (below -70 °C; SG = Pnma (KSiH3); SG = P21/m ( RbSiH3)). During this thesis the vibrational properties of the silyl anion was characterized. The Si–H stretching force constants for the disordered α-phases are around 2.035 Ncm-1 whereas in the ordered b-forms this value is reduced to ~1.956 Ncm-1. The fact that SiH3- possesses stronger Si-H bonds in the α-phases was attributed to dynamic disorder where SiH3- moieties quasi freely rotate in a very weakly coordinating alkali metal ion environment.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry (MMK), Stockholm University, 2017. 104 p.
Keyword
Zintl phases, Hydrogenous Zintl phases, Polyanionic Zintl phases, Interstitial Zintl phases
National Category
Inorganic Chemistry
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-146850 (URN)978-91-7797-006-4 (ISBN)978-91-7797-007-1 (ISBN)
Public defence
2017-10-27, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 13:00 (English)
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Available from: 2017-10-04 Created: 2017-09-13 Last updated: 2017-11-10Bibliographically approved

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