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Investigating Hydrogenous Behavior of Zintl Phases: Interstitial Hydrides, Polyanionic Hydrides, Complex Hydrides, Oxidative Decomposition
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
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. , p. 104
Keyword [en]
Zintl phases, Hydrogenous Zintl phases, Polyanionic Zintl phases, Interstitial Zintl phases
National Category
Inorganic Chemistry
Research subject
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:su:diva-146850ISBN: 978-91-7797-006-4 (print)ISBN: 978-91-7797-007-1 (electronic)OAI: oai:DiVA.org:su-146850DiVA, id: diva2:1141025
Public defence
2017-10-27, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2017-10-04 Created: 2017-09-13 Last updated: 2017-11-10Bibliographically approved
List of papers
1. Hydrogenous Zintl Phases: Interstitial Versus Polyanionic Hydrides
Open this publication in new window or tab >>Hydrogenous Zintl Phases: Interstitial Versus Polyanionic Hydrides
2011 (English)In: Zintl Phases: Principles and Recent Developments / [ed] Thomas F. Fässler, Springer Berlin/Heidelberg, 2011, Vol. 139, p. 143-161Chapter in book (Refereed)
Abstract [en]

Hydrogen may be incorporated in Zintl phases in two different ways: either hydridic where H is exclusively coordinated by electropositive metals (interstitial hydrides), or as part of the polyanion where it acts as a covalently bonded ligand (polyanionic hydride). Both scenarios provide novel coordination environments and bonding scenarios for the atoms involved. This makes hydrogenous Zintl phases important model systems for fundamental studies of hydrogen–metal interactions. Simultaneously, hydrogen-induced chemical structure and physical property changes provide exciting prospects for materials science.

Place, publisher, year, edition, pages
Springer Berlin/Heidelberg, 2011
Series
Structure and Bonding, ISSN 0081-5993 ; 139
Keyword
Main group metal hydrides, Polar intermetallic compounds, Polyanionic hydrides, Zintl phases
National Category
Chemical Sciences
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-146829 (URN)10.1007/430_2010_20 (DOI)978-3-642-21149-2 (ISBN)978-3-642-21150-8 (ISBN)
Available from: 2017-09-12 Created: 2017-09-12 Last updated: 2017-09-13Bibliographically approved
2. Structural properties and superconductivity in the ternary intermetallic compoundsMAB (M=Ca, Sr, Ba; A=Al, Ga, In; B=Si, Ge, Sn)
Open this publication in new window or tab >>Structural properties and superconductivity in the ternary intermetallic compoundsMAB (M=Ca, Sr, Ba; A=Al, Ga, In; B=Si, Ge, Sn)
Show others...
2009 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 80, article id 064514Article in journal (Refereed) Published
Abstract [en]

The ternary intermetallic compounds MAB=CaAlSi, SrAlSi, BaAlSi, CaGaSi, SrGaSi, BaGaSi, SrAlGe, BaAlGe, CaGaGe, SrGaGe, BaGaGe, BaInGe, BaAlSn, CaGaSn, SrGaSn, and BaGaSn have been prepared by arc-melting stoichiometric elemental mixtures and structurally characterized by a combination of x-ray powder and electron diffraction. They crystallize as variants of the simple hexagonal AlB2 structure type where trivalent and tetravalent A- and B-type atoms, respectively, form commonly a planar hexagon layer, and structural variations arise from A/B ordering and/or puckering of hexagon layers. The silicides (B=Si) were previously investigated for their superconducting properties. By dc magnetization measurements it is demonstrated that also the germanides SrAlGe, BaAlGe, SrGaGe, and BaGaGe and the stannide BaAlSn are superconductors above 2 K.

National Category
Chemical Sciences
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-146830 (URN)10.1103/PhysRevB.80.064514 (DOI)
Available from: 2017-09-12 Created: 2017-09-12 Last updated: 2017-11-29Bibliographically approved
3. Structural and dynamic properties of the polyanionic hydrides SrAlGeH and BaAlGeH
Open this publication in new window or tab >>Structural and dynamic properties of the polyanionic hydrides SrAlGeH and BaAlGeH
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2009 (English)In: Solid State Sciences, ISSN 1293-2558, E-ISSN 1873-3085, Vol. 11, no 11, p. 1847-1853Article in journal (Refereed) Published
Abstract [en]

The quaternary aluminium hydrides SrAlGeH and BaAlGeH were synthesized from either hydrogenating the intermetallic AlB2-type precursors SrAlGe and BaAlGe or reacting SrH2 with a mixture of Al and Ge in the presence of pressurized hydrogen. Their structures were characterized by X-ray and neutron powder diffraction of the corresponding deuterides. The compounds crystallize with the trigonal SrAlSiH structure type (space group P3m1, Z = 1, a = 4.2435(2) and 4.3450(2) Å, c = 4.9710(3) and 5.2130(4) Å for SrAlGeH and BaAlGeH, respectively) and feature a two-dimensional polyanion [AlGeH]2− which represents a corrugated hexagon layer built from three-bonded Al and Ge atoms. H is terminally attached to Al. Polyanions [AlGeH]2− are electron precise and, according to electronic structure calculations, the quaternary hydrides display band gaps with sizes between 0.7 and 0.8 eV. Infrared and inelastic neutron scattering spectroscopy show Al–H stretching and bending mode frequencies at around 1250 and 870 cm−1, respectively. SrAlGeH and BaAlGeH are thermally stable up to at least 500 °C. When exposed to air the hydrides decompose rapidly to amorphous, orange colored materials.

Keyword
Zintl phases, Main group metal hydrides, Superconductors, Semiconductors
National Category
Chemical Sciences
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-146831 (URN)10.1016/j.solidstatesciences.2009.08.007 (DOI)
Available from: 2017-09-12 Created: 2017-09-12 Last updated: 2017-09-13Bibliographically approved
4. Hydrogenous Zintl Phase Ba3Si4Hx (x=1-2): Transforming Si-4 Butterfly Anions into Tetrahedral Moieties
Open this publication in new window or tab >>Hydrogenous Zintl Phase Ba3Si4Hx (x=1-2): Transforming Si-4 Butterfly Anions into Tetrahedral Moieties
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2015 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 54, no 3, p. 756-764Article 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.

National Category
Chemical Sciences
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-115692 (URN)10.1021/ic501421u (DOI)000348887400009 ()25247666 (PubMedID)
Note

AuthorCount:7;

Available from: 2015-03-30 Created: 2015-03-27 Last updated: 2017-09-13Bibliographically approved
5. Structural and Vibrational Properties of Silyl (SiH3-) Anions in KSiH3 and RbSiH3: New Insight into Si-H Interactions
Open this publication in new window or tab >>Structural and Vibrational Properties of Silyl (SiH3-) Anions in KSiH3 and RbSiH3: New Insight into Si-H Interactions
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2015 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 54, no 5, p. 2300-2309Article in journal (Refereed) Published
Abstract [en]

The alkali metal silyl hydrides ASiH(3) (A = K, Rb) and their deuteride analogues were prepared from the Zintl phases ASi. The crystal structures of ASiH(3) consist of metal cations and pyramidal SiH3 ions. At room temperature SiH3 moieties are randomly oriented (alpha modifications). At temperatures below 200 K ASiH(3) exist as ordered low-temperature (beta) modifications. Structural and vibrational properties of SiH3- in ASiH(3) were characterized by a combination of neutron total scattering experiments, infrared and Raman spectroscopy, as well as density functional theory calculations. In disordered alpha-ASiH(3) SiH3 ions relate closely to freely rotating moieties with C(3)v symmetry (Si-H bond length = 1.52 angstrom; HSiH angle 92.2 degrees). Observed stretches and bends are at 1909/1903 cm(-1) (nu(1), A(1)), 1883/1872 cm(-1) (nu(3), E), 988/986 cm(-1) (nu(4), E), and 897/894 cm(-1) (nu(2), A(1)) for A = K/Rb. In ordered beta-ASiH(3) silyl anions are slightly distorted with respect to their ideal C-3v symmetry. Compared to a-ASiH(3) the molar volume is by about 15% smaller and the SiH stretching force constant is reduced by 4%. These peculiarities are attributed to reorientational dynamics of SiH3 anions in a-ASiH(3). SiH stretching force constants for SiH3 moieties in various environments fall in a range from 1.9 to 2.05 N cm(-1). These values are considerably smaller compared to silane, SiH4 (2.77 N cm(-1)). The reason for the drastic reduction of bond strength in SiH3- remains to be explored.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:su:diva-116698 (URN)10.1021/ic502931e (DOI)000350391600027 ()25668724 (PubMedID)
Note

AuthorCount:6;

Available from: 2015-04-23 Created: 2015-04-23 Last updated: 2017-09-13Bibliographically approved

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