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NR 20140805

<p>The synthesis, structural characterisation and the properties of the Zintl hydrides AeE₂H₂ and AeAlSiH (Ae = Ba, Ca, Sr; E = Al, Ga, In, Si, Zn) are reported. The first hydride in this class of compounds is SrAl₂H₂ which was discovered under an experiment by Gingl, who hydrogenated SrAl₂ at various temperatures. (Gingl et al, Journal of Alloys and Compounds 306 (2000) 127-132). The intention was to form alanates, e.g. AlH₄^-, by terminating the three dimensional four connected aluminium network in SrAl₂. The new hydride, SrAl₂H₂, has a partially conserved aluminium network. The three dimensional anionic network in SrAl₂ is reduced to two dimensions in the hydride, with aluminium bonded to both aluminium and hydrogen. This type of bonding configuration has not been observed before.</p><p>The hydrogenation of SrAl₂ is straight forward, 190 ^oC and 50 bar, compared to the difficult synthesis of alanates and alane, AlH₃. The latter synthesises uses aluminium in its zero oxidation state in contrast to the synthesis of SrAl₂H₂ from SrAl₂. (In the SrAl₂-precursor aluminium is reduced by the electropositive metal to -I.) Thus, the discovery shows a different route to alanates by using precursors with aluminium in a reduced state. If SrAl₂H₂is further hydrogenated at 250 ^oC the two dimensional network breaks and Sr₂AlH₇forms.</p><p>We wanted to investigate if SrAl₂H₂ was a singularity or if other similar compounds exist. We wanted to study how hydrogenation of precursors similar to the aluminide result in 1) new routes to compounds with high hydrogen content, as alanates, 2) to investigate how the E-H bond is affected as function of the network composition among different ternary hydrides, in particular BaAl_xSi_2-xH_x, and choice of active metal.</p><p>BaGa₂H₂ and SrGa₂H₂, two hydrides isostructural with SrAl₂H₂, were synthesized from its precursors BaGa₂ and SrGa₂. In addition three ternary hydrides BaAlSiH, CaAlSiH and SrAlSiH were manufactured from their related AeAlSi precursors.</p><p>All powders were characterized by neutron and x-ray diffraction methods.</p><p>An increased stability towards water/moisture compared to ordinary saline hydrides was noticed, especially for the ternary hydrides. Heat stability was measured with DSC (differential scanning calorimetry). The hydrides BaGa₂H₂ and SrGa₂H₂ decompose around 300 ^oC at 1 atm. This is similar to isostructural SrAl₂H₂. The ternary hydrides BaAlSiH and SrAlSiH decompose at 600 ^oC, at 1 atm, which is the highest noticed temperature for compounds with Al-H bonds. Inelastic neutron scattering experiments showed that these hydrides Al-H and Sr-H bonds are really weak, even weaker then the Al-H interactions in alanates and alanes. These hydrides are probably stabilized be their lattices. The electric properties among the ternary hydrides were measured with IR-spectroscopy (diffuse reflectance). The ternary hydrides, AeAlSiH, are indirect semi conductors. BaGa₂H₂ and SrGa₂H₂are conductors. The ternary hydrides, AeAl_xSi_2-xH_x, may have adjustable band gaps, which we were not able to determine.</p><p>This work is leading into a new research area within the field of metal hydrides.</p>

The synthesis, structural characterisation and the properties of the Zintl hydrides AeE2H2 and AeAlSiH (Ae = Ba, Ca, Sr; E = Al, Ga, In, Si, Zn) are reported. The first hydride in this class of compounds is SrAl2H2 which was discovered under an experiment by Gingl, who hydrogenated SrAl2 at various temperatures. (Gingl et al, Journal of Alloys and Compounds 306 (2000) 127-132). The intention was to form alanates, e.g. AlH4-, by terminating the three dimensional four connected aluminium network in SrAl2. The new hydride, SrAl2H2, has a partially conserved aluminium network. The three dimensional anionic network in SrAl2 is reduced to two dimensions in the hydride, with aluminium bonded to both aluminium and hydrogen. This type of bonding configuration has not been observed before. The hydrogenation of SrAl2 is straight forward, 190 oC and 50 bar, compared to the difficult synthesis of alanates and alane, AlH3. The latter synthesises uses aluminium in its zero oxidation state in contrast to the synthesis of SrAl2H2 from SrAl2. (In the SrAl2-precursor aluminium is reduced by the electropositive metal to -I.) Thus, the discovery shows a different route to alanates by using precursors with aluminium in a reduced state. If SrAl2H2 is further hydrogenated at 250 oC the two dimensional network breaks and Sr2AlH7 forms. We wanted to investigate if SrAl2H2 was a singularity or if other similar compounds exist. We wanted to study how hydrogenation of precursors similar to the aluminide result in 1) new routes to compounds with high hydrogen content, as alanates, 2) to investigate how the E-H bond is affected as function of the network composition among different ternary hydrides, in particular BaAlxSi2-xHx, and choice of active metal. BaGa2H2 and SrGa2H2, two hydrides isostructural with SrAl2H2, were synthesized from its precursors BaGa2 and SrGa2. In addition three ternary hydrides BaAlSiH, CaAlSiH and SrAlSiH were manufactured from their related AeAlSi precursors. All powders were characterized by neutron and x-ray diffraction methods. An increased stability towards water/moisture compared to ordinary saline hydrides was noticed, especially for the ternary hydrides. Heat stability was measured with DSC (differential scanning calorimetry). The hydrides BaGa2H2 and SrGa2H2 decompose around 300 oC at 1 atm. This is similar to isostructural SrAl2H2. The ternary hydrides BaAlSiH and SrAlSiH decompose at 600 oC, at 1 atm, which is the highest noticed temperature for compounds with Al-H bonds. Inelastic neutron scattering experiments showed that these hydrides Al-H and Sr-H bonds are really weak, even weaker then the Al-H interactions in alanates and alanes. These hydrides are probably stabilized be their lattices. The electric properties among the ternary hydrides were measured with IR-spectroscopy (diffuse reflectance). The ternary hydrides, AeAlSiH, are indirect semi conductors. BaGa2H2 and SrGa2H2 are conductors. The ternary hydrides, AeAlxSi2-xHx, may have adjustable band gaps, which we were not able to determine. This work is leading into a new research area within the field of metal hydrides.

Thesis (Ph.D.) -- McMaster University, 1998. / Includes bibliographical references (leaves 281-296). Also available via World Wide Web.

Thesis (Ph. D.)--University of Texas at Austin, 1999. / Vita. Includes bibliographical references (leaves 199-214). Available also in a digital version from Dissertation Abstracts.

abstract: The discovery of the superconductor MgB2 led to the increase of research activity for more compounds adopting the AlB2 structure type and containing superconductive properties. The prominent successor compounds were the silicide systems, AeAlSi (Ae=Sr, Ba, Ca). Presented here is an extension of this investigation to the germanides, SrAlGe and BaAlGe. The ternary structures were synthesized through arc-melting elemental stoichiometric mixtures and structurally characterized by x-ray powder diffraction. Both crystallize as the hexagonal SrPtSb structure type, a variant of the AlB2 structure type. The low temperature region was measured on a Vibrating Sample Magnetometer (VSM) and both present the onset of superconductivity below 7K. These compounds are susceptible to hydrogen absorption and the new polyanionic hydrides, SrAlGeH and BaAlGeH, structural and dynamic properties are presented. The hydrides were synthesized via two distinct methods. One method is the reaction of SrH2 (BaH2) with elemental mixture of the Al and Ge under pressurized hydrogen and the other is a hydrogenation of the SrAlGe and BaAlGe. Both crystallize in the trigonal SrAlSiH structure type, as determined from Rietveld analysis on powder neutron diffraction measurements. The hydrogen is coordinated by both the active metal and aluminum atoms, providing a unique environment for studying metal-hydrogen interactions. When exposed to air, both the hydrides and alloys transform from a crystalline grey to an amorphous yellow powder accompanied by a dramatic volume increase. Infrared spectroscopy shows the disappearance of the bands associated with the Al-H bond and the appearance of Ge-H and O-H bands. This indicates the material reacts with atmospheric water. / Dissertation/Thesis / M.S. Chemistry 2011

Orientadora: Profa. Dra. Raquel de Almeida Ribeiro / Tese (doutorado) - Universidade Federal do ABC, Programa de Pós-Graduação em Física, 2015. / O crescente interesse em novos materiais com propriedades eletrônicas promissoras, tais como termoeletricidade e supercondutividade entre outras, tem levado a comunidade científica a estudar intensivamente por muitos anos uma classe de materiais conhecida como compostos Zintl. Nesse sentido que propomos nesta Tese o estudo de novos materiais termoelétricos, caracterizando e avaliando suas propriedades, buscando um melhor entendimento das propriedades físicas destes materiais em baixa temperatura, utilizando como ponto de partida à família de compostos Zintl. Compostos do tipo Zintl são governados pelo balanço de cargas entre íons doadores e complexas estruturas, geralmente presentes em grandes células unitárias. Duas famílias complexas de compostos Zintl que tem sido investigadas são formadas por RE14MPn11, AE14MPn11, RE11MPn9 e AE11MPn9 (RE = Yb, Eu; AE = Ca, Sr; M = metal de transição; Pn = pictanídeos). Monocristais de Yb11AlSb9 cristalizam com estrutura ortorrômbica de grupo espacial Iba2 com parâmetros de rede a = 11.76 Å, b = 12.39 Å and c = 16.68 Å. Dados de resistividade elétrica do composto Yb11AlSb9 entre 300 K e 0.4 K, nos mostram um comportamento metálico acima de 100 K e semicondutor abaixo desta temperatura indicando a formação de um estado semicondutor de gap pequeno, apresentando um dependência com a temperatura e com o campo magnético entre 0.4 K > T > 2 K. Medidas de calor específico nos mostram que o composto apresenta uma significante dependência de amostra, como observado também nas medidas de resistividade. Observamos também uma dependência com o campo magnético de Cp para T < 4 K até T = 0.4 K. A contribuião eletrônica foi extraída do ajuste linear da curva Cp/T em função T para T < 2 K e nos mostrou uma dependência do parâmetro de Sommerfeld com a temperatura e com o campo magnético, (T,H), com valores de 230 mJ/mol.K2 e 0 mJ/mol.K2 para H = 0 e 3 T, respectivamente. / The growing interest in new materials with promising electronic properties, such as thermoelectricity and superconductivity among others, has lead the scientific community to intensively study a class of materials known as Zintl compounds for many years. In this sense that we propose in this work the study of new thermoelectric materials, characterizing and evaluating their properties, seeking a better understanding of the physical properties of these materials at low temperatures, using as a starting point the family of Zintl compounds. Zintl compounds are governed by charge balance between donor ions and complex structures, and generally present large unit cells. Tow families of complex Zintl phases that has been recently investigated is comprised of RE14MPn11, AE14MPn11, RE11MPn9 and AE11MPn9 compounds (RE = Yb, Eu; AE = Ca, Sr; M = Transition metal; Pn = Pictogens). Single crystals have been grown by the flux method using Sn flux. Yb11AlSb9 crystallized in an orthorhombic structure (space group Iba2) with lattice parameters a = 11.76 Å, b = 12.39 Å and c = 16.68 Å. The Yb11AlSb9 electrical resistivity data from 300 to 0.4 K shows metallic behavior until 100 K, below which there is an increase indicative of a small gap semiconducting ground state and present H-dependence between 0.4 K > T > 2 K. Specific heat data show us the significant sample and H-dependence,as observed in resistivity measurements. We also observed a dependence with the magnetic field of Cp to T < 4 K at T = 0.4 K. The electronic contribution was extracted from the linear fit of the curve Cp/T depending on T to T < 2 K and showed a dependence on the Sommerfeld parameter with the temperature and the magnetic field, (T,H), with values 230 mJ/mol.K2 and 0 mJ/mol.K2 for H = 0 and 3 T, respectively.

La recherche d’une miniaturisation toujours plus poussée des dispositifs en micro- et opto- électronique a participé au développement des nanotechnologies. A l’échelle nanométrique, la densité des interfaces augmente énormément leur conférant un rôle crucial dans les performances des dispositifs. Dans cette thèse, l’intérêt a été porté sur les interactions aux interfaces entre matériaux hétérogènes lors des premiers stades de leur croissance par épitaxie par jets moléculaires. Chaque chapitre est dédié à l’étude d’une interface spécifique dans le cadre des recherches menées par l’équipe Hétéroépitaxie et Nanostructures de l’Institut des Nanotechnologies de Lyon. Deux approches complémentaires pour l’intégration monolithique du semiconducteur III-V GaAs sur silicium ont été étudiées: les nanofils de GaAs sur Si(111) et la recherche d’une phase Zintl pour la croissance bidimensionnelle (2D) de GaAs sur un substrat SrTiO3/Si(100). Pour ce qui concerne la croissance des nanofils de GaAs, l’étude par spectroscopie de photoémission de l’interface entre le gallium, en tant que catalyseur, et le substrat de silicium (111) recouvert de silice a montré qu’une réaction d’oxydoréduction entre les deux matériaux a lieu et est dépendante de la température pendant la croissance. Ensuite, le mécanisme d’encapsulation / désencapsulation par l’arsenic nécessaire à la protection des nanofils de GaAs lors des transferts, a été étudié structurellement en temps réel grâce à la microscopie électronique en transmission. Enfin, la croissance d'une demi-coquille métallique sur les nanofils de GaAs a été analysée in situ par diffraction et diffusion des rayons X en incidence rasante en utilisant un rayonnement synchrotron. Cette étude exploratoire a montré qu’il était possible d’obtenir une croissance partiellement épitaxiée d’or et d’aluminium sur les facettes des nanofils. Pour ce qui concerne la croissance 2D de GaAs sur un substrat SrTiO3/Si(100), la croissance de la phase SrAl2 proposée théoriquement dans la littérature a été tentée et examinée par photoémission. Une alternative, BaGe2, permettant de mieux pallier aux problèmes d’hétérogénéité chimique a finalement été proposée. / Miniaturization of micro- and opto-electronics devices has led to the development of nanotechnologies. At this scale, the density of interfaces drastically increases explaining their critical role in the device performances. In this thesis, interest has been focused on interactions at the interfaces between heterogeneous materials during their first growth stages by molecular beam epitaxy. Each chapter studies a specific interface with the objective of monolithically integrating III-V semiconductors (GaAs) on silicon substrate, which is a main goal of the INL’s Heteroepitaxy and Nanostructures team. Two complementary approaches have been considered: GaAs nanowires on Si (111) substrate and the research of a Zintl phase as a buffer layer adequate for the two-dimensional (2D) growth of GaAs on a SrTiO3 / Si (100) substrate. In the context of growing GaAs nanowires on Si(111) with gallium as catalyst, the role played by a silica overlayer has been studied by X-Ray Photoelectron Spectroscopy. It has been shown that an oxido-reduction reaction takes place at the interface, reaction which is strongly dependent on the temperature during the process. Besides, the real-time evolution of an As capping/decapping mechanism, which is needed for the protection of GaAs nanowires during transfers, has been studied thanks to electron transmission microscopy. Finally, the growth of a metal half-shell on GaAs nanowires has been investigated by in situ grazing incidence X-ray diffraction using synchrotron radiation. This exploratory study has shown that obtaining a partially epitaxial growth of gold and aluminum on nanowires facets is possible. In the context of obtaining a 2D growth of GaAs on SrTiO3/Si(100) substrate, the growth of the theoretically-suggested Zintl phase SrAl2 was tried by MBE and probed by photoemission, along with an alternative, BaGe2, which appeared more suitable for chemical reasons.

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.