Thèse de doctorat : Science et ingénierie des matériaux : INPL : 2008. / Titre provenant de l'écran-titre.

Chemnitz, Techn. University, Diss., 2003.

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.

Regensburg, Univ., Diss., 2007

Thesis (Ph. D.)--Virginia Commonwealth University, 2010. / Prepared for: Dept. of Chemistry. Title from title-page of electronic thesis. Bibliography: leaves 176-198.

Universiẗat, Diss., 2003--Freiburg (Breisgau).

We present an experimental study of thin films of SrAl₄ on a LaAlO₃ substrate, with special emphasis on the Zintl-Klemm-type properties of the thin films that we grow using molecular beam epitaxy. We quantify the orientation and stoichiometry of the films and the surface morphology using reflection high energy electron diffraction (RHEED), x-ray diffraction (XRD) and atomic force microscopy (AFM). Furthermore, we present measurements of electronic properties using x-ray photoelectron spectroscopy (XPS) and ultraviolet spectroscopy (UPS). We determine the core level shifts due to the chemical environment in SrAl₄-films, which will underline the Zintl-Klemm character of the material. We measure the work function of (001)-oriented SrAl₄. Additionally we analyze the electronic transport properties of the grown thin films including the resistivity, carrier density and mobility. / text

No description available.

This thesis focuses on the synthesis, structures, and physical properties of ternary rare-earth antimonides and germanides. These ternary compounds exhibit diverse polyanionic substructures with classical and non-classical Sb–Sb bonding, Ge–Ge bonding, or both. The Zintl-Klemm concept and band structure calculations were applied to understand their structures and bonding. Electrical resistivities and magnetic properties were measured for these compounds. The compounds RE2Ti7Sb12 (RE = La–Nd) and RE2Ti11–xSb14+x (RE = Sm, Gd, Tb, Yb), which were synthesized by arc-melting, adopt different structures depending on the size of the RE atoms. Both consist of a complex arrangement of TiSbn polyhedra, linked to form a 3D framework with large cavities in which the RE atoms reside. Hypervalent Sb–Sb bonds are manifested in disordered Sb fragments in RE2Ti7Sb12, and 1D linear chains, zig-zag chains, and pairs in RE2Ti11–xSb14+x. A series of compounds, RECrGe3 (RE = La–Nd, Sm), was synthesized by the Sn-flux method. They adopt a hexagonal perovskite structure type, in which chains of face-sharing Cr-centred octahedra are linked by triangular Ge3 clusters. These unusual single-bonded Ge3 substructures can be rationalized simply by the Zintl-Klemm concept. Electrical resistivity measurements show metallic behaviour with prominent transitions coincident with ferromagnetic transitions (Tc ranging from 62 to 155 K) found in magnetic measurements. Band structure calculations show the presence of a narrow, partially filled band with high DOS at Ef, in agreement with the observation that LaCrSb3 is an itinerant ferromagnet. With a different number of d-electrons in the M site, the isostructural REVGe3 compounds exhibit antiferromagnetic behaviour. The doped quaternary compounds LaCr1-xVxGe3 and LaCr1-xMnxGe3 exhibit depressed Curie temperatures. The structures of RECrxGe2 compounds (RE = Sm, Gd–Er) are built up by inserting transition-metal atoms into the square pyramidal sites of a hypothetical “REGe2” host structure (ZrSi2-type). The presence of extensive anionic Ge substructures in the form of 1D zigzag chains and 2D square sheets can be explained by the Zintl-Klemm concept. Magnetic measurements indicated antiferromagnetic ordering with low TN ranging from 3 to 17 K. Compounds involving a p-block element as the second component were prepared. In RE12Ge7–xSb21 (RE = La–Pr), a complex 3D polyanionic framework with Ge pairs, five-atom-wide Sb ribbons, and 2D Ge/Sb layers is present. The bonding exemplifies the competition of valence electron transfer from the RE atoms to metalloids with similar electronegativities. Full electron transfer from the RE atoms to the anionic substructure cannot be assumed. Magnetic measurements on Ce12Ge6.5Sb21 indicate antiferromagnetic coupling. The metal-rich compounds RE5TtxSb3–x (Tt = Si, Ge) adopt the orthorhombic beta-Yb5Sb3-type structure with a range of solid solubility between 0.9 and 1.6. They are not electron-precise and do not obey the Zintl-Klemm concept.

Regensburg, Univ., Diss., 2009.