Investigating Hydrogenous Behavior of Zintl Phases: Interstitial Hydrides, Polyanionic Hydrides, Complex Hydrides, Oxidative Decomposition

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 AlB 2 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 T c 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 Ba 3 Si 4 H x (1 < x < 2) which was obtained from the hydrogenation of the Zintl phase Ba 3 Si 4 . Ba 3 Si 4 contains a Si 4 6- “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 Ba 6 octahedra ( I -Ba 3 Si 4 H), and in the second structure the hydrogen is located inside Ba 5 square pyramids ( P -Ba 3 Si 4 H 2 ). In both scenarios the “butterfly anions appear oxidized and form Si 4 4- tetrahedra. Hydrogenation may also be used as a synthesis technique to produce p-block element rich Zintl phases, such a