Hydrogen incorporation in Zintl phases and transition metal oxides- new environments for the lightest element in solid state chemistry

This PhD thesis presents investigations of hydrogen incorporation in Zintl phases and transition metal oxides. Hydrogenous Zintl phases can serve as important model systems for fundamental studies of hydrogen-metal interactions, while at the same time hydrogen-induced chemical structure and physical property changes provide exciting prospects for materials science. Hydrogen incorporation in transition metal oxides leads to oxyhydride systems in which O and H together form an anionic substructure. The H species in transition metal oxides may be highly mobile, making these materials interesting precursors toward other mixed anion systems. Zintl phases consist of an active metal, M (alkali, alkaline earth or rare earth) and a more electronegative p-block metal or semimetal component, E (Al, Ga, Si, Ge, etc.). When Zintl phases react with hydrogen, they can either form polyanionic hydrides or interstitial hydrides, undergo full hydrogenations to complex hydrides, or oxidative decomposition to more E-rich Zintl phases. The Zintl phases investigated here comprised the CaSi 2 , Eu 3 Si 4 , A Si ( A = K, Rb) and GdGa systems which were hydrogenated at various temperature, H 2 pressure, and dwelling time conditions. For CaSi 2 , a regular phase transition from the conventional 6R to the rare 3R took place and no hydride formation was observed. In contrast, GdGa and Eu 3 Si 4 were very susceptible to hydrogen uptake. Already at temperatures below 100 ºC the formation of hydrides GdGaH 2-x and Eu 3 Si 4 H 2+x was observed. The magnetic properties of the hydrides (antiferromagnetic) differ radically from that of the Zintl phase precursor (ferromagnetic). Upon hydrogenating A Si at temperatures around 100 o C, silanides A SiH 3 formed which contain discrete complex ion units SiH 3 - . The much complicated β – α order-disorder phase transition in A SiH 3 was evaluated with neutron powder diffraction (NPD), 2 H NMR and heat capacity measurements. A systematic study of the hydride reduction of BaTiO 3 leading to perovskite oxyhydrides BaTiO 3-x H x was done. A broad range of reducing agents including NaH, MgH 2 , CaH 2 , LiAlH 4 and NaBH 4 was employed and temperature and dwelling conditions for hydride reduction examined. Samples were characterized by X-ray powder diffraction (XRPD), thermal gravimetric analysis and 1 H NMR. The concentration of H that can be incorporated in BaTiO 3-x H x was found to be very low, which is in contrast with earlier reports. Instead hydr