Novel Interstitial Mg–TM (TM = Ti, Zr, Hf, V, Nb and Ta) Hydrides with an FCC Structure and CaNiH3 with a Perovskite Structure
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
This thesis is focused on structural investigations of novel interstitial ternary alkaline-earth–transition metal hydrides formed by immiscible metals that are held together by the presence of hydrogen atoms. The contents are divided in two parts: Metastable Mg-based hydrides, in which transition-metal additives improve hydrogen storage properties; and a new Ca–Ni hydride, CaNiH3, with a perovskite structure, in which hydrogen enables a Ca:Ni =1:1 ratio although no such alloy composition exists.
New ternary Mg-based hydride with an additive of either Ti, Zr, Hf, V, Nb or Ta were synthesized in a high-pressure anvil cell at 4–8 GPa and 600 ºC. All hydrides had an FCC metal atom structure with an axis of about 4.8 Å. This corresponds to an assumed high-pressure MgH2 phase that is stable at a pressure above 3.6 GPa (P. Vajeeston, P. Ravindran, A. Kjekshus, H. Fjellvåg, Phys. Rev. Lett. 89 (2002) 175506). Ordering of the metal atoms in the lattice leads to a doubling of the FCC unit cell all axis, and the metal atom structure of the hydrides (excluding Zr doped hydride) can be described by the Ca7Ge-type structure. Hydrogen atom arrangements at two tetrahedral sites were found from a powder X-ray diffraction pattern obtained with synchrotron radiation. One of the tetrahedral sites coordinates one TM atom and three Mg atoms, and the other coordinates four Mg atoms.
CaNiH3 crystallizes with a cubic unit cell: a = 3.544(1) Å, V = 44.51(4) Å3 and Z = 1 in space group Pm-3m (No. 221). The hydrogen positions were determined from CaNiD3, using powder neutron diffraction, and the hydride was of an ideal perovskite ABH3 type. It was stabilized by a polyanionic framework of [NiH3]2- counterbalanced by Ca2+ cations. Total-energy calculations of the electronic structure showed that the hydride was a metallic conductor due to a half-filled Ni-H antibonding eg band.
Place, publisher, year, edition, pages
Stockholm: Institutionen för fysikalisk kemi, oorganisk kemi och strukturkemi , 2006. , 52 p.
Metal hydrides, Mg-based hydrides, a new Ca-Ni hydride
IdentifiersURN: urn:nbn:se:su:diva-1016ISBN: 91-7155-272-3OAI: oai:DiVA.org:su-1016DiVA: diva2:189199
2006-06-01, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 8 C, Stockholm, 10:15
Hauback, Bjørn C., Professor
Noréus, Dag, Professor
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