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Structural investigations of two new ternary magnesium–niobium hydrides, Mg6.5NbH~14 and MgNb2H~4
Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
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2006 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 417, no 1-2, 230-234 p.Article in journal (Refereed) Published
Abstract [en]

Two new magnesium–niobium hydrides, Mg6.5NbH14 with a face centred cubic unit cell a = 9.548(1) Å (space group (No. 225), Z = 4, V = 870.49 Å3) and MgNb2H4 with a C-centred monoclinic unit cell a = 5.685 (4) Å, b = 3.2914(6) Å, c = 7.924(2) Å and β = 103.82(3)° (space group C2/m (No. 12), Z = 2, V = 143.98 Å3), were synthesized by reacting MgH2 and Nb in a high-pressure anvil cell at 8 GPa and 600 °C. The structures of the new hydrides were refined from synchrotron radiation data. Mg6.5NbH14 has a structure related to Mg7TiH16 (a = 9.532(2) Å space group (No. 225)) (D. Kyoi, T. Sato, E. Rönnebro, N. Kitamura, A. Ueda, M. Ito, S. Katsuyama, S. Hara, D. Noréus, T. Sakai, J. Alloys Compd. 372 (2004) 213). Where the metal atom structure is of the Ca7Ge type. Mg6.5NbH14, however, has some vacancies on one of the magnesium sites similar to the recently discovered Mg6VH14 (D. Kyoi, T. Sato, E. Rönnebro, Y. Tsuji, N. Kitamura, A. Ueda, M. Ito, S. Katsuyama, S. Hara, D. Noréus, T. Sakai, J. Alloys Compd. 375 (2004) 253). The metal atoms in MgNb2H4 have the AuTe2 type structure.

Place, publisher, year, edition, pages
2006. Vol. 417, no 1-2, 230-234 p.
Keyword [en]
Synchrotron, High-pressure, Metal hydride, Magnesium, Niobium
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:su:diva-22635DOI: 10.1016/j.jallcom.2005.08.068OAI: oai:DiVA.org:su-22635DiVA: diva2:189196
Available from: 2006-05-11 Created: 2006-05-11 Last updated: 2017-12-13Bibliographically approved
In thesis
1. Novel Interstitial Mg–TM (TM = Ti, Zr, Hf, V, Nb and Ta) Hydrides with an FCC Structure and CaNiH3 with a Perovskite Structure
Open this publication in new window or tab >>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)
Abstract [en]

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.
Keyword
Metal hydrides, Mg-based hydrides, a new Ca-Ni hydride
National Category
Chemical Sciences
Identifiers
urn:nbn:se:su:diva-1016 (URN)91-7155-272-3 (ISBN)
Public defence
2006-06-01, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 8 C, Stockholm, 10:15
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Supervisors
Available from: 2006-05-11 Created: 2006-05-11Bibliographically approved

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