Stability of transition metal doped magnesium hydride high-pressure phases

Moser, D

Abstract

MgH2 is hydrogen dense but normally has a rutile structure, which is too
stable with too low a hydrogen diffusion rate for practical applications. At
pressures of several GPa a CaF2 structure phase has been predicted for MgH2 .
With a small fraction of the magnesium atoms substituted by transition
metal atoms such as Ti, this cubic phase remains stable when the high GPa
synthetic pressure is released with typical composition MgyTiHie- How the
metal-hydrogen bonding differs from that in the conventional MgH2 and TiH2
has been investigated in this work.
Using Electron Localisation Function topological analysis, the site preference
for hydrogen entering the host lattice can be predicted. Vibrational
spectra have been calculated with Density Functional Theory (DFT) with the
aim of modelling the incoherent inelastic neutron scattering spectra, which
have been measured at the TOSCA spectrometer at ISIS. DFT calculations
show a weakening in the force constants of the Ti-H and Mg-H bond which
can be directly related to the FCC structures of TiH2 and MgH2 . The structural
similarities of the three FCC systems lead to a better understanding of
the formation process of the new ternary compounds.
The presence of two non-equivalent types of tetrahedral site with different
force constants resulting from the normal modes of vibration suggests a twostep
hydrogenation and dehydrogenation process. Comparison with the TiHx
system is also interesting in terms of hydrogen diffusion inside the host lattice
and a quasi-elastic neutron scattering experiment has been carried out to
investigate this.
Gravimetric measurements allow us to investigate the reversibility of incompletely
dehydrogenated samples and results showing the fast kinetics are
presented. If the partial reversibility could be optimised with respect to TM
selection and addition, it would mean a breakthrough for storing hydrogen
in magnesium hydrides.

Citation

Moser, D. Stability of transition metal doped magnesium hydride high-pressure phases. (Thesis). Salford : University of Salford

• Thesis Type: Thesis
• Deposit Date: Oct 3, 2012
• Award Date: Jan 1, 2010