Stability of transition metal doped magnesium hydride high-pressure phases

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.EThOS - Electronic Theses Online ServiceGBUnited Kingdo