Synthesis and structure of group I and II nitrides as potential hydrogen stores.
PhD thesis, University of Glasgow.
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This thesis describes the synthesis and characterisation of group I and group II nitride materials as potential hydrogen stores. New synthetic routes as well as the development of conventional methods were employed to synthesise binary and ternary nitrides. Ball milling of single phase α-Li3N and of commercial Li3N was performed to induce a phase transformation in order to synthesise β-Li3N which adopts a hexagonal crystal structure (space group P63/mmc). The beta polymorph was initially characterised by
PXD and subsequently by a variable temperature PND experiment, which demonstrated that the phase transformation to α-Li3N began at 600 K. Due to the fact that β-Li3N exhibits the same magnitude of Li+ conductivity as α-Li3N and that lithium ion diffusion is thought to be an important factor in the hydrogen storage potential of the
solid, a further PND investigation to study the interaction of D2 gas with β-Li3N was performed. At lower temperatures, negative thermal expansion can be observed; this
expands following similar trends known for α-Li3N and commercial Li3N. In agreement with the behaviour of Li3N reported in the literature, Li3N reacts with D2 at higher
temperatures resulting in the formation of LiND2 and LiD.
The phase behaviour in the complex Li-N-H system is still being debated despite significant research in this area. This work has focussed on lithium nitride hydride,
which has been reported as a minority phase during the hydrogenation of Li3N. Li4ND was prepared by both conventional means and by a novel synthesis using microwaves; the product was characterised by PXD and PND. During both conventional and microwave syntheses, tetragonal Li4ND (space group I-4) and a new high temperature cubic polymorph formed (space group Fm-3m) where N3-, D- and (ND)2- are disordered across the anti-fluorite anion sites. With regard to providing further evidence for the proposition of a new reaction pathway upon hydrogenation of Li3N, Li4ND and Li2ND were reacted in-situ during a PND investigation forming a solid solution and resulting in synthesis of a cubic ‘quasi-imide’ phase. The ‘quasi-imide’ phase was refined against a modified cubic Li2ND starting model. The presence of D+ and D- ensured that charge balance was maintained. As the stoichiometry increases, the anion distribution changes; the occupancy of N3- and N from (ND)2- on the 4a site increases as does the occupancy of protonic D from (ND)2- on the 192l site.
A ternary nitride, LiCaN (space group Pnma), was prepared by both conventional and novel (via microwaves) means. Optimization of the reaction parameters was the initial
focus of the investigation in order to synthesise single phase LiCaN. Firstly, a PND study was performed in order to ascertain accurate Li positions and ensure the material
was perfectly stoichiometric as made. In order to compare the Li-Ca-N system with reports on the Li-Mg-N system in which both Mg-rich and Li-rich phases as well as stoichiometric LiMgN have been synthesised, attempts were made to synthesise nonstoichiometric Li-Ca-N compounds. This system was investigated by PND.
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