Phase transitions and compositional changes in Ca-Mg-H hydrogen storage systems Ruby Morris and Duncan Gregory School of Chemistry, Joseph Black Building, University of Glasgow, Glasgow G12 8QQ The practical use of metal hydrides for hydrogen storage is restricted/dictated by their intrinsic properties. It is therefore vital to understand the relationship between structure and properties for these materials as therein lies the foundation for their development to meet the criteria for hydrogen storage. Ca-Mg-H materials are among the few bimetallic s-block hydrides to show promise as solid-state stores, with attractive gravimetric and volumetric capacity and good cyclability. Therefore, research on these light metal systems is now focused on destabilisation to tackle kinetic / thermodynamic issues of hydrogen release. For ternary systems containing Ca, Mg and H, only two phases have been identified so far: cubic Ca 19 Mg 8 H 54 and hexagonal Ca 4 Mg 3 H 14 1 . Previous syntheses of these compounds have required high pressure hydrogen, but in this work, we describe how both Ca 19 Mg 8 H 54 and Ca 4 Mg 3 H 14 can be synthesised mechanochemically from the binary hydrides without hydrogen gas.PXD of our mechanochemically-synthesised 4:3 Ca:Mg crystalline sample showed a main ternary phase isostructural to the well-known cubic Ca 19 Mg 8 H 54 2 . However, research in the group has shown that Ca 19 Mg 8 H 54 is not a line phase and that the complex cubic structure can tolerate variable metal content. 3 Rietveld refinement against lab PXD data of the 4:3 sample revealed the main cubic phase to have a composition of Ca 14.5 Mg 12.5 H 54 , extending the possible stoichiometries of the cubic Ca-Mg-H phase still further. We seek to arrive at a better understanding of the processes preceding, during and following dehydrogenation so thermogravimetric-differential thermal analysis (TG-DTA) under Ar was followed ex-situ by PXD and Rietveld refinement. Thermal analysis and subsequent structure refinement of the 4:3 sample heated to 225 °C revealed the coexistence of a Ca 19 Mg 8 H 54 -type phase and a hexagonal Ca 4 Mg 3 H 14 -type phase prior to dehydrogenation. This indicates that the mechanochemically synthesised material first emerges as a cubic phase and then converts to the hexagonal phase at elevated temperatures. It therefore seems increasingly likely that Ca 4 Mg 3 H 14 is a metastable hydride. We are investigating the conditions for the formation and growth of this phase and with the aim of testing the flexibility for non-stoichiometry in the hexagonal structure (as we have seen with the Ca 19 Mg 8 H 54 phase). As we develop a more comprehensive understanding of this system, our strategy will pave the way for methods such as doping, substitution and catalysis towards optimising this material as a reversible solid-state hydrogen store. References 1. Gingl, F.R Bonhomme, K. Yvon, P. Fischer, Tetracalcium trimagnesium tetradecahydride, Ca 4 Mg 3 H 14 : the first ternary alkaline earth hydride, J. Alloys Comp., 1992, 185 , 273-278. 2. Bertheville, K. Yvon, Ca 19 Mg 8 H 54 , a new salt-like ternary metal hydride, J. Alloys Comp. , 1999, 290 , 8-10 3. Reardon, Synthesis, structure, and characterisation of novel lightweight energy materials based on group I & II metal compounds. PhD thesis, University of Glasgow, 2014.
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