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Mechanochemically Driven Non-equilibrium Processes in Amide - Binary Complex Hydride Systems

Mechanochemical transformations of lithium and sodium amides with calcium hydride taken in
different molar ratios lead to the formation of calcium imide, alkali metal monohydrides and
gaseous hydrogen.  In all cases, the overall mechanochemical transformations are equimolar and proceed as one step reactions: MNH2 + CaH2 → CaNH + MH + H2, where M = Li or Na. If the concentrations of starting materials are different from equimolar, then the component whose
concentration in the starting mixture was greater remains in excess and does not transform up to 24 hours of high energy ball milling. The reactions in lithium systems are faster compared to
systems with sodium. Difference in hydrogen release kinetics can be related to differences in
mobilities of lithium and sodium atoms, to differences in strength of ionic bonding of the imides,
and different thermodynamics. Total energies and enthalpies of formation for different reaction
products during the dehydrogenation of CaH2 - MNH2 mixtures indicate that there may exist two dehydrogenation pathways for CaH2 - MNH2 systems: a mechanochemical transformation driven by mechanical ball milling and two-step thermochemical transformations driven by heating.  Compared to thermochemical transformations, which proceed in accordance with
thermodynamic equilibrium, reactions induced by mechanical energy drive the MNH2 – CaH2
systems to nonequilibrium configurations with different final products.

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