M[Y(BH_4)_4] and M_2Li[Y(BH_4) _(6-x)Cl_x] (M = Rb, Cs): New borohydride derivatives of yttrium and their hydrogen storage properties

摘要

Two novel bimetallic borohydrides, Rb[Y(BH__4)_4] and Cs[Y(BH__4)_4], have been synthesised via mechanochemical reactions between MBH_4, M = Rb or Cs and Y(BH_4) _3. Both are ionic compounds comprising complex anions [Y(BH __4)_4]- and alkali metal cations, and they adopt crystal structures which were not observed previously for quasi-ternary borohydrides. LiCl, a by-product from the synthesis of Y(BH_4) _3, is not an unreactive dead-weight but rather it promotes formation of trimetallic borohydride-chlorides, M_2Li[Y(BH_4) _(6-x)Cl_x], which crystallise in the elpasolite-type structures. Formation of Cs_2Li[Y(BH_4)_(6-x)Cl _x] takes place during the mechanochemical synthesis of Cs[Y(BH __4)_4] at room temperature, while that of Rb _2Li[Y(BH_4)_(6-x)Clx] only after heating of the sample above the melting point. The M[Y(BH_4) _4]/M_2Li[Y(BH_4)_(6-x)Clx] mixtures slowly release H_2 when melted (T_m = 170 °C for M = Rb, 210 °C for M = Cs) but the main stage of thermal decomposition occurs at a T_(dec) of ～270 °C, similarly to what is observed for K[Y(BH__4)_4] and NaBH_4/Y(BH_4) _3 composites. We also demonstrate how the T_(dec) of M[Y(BH__4)_4] salts may be tuned by choosing the MBH 4 precursor of the appropriate Lewis basicity.