Solid state synthesis, structural characterization and ionic conductivity of bimetallic alkali-metal yttrium borohydrides MY(BH4)4 (M = Li and Na)

Two bimetallic borohydrides, lithium yttrium tetraborohydride, LiY(BH4)(4), and sodium yttrium tetraborohydride, NaY(BH4)(4), have been synthesized by thermal treatment and quenching of ball-milled precursors. LiY(BH4)(4) crystallizes in the space group P (4) over bar 2c and NaY(BH4)(4) in C222(1) and the compounds are isostructural to LiSc(BH4)(4) and NaSc(BH4)(4), respectively. Both LiY(BH4)(4) and NaY(H-3(4))(4) are built from the complex anions of (Y(BH4)(4))(-). The unit cell volumes per formula unit of the bimetallic compounds LiY(BH4)(4) and NaY(BH4)(4) are significantly larger compared to those of the constituent components (14% and 7.5% unit cell expansion, respectively), which may contribute to the mobility of the Li+ and Na+ cations. The ionic conductivities of LiY(BH4)(4) and NaY(BH4)(4) at room temperature, measured by impedance spectroscopy, are sigma = 1.26 x 10(-6) and 6.92 x 10(-7) S cm(-1) respectively, and are improved compared to those of the precursors. DFT calculations were used to optimize the structures and identify a possible mechanism for the Na ion mobility. In addition, they provide the basis for assignment of the B-11 NMR parameters determined by B-11 MAS NMR for the two boron sites in NaY(BH4)(4).