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

Abstract

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 P2c and NaY(BH₄)₄ in C222₁ and the compounds are isostructural to LiSc(BH₄)₄ and NaSc(BH₄)₄, respectively. Both LiY(BH₄)₄ and NaY(BH₄)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 σ = 1.26 × 10⁻⁶ and 6.92 × 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 ¹¹B NMR parameters determined by ¹¹B MAS NMR for the two boron sites in NaY(BH₄)₄.