Perovskite alkali metal samarium borohydrides: crystal structures and thermal decomposition

Abstract

A new synthesis method of samarium borohydride, Sm(BH₄)₂, using tetrahydrofuran borane, THF-BH₃, and samarium hydride, SmH₂, has been demonstrated and verified. The synthesised Sm(BH₄)₂ was mechanochemically treated with MBH₄, M = K, Rb, Cs. Initially, the formation of KSm(BH₄)₃ is observed while subsequent heat treatment is necessary to form MSm(BH₄)₃, M = Rb, Cs. The new compounds crystallise in orthorhombic unit cells adopting perovskite-type 3D frameworks containing distorted [Sm(BH₄)₆] octahedra. In situ X-ray diffraction studies reveal two second-order polymorphic transitions of α-CsSm(BH₄)₃via a tetragonal intermediate, α′-CsSm(BH₄)₃, into a cubic high-temperature polymorph, β-CsSm(BH₄)₃, resembling an ideal perovskite structure. The new compounds, MSm(BH₄)₃, are thermally stable up to T ∼ 280 °C after which they decompose into mainly MBH₄, SmH₂ and possibly SmB₆ and SmB₁₂H₁₂. Finally, after three cycles of hydrogen release and uptake, the storage capacity was 1.0 wt% for KSm(BH₄)₃ and 0.84 wt% for RbSm(BH₄)₃ and CsSm(BH₄)₃.