Synthesis and thermal stability of perovskite alkali metal strontium borohydrides

Abstract

Three new perovskite-type bimetallic alkali metal strontium borohydride compounds, α-MSr(BH₄)₃ (M = K, Rb, Cs), have been synthesized and investigated by in situ synchrotron radiation powder X-ray diffraction, thermal analysis combined with mass spectrometry and Sievert's measurements. The bimetallic borohydrides were synthesized via an addition reaction between Sr(BH₄)₂ and MBH₄ (M = K, Rb, Cs) by mechanochemical treatment. The Sr(BH₄)₂–NaBH₄ system, which was treated in a similar manner, did not undergo reaction. All three α-MSr(BH₄)₃ compounds crystallize in the orthorhombic crystal system at room temperature: KSr(BH₄)₃ (P2₁cn), a = 7.8967(6), b = 8.2953(7), and c = 11.508(1) Å (V = 753.82(12) ų). RbSr(BH₄)₃ (Pbn2₁), a = 8.0835(3), b = 8.3341(4), and c = 11.6600(5) Å (V = 785.52(6) ų). CsSr(BH₄)₃ (P22₁2₁), a = 8.2068(9), b = 8.1793(9), and c = 6.0761(4) Å (V = 407.87(7) ų). All three compounds are perovskite-type 3D framework structures built from distorted [Sr(BH₄)₆] octahedra. High-temperature polymorphs are identified to form at 258, 220 and 150 °C for MSr(BH₄)₃, M = K, Rb and Cs, respectively. The new compounds are thermally stable and decompose at T > 360 °C into SrB₆, SrH₂ and MBH₄ (M = K, Rb, Cs).