Novel Alkali Earth Borohydride Sr(BH₄)₂ and Borohydride-Chloride Sr(BH₄)Cl

Two novel alkali earth borohydrides, Sr­(BH₄)₂ and Sr­(BH₄)­Cl, have been synthesized and investigated by in-situ synchrotron radiation powder X-ray diffraction (SR-PXD) and Raman spectroscopy. Strontium borohydride, Sr­(BH₄)₂, was synthesized via a metathesis reaction between LiBH₄ and SrCl₂ by two complementary methods, i.e., solvent-mediated and mechanochemical synthesis, while Sr­(BH₄)Cl was obtained from mechanochemical synthesis, i.e., ball milling. Sr­(BH₄)₂ crystallizes in the orthorhombic crystal system, a = 6.97833(9) Å, b = 8.39651(11) Å, and c = 7.55931(10) Å (V = 442.927(10) ų) at RT with space group symmetry Pbcn. The compound crystallizes in α-PbO₂ structure type and is built from half-occupied brucite-like layers of slightly distorted [Sr­(BH₄)₆] octahedra stacked in the a-axis direction. Strontium borohydride chloride, Sr­(BH₄)­Cl, is a stoichiometric, ordered compound, which also crystallizes in the orthorhombic crystal system, a = 10.8873(8) Å, b = 4.6035(3) Å, and c = 7.4398(6) Å (V = 372.91(3) ų) at RT, with space group symmetry Pnma and structure type Sr­(OH)₂. Sr­(BH₄)Cl dissociates into Sr­(BH₄)₂ and SrCl₂ at ∼170 °C, while Sr­(BH₄)₂ is found to decompose in multiple steps between 270 and 465 °C with formation of several decomposition products, e.g., SrB₆. Furthermore, partly characterized new compounds are also reported here, e.g., a solvate of Sr­(BH₄)₂ and two Li–Sr–BH₄ compounds.