Synthesis, structure and properties of new bimetallic sodium and potassium lanthanum borohydrides

Abstract

Two new bimetallic sodium or potassium lanthanum borohydrides, NaLa(BH₄)₄ and K₃La(BH₄)₆, are formed using La(BH₄)₃ free of metal halide by-products. NaLa(BH₄)₄ crystallizes in an orthorhombic crystal system with unit cell parameters, a = 6.7987(19), b = 17.311(5), c = 7.2653(19) Å and space group symmetry Pbcn. This compound has a new structure type built from brucite-like layers of octahedra (hcp packing of anions) with half of the octahedral sites empty leading to octahedral chains similar to rutile (straight chains) or α-PbO₂ (zig-zag chains). K₃La(BH₄)₆ crystallizes in the monoclinic crystal system with unit cell parameters a = 7.938(2), b = 8.352(2), c = 11.571(3) Å, β = 90.19(6)° and space group P2₁/n with a double-perovskite type structure. Thermogravimetric analysis shows a mass loss of 5.86 and 2.83 wt% for NaLa(BH₄)₄ and K₃La(BH₄)₆, respectively, in the temperature range of room temperature to 400 °C. Mass spectrometry shows that hydrogen release starts at 212 and 275 °C for NaLa(BH₄)₄ and K₃La(BH₄)₆, respectively and confirms that no diborane is released. Sieverts’ measurements reveal that 2.03 and 0.49 wt% of hydrogen can be released from the NaLa(BH₄)₄ and K₃La(BH₄)₆, respectively, during the second hydrogen desorption cycle at the selected physical condition for hydrogen absorption.