Crystal structures and properties of solvent-free LiYb(BH4)4−xClx, Yb(BH4)3 and Yb(BH4)2−xClx

Abstract

Metal borohydrides offer high theoretical storage capacity for solid state hydrogen storage. This work is aimed at the mechanochemical synthesis of Yb(BH₄)₃ from LiBH₄ and YbCl₃. Different synthesis routes resulted in three new compounds, LiYb(BH₄)_4−xCl_x, α-Yb(BH₄)₃ and β-Yb(BH₄)₃. Their crystal structures have been determined from lab and synchrotron powder diffraction. LiYb(BH₄)_4−xCl_x takes a primitive tetragonal structure with x = 1.0, a = 6.1729(3) Å and c = 12.4155(10) Å in the space group P2c (no. 112), α-Yb(BH₄)₃ a primitive cubic structure with a = 10.70715(15) Å in the space group Pa (no. 205), and β-Yb(BH₄)₃ a primitive cubic structure with a = 5.44223(3) Å in the space group Pmm (no. 221). Thermal decomposition properties of the materials have been investigated by in situ synchrotron radiation powder X-ray diffraction, thermo gravimetric analysis/differential scanning calorimetry and temperature programmed desorption. The decomposition product, Yb(BH₄)_2−xCl_x, adopting a primitive tetragonal structure with space group P (no. 81) was formed after release of diborane gas, and x increases with increasing temperature. A deuterated sample of this compound was synthesized for powder neutron diffraction. Rietveld refinement gave x = 0.76, a = 6.74763(2) Å and c = 4.28368(2) Å. Another polymorph of Yb(BH₄)_2−xCl_x was synthesized, which adopts a primitive orthorhombic structure with space group Pbca (no. 61), where x = 0.3, a = 13.20997(20) Å, b = 8.26829(12) Å and c = 7.44532(11) Å.