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−xClx, α-Yb(BH4)3 and β-Yb(BH4)3. Their crystal structures have been determined from lab and synchrotron powder diffraction. LiYb(BH4)4−xClx 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−xClx, 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−xClx 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.