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

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-x)Cl-x, alpha-Yb(BH4)(3) and beta-Yb(BH4)(3). Their crystal structures have been determined from lab and synchrotron powder diffraction. LiYb(BH4)(4-x)Cl-x takes a primitive tetragonal structure with x = 1.0, a = 6.1729(3) angstrom and c = 12.4155(10) angstrom in the space group P (4) over bar 2c (no. 112), alpha-Yb(BH4)(3) a primitive cubic structure with a = 10.70715(15) angstrom in the space group Pa (3) over bar (no. 205), and beta-Yb(BH4)(3) a primitive cubic structure with a = 5.44223(3) angstrom in the space group Pm (3) over barm (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-x)Cl-x, adopting a primitive tetragonal structure with space group P (4) over bar (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) angstrom and c = 4.28368(2) angstrom. Another polymorph of Yb(BH4)(2-x)Cl-x was synthesized, which adopts a primitive orthorhombic structure with space group Pbca (no. 61), where x = 0.3, a = 13.20997(20) angstrom, b = 8.26829(12) angstrom and c = 7.44532(11) angstrom.