Structure and thermal properties of composites with RE-borohydrides (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Er, Yb or Lu) and LiBH4

Abstract

RECl₃ (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Er, Yb, Lu) and LiBH₄ ball-milled in the molar ratio of 1 : 6 resulted in formation of numerous rare-earth metal (RE) borohydrides and LiCl in mixtures with excess LiBH₄. The reaction products were characterized by powder X-ray diffraction and infrared spectroscopy, and their hydrogen sorption properties were investigated by thermogravimetric analysis combined with differential scanning calorimetry and in situ synchrotron radiation powder X-ray diffraction. La, Ce, Pr and Nd form LiRE(BH₄)₃Cl compounds which crystallize in the cubic space group I3m. Sm, Gd, Tb, Er and Yb form RE(BH₄)₃ phases crystallizing in the cubic space group Pa, with a possible polymorphic transition to a higher symmetry space group, Pmm. The smaller RE-elements Yb and Lu form tetrahedral [RE(BH₄)₄]⁻ anionic complexes stabilized by Li⁺ cations crystallizing in the tetragonal space group P2c. Additionally Sm and Gd also exhibit a transition to the LiRE(BH₄)₃Cl phases observed for the largest lanthanides. LiSm(BH₄)₃Cl is reduced to Sm(BH₄)₂ upon heating, which takes a previously unreported orthorhombic structure in space group Pbcn. Eu did not form any of these RE-borohydride compounds, but infrared spectroscopy indicates formation of a new borohydride phase. Hydrogen desorption from the composite mixtures starts around 200 °C. Rehydrogenation of the La-system under p(H₂) = 10 MPa at 300 °C showed a partial reversibility, with 18% recovery of the original hydrogen capacity, which increased to 36% at 415 °C. Rehydrogenation under p(H₂) = 10 MPa at 400 °C showed a partial reversibility of 25% in the Er-system. Formation of REH₂ during thermal decomposition destabilizes the remaining LiBH₄ in the composites, thus releasing hydrogen at lower temperatures than for pure LiBH₄.