The influence of LiH on the rehydrogenation behavior of halide free rare earth (RE) borohydrides (RE = Pr, Er)

Abstract

Rare earth (RE) metal borohydrides are receiving immense consideration as possible hydrogen storage materials and solid-state Li-ion conductors. In this study, halide free Er(BH₄)₃ and Pr(BH₄)₃ have been successfully synthesized for the first time by the combination of mechanochemical milling and/or wet chemistry. Rietveld refinement of Er(BH₄)₃ confirmed the formation of two different Er(BH₄)₃ polymorphs: α-Er(BH₄)₃ with space group Pa, a = 10.76796(5) Å, and β-Er(BH₄)₃ in Pmm with a = 5.4664(1) Å. A variety of Pr(BH₄)₃ phases were found after extraction with diethyl ether: α-Pr(BH₄)₃ in Pa with a = 11.2465(1) Å, β-Pr(BH₄)₃ in Pmm with a = 5.716(2) Å and LiPr(BH₄)₃Cl in I3m, a = 11.5468(3) Å. Almost phase pure α-Pr(BH₄)₃ in Pa with a = 11.2473(2) Å was also synthesized. The thermal decomposition of Er(BH₄)₃ and Pr(BH₄)₃ proceeded without the formation of crystalline products. Rehydrogenation, as such, was not successful. However, addition of LiH promoted the rehydrogenation of RE hydride phases and LiBH₄ from the decomposed RE(BH₄)₃ samples.