Synthesis, structure and properties of bimetallic sodium rare-earth (RE) borohydrides, NaRE(BH4)4, RE = Ce, Pr, Er or Gd

Abstract

Formation, stability and properties of new metal borohydrides within RE(BH₄)₃–NaBH₄, RE = Ce, Pr, Er or Gd is investigated. Three new bimetallic sodium rare-earth borohydrides, NaCe(BH₄)₄, NaPr(BH₄)₄ and NaEr(BH₄)₄ are formed based on an addition reaction between NaBH₄ and halide free rare-earth metal borohydrides RE(BH₄)₃, RE = Ce, Pr, Er. All the new compounds crystallize in the orthorhombic crystal system. NaCe(BH₄)₄ has unit cell parameters of a = 6.8028(5), b = 17.5181(13), c = 7.2841(5) Å and space group Pbcn. NaPr(BH₄)₄ is isostructural to NaCe(BH₄)₄ with unit cell parameters of a = 6.7617(2), b = 17.4678(7), c = 7.2522(3) Å. NaEr(BH₄)₄ crystallizes in space group Cmcm with unit cell parameters of a = 8.5379(2), b = 12.1570(4), c = 9.1652(3) Å. The structural relationships, also to the known RE(BH₄)₃, are discussed in detail and related to the stability and synthesis conditions. Heat treatment of NaBH₄–Gd(BH₄)₃ mixture forms an unstable amorphous phase, which decomposes after one day at RT. NaCe(BH₄)₄ and NaPr(BH₄)₄ show reversible hydrogen storage capacity of 1.65 and 1.04 wt% in the fourth H₂ release, whereas that of NaEr(BH₄)₄ continuously decreases. This is mainly assigned to formation of metal hydrides and possibly slower formation of sodium borohydride. The dehydrogenated state clearly contains rare-earth metal borides, which stabilize boron in the dehydrogenated state.