From M(BH₄)₃ (M = La, Ce) Borohydride Frameworks to Controllable Synthesis of Porous Hydrides and Ion Conductors

Rare earth metal borohydrides show a number of interesting properties, e.g., Li ion conductivity and luminescence, and the series of materials is well explored. However, previous attempts to obtain M­(BH₄)₃ (M = La, Ce) by reacting MCl₃ and LiBH₄ yielded LiM(BH₄)₃Cl. Here, a synthetic approach is presented, which allows the isolation of M­(BH₄)₃ (M = La, Ce) via formation of intermediate complexes with dimethyl sulfide. The cubic c-Ce­(BH₄)₃ (Fm3̅c) is isostructural to high-temperature polymorphs of A­(BH₄)₃ (A = Y, Sm, Er, Yb) borohydrides. The larger size of the Ce³⁺ ion makes the empty void in the open ReO₃-type framework structure potentially accessible to small guest molecules like H₂. Another new rhombohedral polymorph, r-M­(BH₄)₃ (M = La, Ce), is a closed form of the framework, prone to stacking faults. The new compounds M­(BH₄)₃ (M = La, Ce) can be combined with LiCl in an addition reaction to form LiM­(BH₄)₃Cl also known as Li₄[M₄(BH₄)₁₂Cl₄]; the latter contains the unique tetranuclear cluster [M₄(BH₄)₁₂Cl₄]^4– and shows high Li-ion conductivity. This reaction pathway opens a way to synthesize a series of A₄[M₄(BH₄)₁₂X₄] (M = La, Ce) compounds with different anions (X) and metal ions (A) and potentially high ion conductivity.