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(BH 4 ) 3 (M = La, Ce) by reacting MCl 3 and LiBH 4 yielded Li M (BH 4 ) 3 Cl. Here, a synthetic approach is presented, which allows the isolation of M(BH 4 ) 3 (M = La, Ce) via formation of intermediate complexes with dimethyl sulfide. The cubic c -Ce(BH 4 ) 3 ( Fm 3̅ c ) is isostructural to high-temperature polymorphs of A(BH 4 ) 3 (A = Y, Sm, Er, Yb) borohydrides. The larger size of the Ce 3+ ion makes the empty void in the open ReO 3 -type framework structure potentially accessible to small guest molecules like H 2 . Another new rhombohedral polymorph, r -M(BH 4 ) 3 (M = La, Ce), is a closed form of the framework, prone to stacking faults. The new compounds M(BH 4 ) 3 (M = La, Ce) can be combined with LiCl in an addition reaction to form LiM(BH 4 ) 3 Cl also known as Li 4 [M 4 (BH 4 ) 12 Cl 4 ]; the latter contains the unique tetranuclear cluster [M 4 (BH 4 ) 12 Cl 4 ] 4– and shows high Li-ion conductivity. This reaction pathway opens a way to synthesize a series of A 4 [M 4 (BH 4 ) 12 X 4 ] (M = La, Ce) compounds with different anions (X) and metal ions (A) and potentially high ion conductivity.