Facile Formation of Thermodynamically Unstable Novel Borohydride Materials by a Wet Chemistry Route

Abstract

A novel wet synthetic method utilizing weakly coordinating anions that yields LiCl-free Zn-based materials for hydrogen storage has recently been reported. Here we show that this method may also be applied for the synthesis of the pure yttrium derivatives, M[Y(BH4)4] (M = K, Rb, Cs). Moreover, it can be extended to the preparation of previously unknown thermodynamically unstable derivatives, Li[Y(BH4)4] and Na[Y(BH4)4]. Importantly, these two H-rich phases cannot be accessed by standard dry (mechanochemical) or solid/gas synthetic methods due to the thermodynamic obstacles. Here we describe their crystal structures and selected important physicochemical properties.

Rare Earth Borohydrides—Crystal Structures and Thermal Properties

“…The Li + atoms have a disorder distributed over half-occupied sites (4k) along the tetragonal c-axis. The same structure type has also been found for LiRE(BH 4 ) 4 where RE = Y [10,40], Yb [29] (with 25%…”

“…The Li + atoms have a disorder distributed over half-occupied sites (4k) along the tetragonal c-axis. The same structure type has also been found for LiRE(BH4)4 where RE = Y [10,40], Yb [29] (with 25% of BH4 − substituted by Cl − ) and Lu [19]. The only difference from the LiSc(BH4)4 structure is that Li + were reported in a fully occupied 2-fold site (2a), tetrahedrally coordinated by 4BH4 − for RE = Y and Yb.…”

“…The Na(BH 4 ) 6 octahedra form edge-sharing chains along the c-axis, with the chains also being interconnected by egde-sharing Sc(BH 4 ) 4 tetrahedra. NaY(BH 4 ) 4 [10,40] 12 (space group Ia3d) are reported to have a garnet-type structure [48]. All other reported bi-and tri-metallic borohydrides with structure types not discussed above, are included in Table 4.…”

“…The Na(BH4)6 octahedra form edge-sharing chains along the c-axis, with the chains also being interconnected by egde-sharing Sc(BH4)4 tetrahedra. NaY(BH4)4 [10,40], NaEr(BH4)4 [42], KHo(BH4)4 [32], o-KY(BH4)4 [43], KEr(BH4)4 [44], and KYb(BH4)4 [45] are isostructural to NaSc(BH4)4. KSc(BH4)4 (space group Pnma) contains similar tetrahedral [Sc(BH4)4] − complexes while the coordination number of alkali metal increases further to eight [46].…”

Synthesis and characterization of a series of mixed-cation borohydrides of scandium: [ Cat ][Sc(BH 4 ) 4 ], [ Cat ] = [Me 4 N], [ n -Bu 4 N], and [Ph 4 P]

“…Here the final product is precipitated in a solvent-mediated metathesis reaction between a salt of weakly-coordinating anion, This process has been successfully tested for zinc-and yttrium-based borohydrides for M=Li-K, and Li-Cs, respectively, and dichloromethane (DCM) used as a solvent [14,15,16].…”