NaSc(BH₄)₄: A Novel Scandium-Based Borohydride

Abstract

A new alkaline transition-metal borohydride, NaSc(BH 4 ) 4 , is presented. The compound has been studied using a combination of in situ synchrotron radiation powder X-ray diffraction, thermal analysis, and vibrational and NMR spectroscopy. NaSc(BH 4 ) 4 forms at ambient conditions in ball-milled mixtures of sodium borohydride and ScCl 3 . A new ternary chloride Na 3 ScCl 6 (P2 1 /n, a ) 6.7375), isostructural to Na 3 TiCl 6 , was identified as an additional phase in all samples. This indicates that the formation of NaSc(BH 4 ) 4 differs from a simple metathesis reaction, and the highest scandium borohydride yield (22 wt %) was obtained with a reactant ratio of ScCl 3 /NaBH 4 of 1:2. NaSc(BH 4 ) 4 crystallizes in the orthorhombic crystal system with the space group symmetry Cmcm (a ) 8.170(2) Å, b ) 11.875(3) Å, c ) 9.018(2) Å, V ) 874.9(3) Å 3 ). The structure of NaSc(BH 4 ) 4 consists of isolated homoleptic scandium tetraborohydride anions, [Sc(BH 4 ) 4 ] -, located inside slightly distorted trigonal Na 6 prisms (each second prism is empty, triangular angles of 55.5 and 69.1°). The experimental results show that each Sc 3+ is tetrahedrally surrounded by four BH 4 tetrahedra with a 12-fold coordination of H to Sc, while Na + is surrounded by six BH 4 tetrahedra in a quite regular octahedral coordination with a (6 + 12)-fold coordination of H to Na. The packing of Na + cations and [Sc(BH 4 ) 4 ] -anions in NaSc(BH 4 ) 4 is a deformation variant of the hexagonal NiAs structure type. NaSc(BH 4 ) 4 is stable from RT up to ∼410 K, where the compound melts and then releases hydrogen in two rapidly occurring steps between 440 and 490 K and 495 and 540 K. Thermal expansion of NaSc(BH 4 ) 4 between RT and 408 K is anisotropic, and lattice parameter b shows strong anomaly close to the melting temperature.

Structure and Characterization of KSc(BH4)4

“…The reaction between KBH 4 and K 3 ScCl 6 observed by in situ SR-PXD corresponds well with the mass loss and thermal events observed in TGA and DSC between 510 and 590 K. Formation of a solid solution was also observed recently during decomposition of NaSc(BH 4 ) 4 . 25 Furthermore, fast dissolution (within minutes) of LiCl in LiBH 4 was observed at 383 K < T < 523 K and relatively slow segregation of LiCl from the solid solution Li(BH 4 ) 1-x Cl x at RT.…”

“…24,25 Here, we report on the synthesis, crystal structure, hydrogen desorption properties, and Raman, infrared, and NMR spectra of another scandium-based bimetallic borohydride, KSc(BH 4 ) 4 .…”

“…25 In both cases, one of the starting products of the synthesis, the alkali metal borohydride, is formed in the first step of hydrogen release. Then, the ternary alkaline scandium chloride formed during the initial synthesis plays a positive role and is involved in the second step of the reaction of hydrogen release.…”

First-principles predicted low-energy structures of NaSc(BH4)4

“…Experimental data are taken from Ref. 12. For these materials, PBE calculations predict very low-density structures to be thermodynamically stable while, at the same time, produce large deviations when optimizing their complicated experimental structures.…”

“…12 Details on these decomposition steps have not been experimentally determined yet. 12 Soon after the experimental synthesis of NaSc(BH 4 ) 4 , several theoretical studies were carried out. Huang et al 13 reported that the Cmcm phase of NaSc(BH 4 ) 4 is an insulator with a band gap of E g = 5.055 eV.…”