Metastability and crystal structure of the bialkali complex metal borohydride NaK(BH4)2

Abstract A new bialkali borohydride, NaK(BH 4 ) 2 , was synthesized by mechanical milling of NaBH 4 and KBH 4 in a 1:1 ratio. The synthesis was conducted based on a prediction from a computational screening of hydrogen storage materials suggesting the potential stability of NaK(BH 4 ) 2 . The new phase was characterized using X-ray diffraction, Raman scattering and magic angle spinning (MAS) nuclear magnetic resonance (NMR). The Raman measurements indicated B–H vibrations of the BH 4 − anion, while magnetic resonance chemical shifts in 23 Na, and 39 K MAS NMR spectra showed new chemical environments for Na and K resulting from the formation of the new bialkali phase. X-ray diffraction spectra indicated a new crystal structure with rhombohedral symmetry, most likely in the space group R 3, distinct from the starting materials NaBH 4 , and KBH 4 . Although in situ XRD measurements indicated the material to be metastable, decomposing to the starting materials NaBH 4 and KBH 4 , the successful synthesis of NaK(BH 4 ) 2 demonstrates the ability of computational screening to predict potential candidates for hydrogen storage materials.