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

A new bialkali borohydride, NaK(BH4)2, was synthesized by mechanical milling of NaBH4 and KBH4 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(BH4)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 BH4− anion, while magnetic resonance chemical shifts in 23Na, and 39K 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 R3, distinct from the starting materials NaBH4, and KBH4. Although in situ XRD measurements indicated the material to be metastable, decomposing to the starting materials NaBH4 and KBH4, the successful synthesis of NaK(BH4)2 demonstrates the ability of computational screening to predict potential candidates for hydrogen storage materials.