Phase Diagram for the NaBH4–KBH4 System and the Stability of a Na1–xKxBH4 Solid Solution

Abstract

The binary system NaBH4–KBH4 has been systematically investigated by several complementary techniques, including in situ synchrotron radiation powder X-ray diffraction (SR-PXD), thermal analysis, and in situ 11B and 23Na magic-angle spinning (MAS) NMR. Full solubility in the system NaBH4–KBH4 and formation of a solid solution, Na1–xKxBH4, 0 110 °C, are observed. The Na1–xKxBH4 solid solution is most clearly observed by 11B MAS NMR, which reveals a binomial distribution of Na+ and K+ ions over the six metal sites in the first coordination sphere of the BH4– unit, i.e., seven almost equally distinct peaks in the range −36.4 to −42.6 ppm with a systematic change of 1.0 ± 0.1 ppm toward lower frequency for each replacement of K+ by Na+. The solid solution Na1–xKxBH4 is metastable at room temperature, and 75% of Na0.5K0.5BH4 has separated into NaBH4 and KBH4 after 24 h at ∼24 °C, as observed by in situ 23Na and 11B MAS NMR. The composition 0.682NaBH4–0.318KBH4 has the lowest m...