Phase Diagram for the NaBH₄–KBH₄ System and the Stability of a Na_1–xK_xBH₄ Solid Solution

The binary system NaBH 4 –KBH 4 has been systematically investigated by several complementary techniques, including in situ synchrotron radiation powder X-ray diffraction (SR-PXD), thermal analysis, and in situ 11 B and 23 Na magic-angle spinning (MAS) NMR. Full solubility in the system NaBH 4 –KBH 4 and formation of a solid solution, Na 1– x K x BH 4, 0 < x < 1, upon heat treatment, T > 110 °C, are observed. The Na 1– x K x BH 4 solid solution is most clearly observed by 11 B MAS NMR, which reveals a binomial distribution of Na + and K + ions over the six metal sites in the first coordination sphere of the BH 4 – 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 Na 1– x K x BH 4 is metastable at room temperature, and 75% of Na 0.5 K 0.5 BH 4 has separated into NaBH 4 and KBH 4 after 24 h at ∼24 °C, as observed by in situ 23 Na and 11 B MAS NMR. The composition 0.682NaBH 4 –0.318KBH 4 has the lowest melting point of T = 458 °C, which gives rise to a lower hydrogen release temperature compared to the reactants, possibly due to enhanced kinetics. A phase diagram for the system NaBH 4 –KBH 4 has been established, summarizing the results in a concise manner.