A low temperature pyrolytic route to amorphous quasi-hexagonal boron nitride from hydrogen rich (NH4)3Mg(BH4)5

Abstract

Pure amorphous quasi-hexagonal boron nitride with minute amounts of amorphous quasi-cubic form was obtained via thermal decomposition of a novel tri-ammonium magnesium penta-borohydride precursor, (NH₄)₃Mg(BH₄)₅, in the temperature range of 220–250 °C, which is significantly lower than 1000–1500 °C applied in industrial approaches. The (NH₄)₃Mg(BH₄)₅ precursor, the most hydrogen rich mixed-cation borohydride salt known to date (21 wt% H), was prepared via low temperature high-energy dry disc-milling. The compound adopts a tetragonal I4/mcm unit cell isostructural with Rb₃Mg(BH₄)₅ and Cs₃Mg(BH₄)₅. The multi-step thermal decomposition yields hydrogen contaminated with B₂H₆ and borazine volatiles. The solid residue rinsed with water corresponds to amorphous boron nitride of high purity as evidenced by ¹¹B MAS NMR, PXRD, FTIR and EDX analyses.