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

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)(3)Mg(BH4)(5), in the temperature range of 220-250 degrees C, which is significantly lower than 1000-1500 degrees C applied in industrial approaches. The (NH4)(3)Mg(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 B-11 MAS NMR, PXRD, FTIR and EDX analyses.