Oxide compound formation between boron and excess magnesium

Mixtures of boric anhydride (B₂O₃) or orthoboric acid (B(OH)₃) with MgO in 1:1, 1:10 and 1:100 ratios were heated in alumina crucibles from 400°C to 1000°C. The resulting crystalline compounds were identified by powder X-ray diffraction. Solid state ¹¹B NMR and FT-IR were used to identify the boron bonding environment. Three- and four-coordinate boron species were found in the mixtures at the various temperatures. Temperature had the greatest effect on which compounds could be identified, with no effect observed below 800°C. It was found that mixtures of boron oxide and magnesium oxide produced a magnesium hexaborate (MgB₆O₁₀·7H₂O) and suanite (Mg₂B₂O₅), whereas mixtures of boric acid and magnesium oxide produced kotoite (Mg₃(BO₃)₂) and suanite. Thus, the type of boron starting material in the solid mixture determined what compounds formed at higher temperatures. The implication is that, at low levels, the boron reagents combine with magnesium oxide differently during heating and with boric acid, before dehydration occurs.