Mixtures of boric anhydride
(B
$_{2}$
O
$_{3}$
) 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
$^{11}$
B 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
(MgB
$_{6}$
O
$_{10}$
·7H
$_{2}$
O)
and suanite
(Mg
$_2$
B
$_{2}$
O
$_{5}$
),
whereas mixtures of boric acid and magnesium oxide produced kotoite
(Mg
$_{3}$
(BO
$_{3}$
)
$_{2}$
)
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.
