Selective Formation of Yttrium Manganese Oxides through Kinetically Competent Assisted Metathesis Reactions

The synthesis of complex oxides requires
high temperatures to overcome
barriers imparted by solid-state diffusion; as such, reactions typically
yield the most stable polymorph for a given composition. To synthesize
new or metastable complex oxides, kinetically competent reactions
with lower initial energy barriers must be devised to control the
reaction pathway and resulting products. This contribution details
the selective synthesis of different yttrium manganese oxides through
assisted metathesis reactions between Mn2O3,
YCl3, and A2CO3 under flowing oxygen;
where A = Li, Na, K. With lithium carbonate, the orthorhombic perovskite
o-YMnO3 (o-YMnO3+δ) forms over the temperature
range of 550–850 °C. With sodium carbonate, the pyrochlore
Y2Mn2O7 forms at 650 °C. No
apparent selectivity is observed with K2CO3,
and all alkalis yields hexagonal YMnO3 at T > 950 °C. The alkali species modify the reaction pathway
and
thus impart kinetic control in the formation of both phases.