We have studied the effect of reducing the particle size on the crysta
l structures of copper oxide and cerium oxide. Below 25 nm, the cubic
Cu2O is found to be more stable than the monoclinic CuO. This confirms
an earlier conjecture that the ionic character of a solid tends to in
crease with a reduction in the particle size. As a result, high-symmet
ry crystal structures are more likely to be stable at smaller sizes. F
or cerium oxide, the bulk (cubic) CeO2 phase remains stable down to 4.
8 nm and, as expected, does not undergo a transition to any of the oth
er available structures with lower symmetry.