Size-induced structural transitions in the Cu-O and Ce-O systems

We have studied the effect of reducing the particle size on the crystal structures of copper oxide and cerium oxide. Below 25 nm, the cubic ${\mathrm{Cu}}_{2}$O is found to be more stable than the monoclinic CuO. This confirms an earlier conjecture that the ionic character of a solid tends to increase with a reduction in the particle size. As a result, high-symmetry crystal structures are more likely to be stable at smaller sizes. For cerium oxide, the bulk (cubic) ${\mathrm{CeO}}_{2}$ phase remains stable down to 4.8 nm and, as expected, does not undergo a transition to any of the other available structures with lower symmetry. \textcopyright{} 1996 The American Physical Society.