Intrinsic focusing of the particle size distribution in colloids containing nanocrystals of two different crystal phases.

We have studied the Ostwald ripening of colloids containing nanocrystals of two different crystal phases of the same material. Ostwald ripening in such polymorphic systems is shown to result in an intrinsic focusing of the particle size distribution of the thermodynamically preferred phase while the particles of the less stable phase completely dissolve. Experimentally, a colloidal system of this kind was realized by mixing small NaEuF4 nanocrystals of the cubic α-phase with particles of the hexagonal β-phase having the same mean size and size distribution. The temporal evolution of the particle sizes of both phases can be understood and numerically simulated within the framework of LSW theory. The simulations show that small differences in the bulk solubility or the surface energy of the two phases are sufficient to explain the experimentally observed narrowing of the particle size distribution.