The growth of NdPO4 nanocrystals in diphenyl ether based reaction medium has been monitored by optical spectroscopy. Since the crystal field splitting and the intensity of the Nd3+ f−f transitions are affected by the coordination of the Nd3+ ions, different stages of the growth of NdPO4 nanocrystals can be distinguished by UV−visible absorption spectroscopy. It is found that a first reaction in homogeneous solution occurs already at room temperature when the phosphate-containing precursor solution is combined with the solution of the molecular Nd3+ precursor. Our results clearly indicate that a sol of NdPO4 nanoparticles with a mean particle diameter below 3 nm is thereby formed as the first product. Upon heating to 200 °C, this first product undergoes a complex sol−gel−sol transition, leading to a colloidal solution of 4−5 nm diameter NdPO4 nanocrystals. Further heating at 200 °C leads to annealing of the nanocrystalline lattice, but not to further particle growth if the amine concentration is sufficiently high.

The evolution of the geometric and electronic structures within the entire series of lanthanide orthophosphate nanoparticles ͑ϳ2-ϳ 5 nm͒ has been determined experimentally with X-ray diffraction and near edge X-ray absorption fine structure spectroscopy. In particular, the interplay between electronic structure, crystal morphology, and crystal phase has been systematically studied. A missing local order in the crystal structure accompanied by multiple ion sites in the nanoparticles was revealed to be due to the small crystal size and large surface contribution. All lanthanide ions were found to be in "3+" configuration and accommodated in three different crystallization states: the larger lanthanide ions ͑La, Ce, Pr, Nd, Sm͒ in the monoclinic phase, the smaller ones ͑Er, Tm, Yb, Lu͒ in the tetragonal phase, and the intermediate lanthanide ions ͑Eu, Gd, Tb, Dy, Ho͒ in a "mixed phase" between monoclinic and tetragonal phases.

By varying the type of the amines used in the synthesis of CePO4:Tb nanoparticles it is possible to direct the colloidal solubility in solvents of different polarity (see image). Moreover, it is shown that the resulting particle size is influenced by the concentration of the amine used in synthesis. Particles in the range from 4 to 24 nm in diameter are prepared.

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