The effects of the NaF flux on the oxidation state and localisation of praseodymium in Pr-doped zircon pigments

The role that NaF plays in the preparation of Pr-doped zircon pigments was studied through the analysis of the nature and localisation of the Pr cations into the zircon matrix in samples prepared in the absence and in the presence of NaF. As previously observed, the addition of NaF caused a decrease of the minimum temperature required for zircon formation from 1400 to 1100°C, and an increase of the yellow colour intensity. In the absence of NaF, the Pr cations mainly presented a threefold oxidation state, being located out of the zircon lattice as Pr 2Zr 2O 7, whereas in the presence of this flux, most of the Pr cations showed a fourfold valence and formed a solid solution with the zircon lattice, which was then the main responsible for the stronger yellow colour observed in this case. After heating this pigment at 1400°C, we detected an exsolution of the Pr (IV) cations as Pr 8Si 6O 24 which was accompanied by a decrease of the yellow colour intensity. Therefore, it was concluded that the main role of NaF in the preparation of yellow Pr-zircon pigments is to decrease the temperature of zircon formation to the range in which the chromophore responsible for the bright yellow colour, i.e. the Pr (IV)-zircon solid solution, is stable.