Thermal behaviour of the NaYF4:Yb3+,R3+ materials

Abstract

The NaYF4:Yb3+,R3+ (R: none, Pr, Nd, Sm, Eu, Tb or Dy) materials were prepared with selected dopant concentrations, using co-precipitation synthesis, to study the effect of the dopant and its concentration on the structure of these materials. The thermal behaviour of the as-prepared materials was studied with differential scanning calorimetry (DSC) and thermogravimetry. The structures prior to and after annealing were identified with X-ray powder diffraction. The materials were mainly hexagonal with occasional slight cubic impurity. The DSC curves revealed the cubic-to-hexagonal phase transition at 400–450 °C which temperature changes irregularly with the R3+ dopant and its concentration. The specific enthalpy of this transition varies also in a complicated way but may be correlated with the completeness of the transition. The hexagonal-to-cubic transition temperature (ca. 670 °C) is rather constant regardless of the R3+ dopant or its concentration. The temperatures and specific enthalpies of the phase transitions are useful when choosing the optimum dopant concentrations. It is also possible to estimate the structure prior to annealing with significant savings in use of resources. Only with Sm3+ and Dy3+ doping, no visible up-conversion luminescence was observed—in addition to the Er3+ and Tm3+ impurity emission. Eventually, it was found that the hexagonal form gives much stronger up-conversion luminescence and changes in the rare earth concentration allows the formation of this form.