Caracterização estrutural e óptica do composto NaYP2O7 dopado com terras raras

Abstract

The aims of this work were to produce and investigate the properties of NaYP2O7 and NaY0.99Ln0.01P2O7 (Ln = Ce, Sm, Eu, Tb and Yb) polycrystalline samples. The synthesis was done via a modified sol-gel method using PVA as polymeric agent. In order to obtain single crystalline phase, it was necessary to use molar ratio Na:Y:P = 1.15:1:2, which is not the stoichiometric, with two calcination steps at 400 ºC / 4 h and 600 ºC / 4 h. Structural characterizations of the synthesized material were performed by X-ray Diffraction and Absorption (XRD and XAS). It was observed that the dopant ions were incorporated into the matrix without changing the structure. Under VUV excitation, the undoped sample presented a luminescent emission spectrum which was attributed to self-trapped exciton recombination. On the other hand, the photoluminescence (PL) of doped samples presented the emission lines characteristic of each dopant. From the photoluminescence excitation curves, 4f-5d transitions were identified for Ce3+, Sm3+ and Tb3+ doped samples and charge transfer processes were noticed for that of Eu3+, Sm3+ and Yb3+. The experimental data of PL was used to determine the positions of the energy levels of all lanthanides (with oxidation states of 3+ or 2+) relative to the valence and conduction bands of NaYP2O7. The model successfully provided the positions of electronic levels within the bandgap, with excellent agreement with the experimental spectra. Thermoluminescence (TL) glow curves were measured for co-doped samples NaYP2O7:Ce3+,Ln3+ (Ln = Dy, Ho and Sm) in order to investigate the nature of the trapping centres. The values determined for the activation energies were comparable with the position of dopant-induced electronic levels, whereas the frequency factor related to the TL peak of Ho-doped sample agreed with the phonon frequency for the lattice. The results indicate that the Ce3+ acts as recombination centre whereas the codopants create electron trap levels.