Spectroscopic study of the 4 f n-1 5 d transitions of LaPO 4 doped with Pr 3+  or co-doped with Pr 3+  and Gd 3+  in the vacuum ultra violet region.

The energy of f-d transitions depends on the crystalline field in which the lanthanide ion is inserted. Depending on the experimental setup, these transitions could occur at high energy, so several studies regarding theoretical data have been conducted. Here, we present the experimental determination of the energy of interconfigurational 4 f n  → 4 f n -1 5 d (f-d) transitions from Pr 3+ ions to the lanthanum orthophosphate LaPO 4 matrix; we have also determined the bandgap value for this host. The experiments were carried out at the Synchrotron setup of the Brazilian LNLS laboratory. Specifically, we synthesized LaPO 4 :Pr 3+ and LaPO 4 :Pr 3+ /Gd 3+ by the hydrothermal method under different pH conditions or by spray pyrolysis. The particles resulting from hydrothermal synthesis had different morphologies and the influence of pH value was showed: the reaction medium was controlled along the process, which changed the surface potential. On the basis of Raman spectroscopy and x-ray diffraction analyses, we found that the crystalline phase was monoclinic monazite for all the samples. We studied the 4 f 5 d level and bandgap transitions at high energy by absorption analysis in the VUV range. The experimental results were 7.5 eV (LaPO 4 bandgap) and 5 eV (4 f n → 4 f n -1 5 d transition of the Pr 3+ ion), which were close to the theoretical values reported in the literature for this ion and this matrix.