Electronic and Vibronic Spectra of Ce3+ in YPO4 and LuPO4

Rather sharp zero-phonon lines accompanied by many vibronic lines are observed in the near-ultraviolet luminescence and excitation spectra due to the 4f-5d transitions of Ce 3+ ion in YPO 4 and LuPO 4 crystals at low temperature. Comparing the vibrational energies of the vibronic lines with the results of infrared and Raman measurements of YPO 4 crystals, it is shown that these vibronic lines can be separated into two parts; the one is due to the lattice (external) vibration of Y 3+ and PO 4 3- ions and the other is due to the coupled molecular (internal) vibration of PO 4 complexes. The coupling scheme of these vibrations with the 4f or 5d electron of Ce 3+ ion is discussed.