Site-selective spectroscopy and determination of energy levels in Eu3+-doped strontium fluorophosphate

Absorbance and site-selective fluorescence studies of Eu3+:Sr5(PO4)3F (Eu:SFAP) provide a nearly complete characterization of the Eu3+ energy levels. These results are compared with calculated energies generated by lattice-sum analysis. Eu3+ is found to occupy one site predominantly, but numerous minority sites are also evident; polarization studies of absorption and fluorescence lines provide evidence for the principle site having Cs symmetry. Vibronic transitions occur in spectra as well. Interpretation of the lattice location and charge compensation mechanism for the principal site is discussed in the context of obstacles arising from spectral interpretation, high crystal fields in SFAP, low site symmetry, low J-numbers in Eu3+ energy manifolds, and lattice covalency. Eu2+ emission upon ultraviolet excitation is also demonstrated; valleys in the emission profile are attributed to intracrystal absorption by Eu3+.