Luminescent manifestations of ytterbium ions in the crystal structure of silicate apatite

The manifestation of ytterbium ions in the microcrystals of the Sr2Y6.8YbSi6O26:Er0.2 solid solution with apatite structure was studied on the basis of low-temperature photoluminescence (PL) spectroscopy and spectral-kinetic measurement data. Intracenter down-conversion luminescence of Er3+ ions in the visible and IR ranges was detected. Up-conversion luminescence of Er3+ ions is observed upon IR excitation due to excitation of Yb3+ ions and subsequent energy transfer Yb3+ -> Er3+. Overlapping bands at 412 and 430 nm with a Stokes shift of less than 0.3 eV are observed in the PL spectra at room temperature. These emission bands are assigned to the spinallowed and spin-forbidden 4f135d -> 4f14 transitions from the low-spin and high-spin exciting states of Yb2+ ion, respectively. The PL decay kinetics for transitions from the low-spin states is characterized by a dominant component tau = 0.39 ns. An alternative model of Yb3+ luminescence center associated with a charge transfer band does not explain the data of low-temperature (5K) PL spectroscopy. At T = 5 K, a new wide emission band with a maximum at 696 nm and a Stokes shift of 1.1 eV appears in the PL spectrum. Two options of this PL band nature are considered. The first is the luminescence of defects having a vacancy nature. The second possible option is the manifestation of anomalous luminescence of Yb2+ ions, which occupy a different crystallographic position in the apatite crystal structure.