Energy migration and $Gd^{3+}$ ↔ $Ce^{3+}$ transfer in $Ce^{3+}$ -doped $GdP_3O_9$ metaphosphate

Luminescence-kinetics studies of Ce3+-doped GdP3O9 metaphosphate, which was prepared by a melt-solution technique, are performed using time-resolved luminescence spectroscopy techniques at T = 4.2–300 K. The processes of energy migration along the Gd3+ sub-lattice and energy transfer between Gd3+ and Ce3+ ions are investigated. At low temperatures, the emission of Gd3+ is caused by the radiative decay of its lowest-energy relaxed excited 6P7/2 state, and the Gd3+ → Ce3+ energy transfer is absent. As the temperature increases, the thermally stimulated population of the higher energy 6P5/2, 6P3/2 levels of the Gd3+ 6PJ-excited state takes place, which reveals itself in a higher energy shift of the Gd3+ 6PJ → 8S7/2 emission band. The phonon-assisted population of the Gd3+ 6P5/2 excited level at T < 150 K is suggested to be responsible for the increase in probability of energy migration along the Gd3+ sub-lattice followed by the Gd3+ → Ce3+ energy transfer. The more effective Gd3+ → Ce3+ energy transfer at T > 150 K takes place owing to the thermally stimulated population of the highest-energy 6P3/2 excited level together with the lower energy shift of the Ce3+ 4f–5d1 absorption band. Unlike the Gd3+ → Ce3+ transfer, the reverse Ce3+ → Gd3+ energy transfer is observed at any temperature