Energy migration and relaxation through Gd3+ sublattice in NaGdF4

Time-resolved spectra of luminescence from the lowest excited state of Gd3+ (6P7/2) have been studied in various Gd compositions in NaY1−xGdxF4 (0.1⩽x⩽1). Two emission bands are observed at room temperature: 311 nm band corresponding to an intrinsic state on Gd sublattice, and 313 nm band corresponding to a trap state on irregular Gd sites. In NaGdF4, a temporal profile of the 311 nm band shows an apparent rise at about 100 μs and a long decay time of about 10 ms, while that of the 313 nm band shows a simple exponential decay with about 3 μs. When Gd concentration is diluted with Y3+, the rise part of the 311 nm band decreases and the decay time of the 313 nm band becomes long, though the rise time of the 311 nm band does not change. At low temperature, the 311 nm band disappears and the decay time of the 313 nm band is much prolonged in NaGdF4. These results are discussed in terms of the energy transfer from the trap state to the intrinsic Gd state in the Gd sublattice. The trap state plays an important role in the dynamics of the energy relaxation.