GdTaO4 and (Gd1-xTbx)TaO4 (x = 0, 0.001, 0.02, 0.04, 0.10, and 0.16) single crystals were grown by the micro-pulling down method. The crystallographic system of the grown crystals was identified to be monoclinic (I2/a), which is the stable low-temperature phase of GdTaO4. In the photoluminescence (PL) spectra, intense emission was observed at 549 nm (5D3→7F4 transition of Tb3+) for every Tb doped GdTaO4. In the excitation spectra for the 549 nm emission, several excitation paths to the Tb3+ were attributed: 4f-4f transitions of Gd3+, 4f8-4f75d transitions of Tb3+ and charge transfer of TaO4 3− group. Radiolumincece spectra under X-ray irradiation were taken, and the highest emission peak was observed at 544 nm, which was ascribed to the 5D4→7F5 transition of Tb3+. The Tb 2% doped GdTaO4 showed the highest emission intensity at the 544 nm, and as increasing dopant concentration, the intensity decreased due to the cross-relaxation of Tb3+.
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