Rare-Earth Tantalates and Niobates Single Crystals: Promising Scintillators and Laser Materials

Renqin Dou, Qingli Zhang, Jinyun Gao, Yuanzhi Chen, Shoujun Ding, Fang Peng, Wenpeng Liu, Dunlu Sun
2018 Crystals  
Rare-earth tantalates, with high density and monoclinic structure, and niobates with monoclinic structure have been paid great attention as potential optical materials. In the last decade, we focused on the crystal growth technology of rare-earth tantalates and niobates and studied their luminescence and physical properties. A series of rare-earth tantalates and niobates crystals have been grown by the Czochralski method successfully. In this work, we summarize the research results on the
more » ... esults on the crystal growth, scintillation, and laser properties of them, including the absorption and emission spectra, spectral parameters, energy levels structure, and so on. Most of the tantalates and niobates exhibit excellent luminescent properties, rich physical properties, and good chemical stability, indicating that they are potential outstanding scintillators and laser materials. Crystals 2018, 8, 55 2 of 21 studied systematically with polycrystalline samples produced by solid-state reaction [9] . Although the optical absorption and luminescence of RETaO 4 crystals are described, the crystal size is too small, only 7 mm × 7 mm × 1 mm [10] . It is urgent to develop the growth technology of large RETaO 4 crystals for the device application. Recently, our group has made great progress in the Cz growth of large-size tantalite crystals and obtained big and fine GdTaO 4 single crystals, which would promote the growth and practical application of RETaO 4 [16] . In addition, REBO 4 have been considered appropriately as host matrices and have also attracted wide interests as potential rare earth-doped laser hosts [17] . They belong to monoclinic system and have large crystal field energy. The active ions occupy the site with C 2 symmetry (RE sites), which is advantageous to remove the parity-forbidden rule of f-f transition. Therefore, it is beneficial for the realization of new emission and tunable wavelength. Additionally, their mechanical and thermal properties are also sufficient for laser applications. Recently, on the one hand, our group have finished a lot of research works on scintillation characteristics of RETaO 4 , such as GdTaO 4 , Tb:GdTaO 4 , LuTaO 4 , Nd:LuTaO 4 , and so on. The luminescence results indicate that GdTaO 4 and Nd:LuTaO 4 are the promising scintillator candidates. On the other hand, the emission spectra of active ions (Yb, Nd, Er, Ho) doped GdBO 4 , YBO 4 , and mixed GdYBO 4 crystals show that they can be used as the new materials for visible, near, and middle infrared lasers. Meanwhile, near infrared lasers are realized on the Yb and Nd-doped ReBO 4 successfully. Experiments Polycrystalline Synthesis In this paper, LuTaO 4 and Nd-doped LuTaO 4 series samples were prepared by solid-state reaction method using Lu 2 O 3 (99.99%), Ta 2 O 5 (99.99%), and Nd 2 O 3 (99.999%) as starting materials. First, the raw materials were weighed accurately according to the appropriate stoichiometric ratio and then mixed and ground in a mortar. The mixed powders were then heated to 1500 • C at a heat rate of approximately 2.3 • C/min and calcined at 1500 • C for 24 h. Finally, the calcined samples were slowly cooled to room temperature and then carefully ground for the measurements. Single Crystal Growth All of the crystals, mentioned in the text, were grown by the Czochralski (Cz) method. Raw materials, with high purity (>4 N), were weighed stoichiometrically, mixed thoroughly, and then pressed into disks. The disks were put into an iridium crucible and melted in a JGD-60 furnace with an automatic diameter controlled system. Using a seed, the crystals were grown in nitrogen atmosphere with a rotation speed of 3.0-10.0 rpm and pulling rate of 0.35-1 mm/h. The growth process of all crystals is similar. The as-grown crystals and some samples are shown in Figure 1 .
doi:10.3390/cryst8020055 fatcat:riotfkzwbrdcdcdqzjmkwt6xai