Probing Site Symmetry Around Eu~(3+) in Nanocrystalline ThO_2 Using Time Resolved Emission Spectroscopy

摘要

ThO_2:Eu~(3+) nanoparticles were synthesized at 300℃ by combustion route using urea as a fuel and characterized by thermo-gravimetric/differential thermal analysis, X-ray diffraction, transmission electron microscopy, and photoluminescence techniques. To investigate the effect of anneah'ng temperature, as synthesized powder were heated further at 500℃, 700℃, and 900℃. It was observed that extent of asymmetry around Eu~(3+) at 700℃/900℃ is very high as compared to as-prepared or 500℃ annealed sample. Based on the time resolved emission spectroscopic investigations, it was inferred that two different types of Eu~(3+) ions were present in the ThO_2 nanoparticles. In ThO_2 structure, Eu~(3+) ions occupy two sites; cubic (O_h) and noncubic (＜C_(2v)) as can be confirmed from our emission studies. Short-lived species T_1 (～1.3-3.4 ms) predominates at higher annealing temperature arises because of Eu~(3+) ions occupying noncubic (＜C_(2v)) sites without inversion symmetry, whereas long-lived species T_2 (～4.6-6.6 ms) can be ascribed to Eu~(3+) ions occupying cubic sites (O_h) with inversion symmetry.