Anomalous luminescence dynamics ofEu3+in BaFCl microcrystals

Abstract

Spectroscopic properties of ${\mathrm{Eu}}^{3+}$ ions $(l0.1\phantom{\rule{0.3em}{0ex}}\mathrm{at}\phantom{\rule{0.2em}{0ex}}%)$ at two main sites of BaFCl microcrystals are investigated. For ${\mathrm{Eu}}^{3+}$ ions at site I the energy levels and luminescence intensity distribution are similar to most other normal systems ever reported. But the $^{5}D_{0}$ lifetime of ${\mathrm{Eu}}^{3+}$ at site I decreases rapidly with the temperature and the nonradiative energy transfer (ET) rate is observed to be ${T}^{2}$ dependence. In contrast, ${\mathrm{Eu}}^{3+}$ at site II shows unusual energy level structure and luminescence intensity distribution with the strongest $^{5}D_{0}\ensuremath{\rightarrow}^{7}F_{0}$ transition. The nonradiative ET rate of ${\mathrm{Eu}}^{3+}$ at site II obeys a ${T}^{9}$ dependence. Spectroscopic results and theoretical analyses indicate that site II is associated with the charge compensator ${\mathrm{O}}^{2\ensuremath{-}}$ occupying the nearest ${\mathrm{F}}^{\ensuremath{-}}$ site around ${\mathrm{Eu}}^{3+}$. The mechanisms for anomalous luminescence properties at both sites are further discussed based on the model of charge transfer vibronic exciton (CTVE) and charge-imbalance induced large linear crystal-field potential.