Correlational analysis of Eu3+ charge transfer state using La effective charge in La-based mixed-anion host compounds

Abstract

A prediction of the Eu3+ charge transfer state (ECT) was attempted in La-based mixed-anion host compounds. We paid attention to La3OF3S2:Eu, since it is expected to have a more covalent La site than La2O2S. The La effective charge (La EC) was proposed as the index factor of covalency and/or ionicity. The correlation between the experimental ECT and the calculated La EC was systematically analyzed for La2S3, LaFS, La2O2S, La2O3, LaOF, and LaF3 host materials, and good approximation was obtained using the single exponential function with a variable number of La ECs. According to the fitting curve, the ECT of La3OF3S2:Eu was predicted to be 5.8 and 2.1 eV for Eu3+ centers activated at ionic and covalent sites, respectively. To confirm the prediction accuracy, La3OF3S2:Eu phosphor powder samples were synthesized by solid-state reaction. From the photoluminescence excitation and absorption measurements, the ECT values of about 4.7 eV (ionic La site) and 2.4 eV (covalent La site) were obtained. Even though the energy difference between the predicted and experimental values is large for the higher ECT, La EC is the useful index factor for estimating ECT. In addition, it indicates that the estimation can be applied to phosphor materials having multication sites.