First-principles study of luminescence in Eu${}^{2+}$-doped inorganic scintillators

Luminescence in Eu${}^{2+}$ activated materials corresponds to a transition from an excited state where the lowest Eu 5$d$ level is filled with one electron [often called the (Eu${}^{2+}$)${}^{*}$ state] to the ground state with a half-filled 4$f$ shell with seven electrons of the same spin. We have performed theoretical calculations based on density functional theory to determine the ground state band structure of Eu-doped materials as well as study the (Eu${}^{2+}$)${}^{*}$ excited state. Calculations were performed on Eu-doped materials, experimentally known to be either scintillators or nonscintillators, in order to relate theoretically calculable parameters to experimentally observed properties. Applying criteria previously developed for Ce-doped systems [A. Canning, A. Chaudhry, R. Boutchko, and N. Grønbech-Jensen, Phys. Rev. B 83, 125115 (2011)] to new Eu-doped materials, we developed a list of candidate materials for new bright Eu-activated scintillators. Ba${}_2$CsBr${}_5$:Eu is an example of a new bright scintillator from our candidate list that has been synthesized in microcrystalline powder form. As discussed in our previous paper on Ce-doped materials, this approach was designed as a systematic high-throughput method to aid in the discovery of new bright scintillator materials by prioritization and down-selection on the large number of potential new materials.

Figure 1

Density of states calculated for BaI${}_2$:Eu with (a) $U_{\mathrm{eff}}=2.2$ eV and (b) $U_{\mathrm{eff}}=6$ eV. Eu 4$f$ (spin up) states are clearly below the VBM for $U_{\mathrm{eff}}=6$ eV. Fermi level is set at 0 eV.

Figure 2

Lowest excited state plots for Eu-doped scintillators and nonscintillators (a) SrI${}_2$, (b) Ba${}_2$AlS${}_4$, (c) SrGa${}_2$O${}_4$, (d) BaClBr, (e) Ba${}_2$CsI${}_5$, and (f) Ba${}_2$HfS${}_4$. Plots show the charge density isosurfaces of the excited state at 50% threshold. The excited state is delocalized for the nonluminescent compound SrGa${}_2$O${}_4$:Eu and has a predominantly Ga $s$ character, while the excited state has $d$ character for other compounds. Ba${}_2$HfS${}_4$:Eu is a predicted nonscintillator, wherein the first excited state has Hf 5$d$ character.