First-principles search for efficient activators for LaI3

Abstract

First-principles calculations were performed using density functional theory with Hubbard corrections or hybrid exchange-correlation functionals, as well as the GW approximation, to predict dopants that could serve as efficient activators for LaI3, a potentially very bright scintillator for which an appropriate activator has not been identified yet. The dopants considered in this work included a series of lanthanide ions (Ce, Pr, Nd, Eu, Gd, and Tb) and several ns2 ions (Tl, Pb, Bi, and Sb). Based on both ground-state calculations and constrained DFT calculations to simulate excited states, the trivalent lanthanide dopants were shown not to constitute an improvement over Ce3+, for which experimental data exist, as they showed occupied 4f states below the valence band maximum (VBM) and 5d states above the conduction band maximum (CBM). In contrast, the only divalent lanthanide considered, Eu2+, displayed occupied 4f states within the band gap of the host, but its 5d states were calculated to be above the CBM. Eu2+ could nonetheless be exploited as an activator by increasing the band gap energy slightly through substitution of iodide ions by bromide ions. Similar results were obtained for Tl+. Finally, Bi3+ and Sb3+ were predicted to be efficient activators in LaI3 by virtue of having unoccupied p states below the CBM, which can serve as electron traps and can combine with holes at the VBM to form localized luminescence centers. Therefore, Bi- and Sb-doped LaI3 should be grown and tested experimentally. Comparison with empirical relationships of the relative positions of the energy levels of rare-earth dopants and implications for efficient doping schemes and scintillation mechanisms in LaI3 are also discussed.