First-principles study of complex halide scintillators for radiation detection*

Current demands for cost-effective and high-performance scintillators have led to a discernible shift from simple binary halides (e.g., NaI, CsI) toward host compounds that are structurally and electronically more complex. Eu-doped SrI$_2$ is a prominant example. Despite its advanced properties, improvements are needed for extensive deployment at low cost. Codoping techniques are often useful to improve the electronic response of such insulators. Using first-principles based approach we report on the influence of codoping with aliovalent and isovalent impurities. We find all codopants induce deep levels, show amphoteric character, and may bind with I-vacancy forming charge compensated donor-acceptor pairs. Lack of deep-to-shallow behavior upon codoping and its ramifications will be discussed. We studied another set of stable monoclinic phase of ternary ns$^2$ containing iodides, e.g. TlBa$_2$I$_5$. One objective is to explore them as scintillators where ns$^2$ ions play a central role. Interestingly, we predict Eu$^{2+}$ activation will be rendered ineffective in these compounds, caused by changes in the valence and conduction band edges. However, the prospect of fast electron capture at ns$^2$ sites and self-activated scintillation could be important for detector applications.