First-principles study of rare-earth-free Cs4SrI6:Tl, a zero-dimensional halide perovskite for scintillation applications.

In recent years, rare-earth-activated perovskite scintillators have drawn significant research attention owing to their characteristic and efficient 4f n ↔ 4f n-15d electronic transitions. However, the high cost and limited readiness of rare-earth elements motivate the search for alternative activators. In this context, thallium ions (Tl+) can offer a promising activator that can act as a two-level luminescent center, yielding characteristic emissions through 6s16p1 → 6s2 electronic transitions, similar to the well-known NaI:Tl scintillator while remaining cost-effective. In this investigation, density functional theory (DFT) calculations were performed to explore the opto-electronic and scintillation-relevant properties of the undoped and Tl+-doped Cs4SrI6 halide perovskites. The computed band structures demonstrated the reduction of band gap with the increasing Tl+ concentrations. The observed relatively flat bands near the band edges suggested large carrier effective masses, which may suppress carrier mobility and promote carrier localization. In halide systems with strong electron-phonon coupling, such localization can lead to the formation of self-trapped excitons that may recombine radiatively through Tl+ activator centers, enhancing the emission efficiency. More specifically, Tl+ introduces localized Tl+-6s* and Tl+-6p* states above valence band maximum (VBM) and below conduction band minimum (CBM) of the host Cs4SrI6, respectively. As the Tl+ concentration increases, these states (especially Tl+-6s*) become more prominent and enhance the probability of efficient and characteristic radiative emission from Tl+-6p*(6s16p1) → Tl+-6s* (6s2). urthermore, the estimated values of radiant quantum efficiency were 39.52%, 41.41%, and 43.70% for Cs4SrI6 doped with 1.52% Tl+, 3.03% Tl+, and 4.55% Tl+, respectively, which showed enhanced energy conversion efficiency. Additionally, the reduction in band gap with the increasing Tl+ concentration led to an increase in the theoretical upper limit of light yield (LY), exceeding 108 401 photons/MeV for the 4.55% Tl+ dopant under ideal conditions. Moreover, this investigation revealed an Auger-free luminescence process in Tl+-doped compounds, which reduced self-absorption losses. These results demonstrated that Tl+ in Cs4SrI6 offers a cost-effective and rare-earth-free alternative activator for perovskite-based scintillators, with the favorable characteristics of light emission and promising scintillation performance.