Ab initio prediction of fast non-equilibrium transport of nascent polarons in SrI₂: a key to high-performance scintillation [First-principles study of hole polaron formation and migration in strontium iodide]

Journal Article · Fri Aug 12 04:00:00 UTC 2016 · npj Computational Materials

DOI: https://doi.org/10.1038/npjcompumats.2016.22 · OSTI ID:1305858

Zhou, Fei ^[1]; Sadigh, Babak ^[1]; Aberg, Daniel ^[1]; Erhart, Paul ^[2]

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1. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States). Physical and Life Science Directorate
2. Chalmers Univ. of Technology, Gothenburg (Sweden). Dept. of Applied Physics

The excellent light yield proportionality of europium-doped strontium iodide (SrI₂:Eu) has resulted in state-of-the-art γ-ray detectors with remarkably high-energy resolution, far exceeding that of most halide compounds. In this class of materials, the formation of self-trapped hole polarons is very common. However, polaron formation is usually expected to limit carrier mobilities and has been associated with poor scintillator light-yield proportionality and resolution. Here using a recently developed first-principles method, we perform an unprecedented study of polaron transport in SrI₂, both for equilibrium polarons, as well as nascent polarons immediately following a self-trapping event. We propose a rationale for the unexpected high-energy resolution of SrI₂. We identify nine stable hole polaron configurations, which consist of dimerised iodine pairs with polaron-binding energies of up to 0.5 eV. They are connected by a complex potential energy landscape that comprises 66 unique nearest-neighbour migration paths. Ab initio molecular dynamics simulations reveal that a large fraction of polarons is born into configurations that migrate practically barrier free at room temperature. Consequently, carriers created during γ-irradiation can quickly diffuse away reducing the chance for nonlinear recombination, the primary culprit for non-proportionality and resolution reduction. We conclude that the flat, albeit complex, landscape for polaron migration in SrI₂ is a key for understanding its outstanding performance. This insight provides important guidance not only for the future development of high-performance scintillators but also of other materials, for which large polaron mobilities are crucial such as batteries and solid-state ionic conductors.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE
• Grant/Contract Number: AC52-07NA27344
• OSTI ID: 1305858
• Report Number(s): LLNL-JRNL--678128
• Journal Information: npj Computational Materials, Journal Name: npj Computational Materials Vol. 2; ISSN 2057-3960
• Publisher: Nature Publishing GroupCopyright Statement
• Country of Publication: United States
• Language: English