Abstract
AbstractThe 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 non-linear 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.
Highlights
A small, or self-trapped, polaron is a quasi-particle consisting of a localised charge carrier that has strongly polarised its immediate surrounding lattice
Polaron formation and transport are crucial for understanding important quantum-mechanical processes occurring in scintillations,[1] as well as batteries,[2] and solid ion conductors
Polaron migration typically proceeds by a hopping mechanism without delocalisation
Summary
A small, or self-trapped, polaron is a quasi-particle consisting of a localised charge carrier that has strongly polarised its immediate surrounding lattice. The polaron can be centred on a specific atom, e.g., hole and electron polarons in TiO2 or SrTiO3,3,4 or atomic bonds, e.g., in the case of VK-centres in alkali halides.[5] The latter consists of a hole localised on a dimerised nearest-neighbour halide–ion pair. For small polarons such as these, the adiabatic potential energy landscape comprises potential wells that are usually o1 eV deep (corresponding to the polaron-binding energy) and separated by energy barriers that are generally lower than the binding energy. As standard density functional theory (DFT) fails to stabilise polaron states,[6,7] mainly due to the large self-interaction error of the localised state, several alternatives have been suggested
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