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The excellent light yield proportionality of europium-doped strontium iodide(SrI$_2$:Eu) has resulted in state-of-the-art $\gamma$-ray detectors withremarkably high energy resolution, far exceeding that of most halide compounds.In this class of materials the formation of self-trapped hole polarons is verycommon. However, polaron formation is usually expected to limit carriermobilities and has been associated with poor scintillator light-yieldproportionality and resolution. Here, using a recently developedfirst-principles method, we perform an unprecedented study of polaron transportin SrI$_2$, both for equilibrium polarons, as well as nascent polaronsimmediately following a self-trapping event. As a result, we put forward arationale for the unexpected high energy resolution of SrI$_2$. We identifynine stable hole polaron configurations, which consist of dimerized iodinepairs with polaron binding energies of up to 0.5\,eV. They are connected by acomplex potential energy landscape that comprises 66 unique nearest-neighbormigration paths. \textit{Ab initio} molecular dynamics simulations reveal thata large fraction of polarons is born into configurations that migratepractically barrier free at room temperature. As a result, carriers createdduring $\gamma$-irradiation can quickly diffuse away reducing the chance fornon-linear recombination reactions that are the primary culprits fornon-proportionality and resolution reduction. This leads to the conclusion thatthe flat, albeit complex, landscape for polaron migration in SrI$_2$ is key forunderstanding its outstanding performance. This insight provides importantguidance not only for the future development of high-performance scintillatorsbut also of other materials, for which large polaron mobilities are crucialsuch as batteries and solid state ionic conductors.

Ab-initio prediction of fast non-equilibrium transport of nascent polarons in SrI$_2$: A key to high-performance scintillation