  The excellent light yield proportionality of europium-doped strontium iodide
(SrI$_2$:Eu) has resulted in state-of-the-art $\gamma$-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 SrI$_2$, both for equilibrium polarons, as well as nascent polarons
immediately following a self-trapping event. As a result, we put forward a
rationale for the unexpected high energy resolution of SrI$_2$. We identify
nine stable hole polaron configurations, which consist of dimerized 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-neighbor
migration paths. \textit{Ab initio} molecular dynamics simulations reveal that
a large fraction of polarons is born into configurations that migrate
practically barrier free at room temperature. As a result, carriers created
during $\gamma$-irradiation can quickly diffuse away reducing the chance for
non-linear recombination reactions that are the primary culprits for
non-proportionality and resolution reduction. This leads to the conclusion that
the flat, albeit complex, landscape for polaron migration in SrI$_2$ is 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.

Materials Science