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

TL;DR

This study uses first-principles methods to analyze polaron transport in SrI$_2$, revealing rapid, barrier-free migration of nascent polarons that explains its high scintillation performance and energy resolution.

Contribution

It provides the first detailed ab initio analysis of both equilibrium and nascent polaron transport in SrI$_2$, uncovering the mechanisms behind its exceptional scintillation properties.

Findings

Nine stable hole polaron configurations identified.

Large fraction of polarons migrate barrier-free at room temperature.

Complex potential landscape facilitates high polaron mobility.

Abstract

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.