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

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.

14 pages, 6 figures, Computational Materials (2016)

References in corpus (7)

• Lattice anharmonicity and thermal conductivity from compressive sensing of first-principles calculations
• Polaronic hole localization and multiple hole binding of acceptors in oxide wide-gap semiconductors
• Efficacy of the DFT+U formalism for modeling hole polarons in perovskite oxides
• A variational polaron self-interaction corrected total-energy functional for charge excitations in wide-band gap insulators
• Origin of resolution enhancement by co-doping of scintillators: Insight from electronic structure calculations
• A first-principles study of co-doping in lanthanum bromide
• Quasi-particle spectra, absorption spectra, and excitonic properties of sodium iodide and strontium iodide from many-body perturbation theory

Cited by in corpus (2)

• Compressive sensing lattice dynamics. I. General formalism
• Dynamic Competition Between Hubbard and Superexchange Interactions Selectively Localizes Electrons and Holes Through Polarons