Ab initio prediction of fast non-equilibrium transport of nascent polarons in SrI2: 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 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 SrI 2 , 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 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. 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 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.

Analysis of self-trapped hole mobility in alkali halides and metal halides

“…This linear relationship certainly helps to make a more specific conclusion. with a broad range of the distances, 3.9-5.0 Å [32]. From our analysis for the iodides (Fig.5), we expect that migration temperatures of Vk centers in SrI2 fall into range 50 -100 K, respectively.…”

Exploring Polaronic, Excitonic Structures and Luminescence in Cs4PbBr6/CsPbBr3

“…Preliminary nudged elastic band calculations about the diffusion of an electron polaron between two equivalent sites yield a relatively low barrier height of ∼0.2 eV (66-atom, Γ-point only; see Figure S4), despite the disconnected octahedra in Cs 4 PbBr 6 . Ease of polaron diffusion aided by low barriers seems to be an interesting characteristic of luminescent materials such as in NaI (∼0.2 eV) , and SrI 2 (∼0.12 eV) . Semiconducting halides such as Tl 6 SeI 4 and InI, who are not emissive, have much higher defect diffusion barrier (>0.5 eV). , When localized charge carriers diffusing through Cs 4 PbBr 6 encounter large and fairly even offset at the conduction and valence band edges (Figure ), they will likely dissociate, and it may serve to concentrate carriers in CsPbBr 3 .…”

“…Ease of polaron diffusion aided by low barriers seems to be an interesting characteristic of luminescent materials such as in NaI (∼0.2 eV) 43,44 and SrI 2 (∼0.12 eV). 45 Semiconducting halides such as Tl 6 SeI 4 and InI, who are not emissive, have much higher defect diffusion barrier (>0.5 eV). 46,47 When localized charge carriers diffusing through Cs 4 PbBr 6 encounter large and fairly even offset at the conduction and valence band edges (Figure 4), they will likely dissociate, and it may serve to concentrate carriers in CsPbBr 3 .…”

Accurately predicting optical properties of rare-earth, aluminate scintillators: influence of electron–hole correlation

“…They are an important subset of the broader class of luminescent materials, and these have key applications in radiation detection for national security, high energy physics, lasers and light emitting diodes, medical imaging and therapy, oil drilling, and more. 2 In light of this, accurate predictions of scintillator performance and fundamental notions for how to tune that performance to the desired application have become increasingly sought after, [3][4][5][6][7] but achieving this goal requires accounting for radiative and non-radiative decay channels in complex, quantum light-matter interactions. 8 Properties at the macroscopic level such as light yield, response time, and stability under irradiation conditions are of pivotal importance for scintillators, 9,10 yet a full quantum mechanical treatment of the properties remains a significant theoretical challenge.…”