Ab initio prediction of fast non-equilibrium transport of nascent polarons in SrI₂: a key to high-performance scintillation （对SrI₂中新生极化子快速非平衡传输的从头预测：高性能闪烁的关键） 

Fei Zhou, Babak Sadigh, Paul Erhart & Daniel Åberg
npj Computational Materials 2, Article number: 16022 (2016)
doi:10.1038/npjcompumats.2016.22
Abstract| Full Text | PDF OPEN

摘要：铕掺杂碘化锶 (SrI₂:Eu) 的光产额极高，能赋予γ-射线探测器极高的高能分辨率，远远超过大多数卤素化合物。对这一类材料而言，其内部常常形成自扑捉空穴极化子，但通常认为极化子的形成限制了载流子移动，而且与闪烁光低产率和低分辨率相关。采用最近开发的第一原理方法，我们首创对SrI₂平衡极化子和自陷事件后即刻形成的新生极化子进行了研究。本研究对SrI₂意外高能分辨率提出了理论解释，确定了九种稳定的空穴极化子组态，包括二聚化碘对，结合能高达0.5 eV。这些空穴极化子组态通过复杂的势能景貌连结，这个势能景貌有66个独特的最邻近迁移路径。从头预测的分子动力学模拟表明，极化子的很大一部分产生后就进入了室温下几乎可自由迁移的组态中。这样，在γ-辐照过程中产生的载流子可以迅速扩散，减少了非线性重组的机会，成为非均衡性和低分辨率的主因。本研究认为，SrI₂ 中极化子平坦而复杂的能量景貌是理解其卓越闪烁性能的关键。这种认识不仅为将来开发高性能闪烁体提供了重要的指导，也为研发高迁移率极化子的其他材料，如电池和固态离子导体，提供了重要的参考。 

Abstract: The excellent light yield proportionality of europium-doped strontium iodide (SrI₂: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 SrI₂, 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 SrI₂. We identify nine stable hole polaron configurations, which consist of dimerised 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-neighbour 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 SrI₂ is a 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. 

Editorial Summary

Scintillation: A fast move for gamma ray detectors (闪烁：伽玛射线探测器中的快速移动)

An international team of researchers has uncovered evidence that may explain the excellent scintillation properties of strontium iodide. Daniel Åberg and colleagues at Lawrence Livermore National Laboratory in the United States and Chalmers University in Sweden looked at the role of polarons — a “quasi-particle” consisting of a charge carrier and the surrounding displaced lattice — in the performance of gamma ray scintillators, which absorb radiation and convert it into visible light. In “good” scintillators like strontium iodide, which is used in gamma ray detectors, it is imperative that the polarons move quickly away from the region of their creation. The team computed the properties of polarons in strontium iodide and found that generated polarons moved unusually fast and had a variety of energy configurations. These observations are thought to explain the high energy resolution of strontium iodide.  

一个国际研究小组发现了可以解释碘化锶优良闪烁性能的证据。工作于美国Lawrence Livermore国家实验室和瑞典Chalmers理工大学的Daniel Åberg及其同事，研究了极化子（一种“准粒子”，由电荷载体和其周边伴生晶格畸变组成）在γ-射线闪烁体中的作用，极化子吸收辐射并将其转换成可见光。类似于碘化锶那样“好”的、应用于γ-射线探测器中的各种闪烁体来说，需要极化子迅速远离他们所产生的区域。该团队计算了碘化锶中极化子的性质，发现产生的极化子移动异常迅速，并有各种能量组态。他们研究认为，这些结果解释了碘化锶高能分辨的机理。