In this work, we investigated the electronic, optical, phonon, polaron, and transport properties of cesium hafnium halides Cs2HfX6, X=Cl, Br and I and estimated their upper light yield. Using First-principles calculations combined the exchange-correlation energy of the Generalized Gradient Approximation GGA with the modified Becke–Johnson exchange potential (mBJ) and the spin-orbit coupling (SOC), these materials demonstrated discrete valence and conduction bands with low band dispersion and direct band gap decreasing as Cs2HfCl6 (5.48 eV)→ Cs2HfBr6(4.18eV)→ Cs2HfI6 (2.72 eV). These obtained values are very close to the available experimental ones. Likewise, the band edge curvature of both valence band maximum (VBM) and conduction band minimum (CBM) reflects the heavier effective mass of electron at CBM Cs2HfCl6(7.58m0),Cs2HfBr6(5.94m0),Cs2HfI6(3.92m0) compared to that of the hole at VBM Cs2HfCl6(6.12m0),Cs2HfBr6(3.72m0),Cs2HfI6(2.34m0). Moreover, these materials exhibited a high transmittance rate, in their emission range, about: 93.57 % for Cs2HfCl6 91.25 % for Cs2HfBr6 and 88.1 % for Cs2HfI6, which is adequate to minimize the scintillated photons loss. The phonon dispersion and the normal modes for Γ point were calculated and discussed. Only three longitudinal optical phonon modes were identified active and their contribution to the ionic dielectric constant was calculated. Having First-principles results, polaron properties, the temperature dependence of polaron mobility, and relaxation time were calculated. These materials showed moderate electron (hole)-phonon couplings αe=6.72(αh=6.04) for Cs2HfCl6,αe=8.26(αh=6.54) for Cs2HfBr6 and αe=6.16(αh=4.76) for Cs2HfI6 compared to other polar materials leading to large polaron mass, high scattering rate, small relaxation time, and low electron (hole) room-temperature mobility 0.14cm2V−1s−1 (0.23 cm2V−1s−1) for Cs2HfCl6 0.14cm2V−1s−1 (0.45 cm2V−1s−1) for Cs2HfBr6 and 0.62cm2V−1s−1 (1.81 cm2V−1s−1) for Cs2HfI6which has only a positive role via the increase of the recombination yield. While such low mobility definitely limited their use in solar cell applications. Moreover, the upper light yield of these materials was estimated for the polaron and simple phenomenological models by taking the needed material inputs to these models from First-principles calculations. The results of latter one were 58,290 photon/MeV for Cs2HfCl6 76,685photon/MeV for Cs2HfBr6 and 118,000photon/MeV for Cs2HfI6 compared to the maximum experimental values 54,000 photon/MeV for Cs2HfCl6 61,500photon/MeV for Cs2HfBr6 and 70,000photon/MeV for Cs2HfI6. These computational findings may add strong evidence for suggesting experimental improvement of the crystal growth conditions and explain the remarkable scintillation properties of these materials as new and efficient gamma- and X-rays scintillators.

中文翻译：

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卤化铯铪 Cs2HfX6 的电子、光学、声子、极化子、载流子传输和闪烁特性（X = CI、Br 和 I）


在这项工作中，我们研究了卤化铪铪 Cs2HfX6、X=Cl、Br 和 I 的电子、光学、声子、极化子和传输特性，并估计了它们的上光产额。使用第一性原理计算将广义梯度近似 GGA 的交换相关能与改进的贝克-约翰逊交换电位 （mBJ） 和自旋轨道耦合 （SOC） 相结合，这些材料表现出离散价和导带，具有低频带色散和直接带隙，随着 Cs2HfCl6 （5.48 eV）→Cs2HfBr6 （4.18eV）→ Cs2HfI6 （2.72 eV）。这些获得的值非常接近可用的实验值。同样，价带最大值 （VBM） 和导带最小值 （CBM） 的带边曲率反映了 CBM Cs2HfCl6 （7.58m0）、Cs2HfBr6（5.94m0）、Cs2HfI6（3.92m0） 处的电子有效质量比 VBM Cs2HfCl6 （6.12m0）、Cs2HfBr6（3.72m0）、Cs2HfI6（2.34m0） 处的电子有效质量更重。此外，这些材料在其发射范围内表现出高透射率，大约为：Cs2HfCl6 为 93.57 %，Cs2HfBr6 为 91.25 %，Cs2HfI6 为 88.1 %，这足以最大限度地减少闪烁光子的损失。计算并讨论了Γ点的声子色散和法向模态。只有三种纵向光学声子模式被确定为活跃的，并计算了它们对离子介电常数的贡献。计算了第一性原理结果、极化子性质、极化子迁移率的温度依赖性和弛豫时间。这些材料对 Cs2HfCl6 表现出中等的电子（空穴）-声子耦合 αe=6.72（αh=6.04），Cs2HfBr6 的 αe=8.26（αh=6.54），αe=6.16（αh=4。76） 对于Cs2HfI6 与其他极性材料相比，导致极化子质量大、散射速率高、弛豫时间短和电子（空穴）室温迁移率 Cs2HfCl6 为 0.14cm2V-1s-1 （0.23 cm2V-1s-1） Cs2HfBr6 为 0.14cm2V-1s-1 （0.45 cm2V-1s-1） 和 Cs2HfI6 为 0.62cm2V-1s-1 （1.81 cm2V-1s-1），仅通过增加复合产率发挥积极作用。虽然如此低的迁移率无疑限制了它们在太阳能电池应用中的使用。此外，通过从第一性原理计算中获取这些模型所需的材料输入，估计了极化子和简单现象模型这些材料的上部光产率。后一个的结果是 Cs2HfCl6 为 58,290 光子/MeV，Cs2HfBr6 为 76,685 光子/MeV，Cs2HfI6 为 118,000 光子/MeV，而 Cs2HfCl6 的最大实验值为 54,000 光子/MeV，Cs2HfBr6 为 61,500 光子/MeV 和 Cs2HfI6 为 70,000 光子/MeV。这些计算结果可能为表明晶体生长条件的实验改进提供了强有力的证据，并解释了这些材料作为新型高效的伽马射线和 X 射线闪烁体的显着闪烁特性。