动态晶体学揭示立方GeTe中的自发各向异性
Dynamic crystallography reveals spontaneous anisotropy in cubic GeTe
AI总结：
本文开发能量分辨可变快门对分布函数技术，揭示立方GeTe在所有温度下均为晶体结构，高温下呈现各向异性非谐动力学和铁电涨落，并通过Ginzburg-Landau模型解释其自发各向异性的来源。
摘要：
立方能源材料（如热电材料或杂化钙钛矿材料）通常被认为高度无序。在GeTe及相关的IV-VI族化合物中，这种无序被认为提供了热电应用所需的低热导率。由于传统晶体学无法区分静态无序与原子运动，我们开发了能量分辨可变快门对分布函数技术。该技术在原子运动相关的时间尺度上，以不同的曝光时间采集结构快照。与以往的解释不同，我们发现GeTe的时间平均结构在所有温度下均为晶体状态，但在较高温度下表现出各向异性的非谐动力学，在快速快门速度下类似于静态无序，并伴随沿$<$100$>$c方向的相关铁电涨落。我们证明，这种各向异性自然产生于一个Ginzburg-Landau模型，该模型通过长程弹性相互作用耦合极化涨落。通过获取能源材料中随时间变化的原子关联，我们解决了局域结构探针与平均结构探针之间长期存在的分歧，并表明自发各向异性在立方IV-VI族材料中普遍存在。
英文摘要：
Cubic energy materials such as thermoelectrics or hybrid perovskite materials are often understood to be highly disordered. In GeTe and related IV-VI compounds, this is thought to provide the low thermal conductivities needed for thermoelectric applications. Since conventional crystallography cannot distinguish between static disorder and atomic motions, we develop the energy-resolved variable-shutter pair distribution function technique. This collects structural snapshots with varying exposure times, on timescales relevant for atomic motions. In disagreement with previous interpretations, we find the time-averaged structure of GeTe to be crystalline at all temperatures, but with anisotropic anharmonic dynamics at higher temperatures that resemble static disorder at fast shutter speeds, with correlated ferroelectric fluctuations along the $<$100$>$c direction. We show that this anisotropy naturally emerges from a Ginzburg-Landau model that couples polarization fluctuations through long-range elastic interactions. By accessing time-dependent atomic correlations in energy materials, we resolve the long-standing disagreement between local and average structure probes, and show that spontaneous anisotropy is ubiquitous in cubic IV-VI materials.
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