Diamond anvil cell techniques have been improved to allow access to the multimegabar ultrahigh-pressure region for exploring novel phenomena in condensed matter. However, the only way to determine crystal structures of materials above 100 GPa, namely, X-ray diffraction (XRD), especially for low Z materials, remains nontrivial in the ultrahigh-pressure region, even with the availability of brilliant synchrotron X-ray sources. In this work, we perform a systematic study, choosing hydrogen (the lowest X-ray scatterer) as the subject, to understand how to better perform XRD measurements of low Z materials at multimegabar pressures. The techniques that we have developed have been proved to be effective in measuring the crystal structure of solid hydrogen up to 254 GPa at room temperature [C. Ji et al., Nature 573, 558–562 (2019)]. We present our discoveries and experiences with regard to several aspects of this work, namely, diamond anvil selection, sample configuration for ultrahigh-pressure XRD studies, XRD diagnostics for low Z materials, and related issues in data interpretation and pressure calibration. We believe that these methods can be readily extended to other low Z materials and can pave the way for studying the crystal structure of hydrogen at higher pressures, eventually testing structural models of metallic hydrogen.

中文翻译：

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超高压下低Z材料的晶体学：固体氢的案例研究

金刚石砧座电池技术已经得到改进，可以进入多兆巴超高压区域，以探索凝聚态物质中的新现象。然而，确定 100 GPa 以上材料的晶体结构的唯一方法，即 X 射线衍射 (XRD)，特别是对于低 Z 材料，在超高压区域仍然很重要，即使有明亮的同步加速器 X 射线的可用性来源。在这项工作中，我们进行了系统研究，选择氢（最低的 X 射线散射体）作为研究对象，以了解如何在多兆巴压力下更好地对低 Z 材料进行 XRD 测量。我们开发的技术已被证明可有效测量室温下高达 254 GPa 的固体氢的晶体结构 [C. Ji et al., Nature 573, 558–562 (2019)]。我们介绍了我们在这项工作的几个方面的发现和经验，即金刚石砧选择、超高压 XRD 研究的样品配置、低 Z 材料的 XRD 诊断以及数据解释和压力校准中的相关问题。我们相信这些方法可以很容易地扩展到其他低 Z 材料，并可以为在更高压力下研究氢的晶体结构铺平道路，最终测试金属氢的结构模型。