A major stumbling block in our understanding of glass and glass phenomena is the elusive relationship between relaxation dynamics and glass structure. A team led by Dr. Qiaoshi Zeng from HPSTAR recently developed a new in situ high-pressure wide-angle x-ray photon correlation spectroscopy method to enable atomic-scale relaxation dynamics studies in metallic glass systems under extreme pressures. The study is published in the latest Proceedings of the National Academy of Sciences (PNAS) edition.

Metallic glasses (MGs), with many superior properties to both conventional metals and glasses, have been the focus of worldwide research. As thermodynamically metastable materials, like typical glasses, MGs spontaneously evolve into their more stable states all the time through various relaxation dynamic behaviors. These relaxation behaviors have significant effects on the physical properties of MGs. Still, until now, scientists’ ability to deepen the understanding of glass relaxation dynamics and especially its relationships with atomic structures has been limited by the available techniques.

"Thanks to the recent improvements in synchrotron x-ray photon correlation spectroscopy (XPCS), measuring the collective particle motions of glassy samples with a high resolution and broad coverage in the time scale is possible, and thus, various microscopic dynamic processes otherwise inaccessible have been explored in glasses,” said Dr. Zeng. “However, the change in atomic structures is subtle in previous relaxation process measurements, which makes it still difficult to probe the relationship between the structure and relaxation behavior. To overcome this problem, we decided to employ high pressure because it can effectively alternate the structure of various materials, including MG.”

To this end, the team developed in situ high-pressure synchrotron wide-angle XPCS to probe a cerium-based MG material during compression. In situ high-pressure wide-angle XPCS revealed that the collective atomic motion initially slows down, as generally expected with increasing density. Then, counter-intuitively it accelerates with further compression, showing an unusual non-monotonic pressure-induced steady relaxation dynamics crossover at ~3 GPa. Furthermore, by combining these results with in situ high-pressure synchrotron x-ray diffraction, the relaxation dynamics anomaly closely correlates with the dramatic changes in local atomic structures during compression, rather than monotonically scaling with either the sample density or overall stress level.

"With density increases, atoms in glasses generally get more difficult to move or diffuse, slowing down its relaxation dynamics. This is what we normally expect from hydrostatic compression,” Dr. Zeng explained. “So the non-monotonic relaxation behavior observed here in the cerium-based MG under pressure is quite unusual, which indicates besides density, structural details could also play an important role in glass relaxation dynamics." Dr. Zeng explained.

These findings demonstrate that there is a close relationship between glass relaxation dynamics and atomic structures in MGs. The technique Dr. Qiaoshi Zeng’s group developed here can also be extended to explore the relationship between relaxation dynamics and atomic structures in various glasses, especially those significantly tunable by compression, offering new opportunities for glass relaxation dynamics studies at extreme conditions.

Caption: Two-time correlation functions of the ce-based MG measured by HP-XPCS at different pressures during compression. At each pressure, the width of the reddish diagonal contour is proportional to the relaxation time, which broadens below 2.9 GPa and then narrows during further compression.

Media report│Phys.org:  New spectroscopy method reveals accelerated relaxation dynamics in compressed cerium-based metallic glass

玻璃是一类典型的复杂材料体系。它们在原子结构上高度无序；在能量上处于亚稳态；在时间上具有一直向能量更低状态自发演变的特征，即弛豫行为。玻璃弛豫动力学与其微观结构之间的关联一直是凝聚态物理和材料科学领域极具争议和挑战的重要科学问题。近日，北京高压科学研究中心的曾桥石研究员带领的研究团队通过发展高压原位同步辐射高能广角X射线光子关联谱(HP-XPCS)技术，实现了对金属玻璃在高压下的原子尺度弛豫动力学的原位探测，发现铈基金属玻璃在压缩过程中，其稳态弛豫动力学并非随常规设想的那样随密度增加而持续变慢，而是和原子结构变化的细节密切相关，表现出先变慢后又变快的非单调变化行为。相关研究结果以“Pressure-induced non-monotonic crossover of steady relaxation dynamics in a metallic glass”， 为题于6月5日发表于《美国科学院院刊》。