Recently, the new concept of "Pressure Aging" proposed by SHARPS Lu Xujie's research group of Ho- Kwang Mao' team was published in the Proceedings of the National Academy of Sciences PNAS. This important research result can lock the structure of materials and their enhanced properties under high pressure to normal pressure conditions, thus providing a new way to solve the above problems, and will have a profound impact on high pressure chemistry and materials science.
The researchers demonstrated the strategy of PA using the two-dimensional ferroelectric material CuInP₂S₆ as an example (Figure 1). By holding the material at a pressure of 3.3 GPa for 24 hours, a permanent change in the configuration of Cu ions was achieved. This results in a 2.5 fold increase in the residual polarization intensity and a substantial increase in the Curie temperature (Tc) from 317 K to 583 K. In contrast, the structure and physical properties of samples undergoing a simple compression-decompression cycle without pressure aging are essentially reversible.
The research team used a variety of in-situ characterization methods to reveal the structure evolution and performance enhancement mechanism during pressure aging by in-situ monitoring of second harmonic response (SHG), Raman spectroscopy, photoluminescence (PL), and synchrotron radiation X-ray diffraction (XRD) (Figure 2). The SHG strength of CuInP₂S₆ increased with increasing pressure, and was increased by 12 fold compared to the initial value at 3.3 GPa. During decompression, SHG shows reversible changes. However, after 24 hours of pressure aging, the enhancement of SHG can be retained after pressure release. This indicates that the dynamic processes involved in high pressure aging and conventional high pressure treatment are different.
Using a combination of experimental techniques such as monocrystalline XRD and Raman spectroscopy, the team elucidated the PA-induced structural changes. XRD analysis showed that the Cu position and cu-upward /Cu-downward ratio of CuInP₂S₆ changed significantly after PA treatment and pressure release (Figure 2e). Raman spectroscopy further confirmed the irreversible displacement of certain vibration modes, confirming the retained structural changes. These results demonstrate the ability of aging treatments to produce lasting changes at the atomic scale.
Further, the team used the Kohlrausch-Williams-Watts (KWW) function to study the relaxation dynamics of the PA process. The analysis shows that the change of structure (Raman) and performance (SHG) in PA process has a common mechanism, and the relaxation process can be effectively described by KWW function. For CuInP₂S₆, the relaxation time (τ) was about 6 hours and the PA index (β) was about 1.5, which is close to the values observed in soft materials. The PA process involves being held at a specific pressure for a long period of time, during which the material undergoes structural relaxation and rearrangement. As an example of the Cu cation in CuInP₂S₆, high-pressure conditions caused it to move and reach a new equilibrium state. Over time, these new configurations become more stable. When the pressure is released, the material retains these structural changes, allowing the performance enhancements to be preserved as well.
The feasibility of the pressure aging strategy was further validated in materials with different structural characteristics (Figure 3), including 3D MHyPbBr₃, 2D (BA)₂(GA) Pb₂I₇, 1D (CH₃CH₂CH₂NH₃)₂SbBr₅, and 0D (MePh₃P)₂SbCl₅. In these systems, PA treatment resulted in permanent structural changes and improved properties after pressure release, while samples without PA treatment showed reversible changes in the pressure-release process. The analysis of KWW function shows that different materials have different response rates, that is, the relaxation time (τ), and the temperature increase can promote the process, while their relaxation index (β) remains the same.
In summary, the concept of pressure aging introduces a groundbreaking new approach to the study of high-pressure materials and provides a new way to solve the problem of how to preserve high-pressure structures and properties. By revealing the importance of duration at target pressure, it provides a way to preserve the high-pressure enhanced properties of materials. This will prompt researchers to pay more attention to the synergistic effects of pressure and time on material properties in future studies, so as to develop more high-pressure new structures and new materials with unique properties.
近期，由SHARPS毛河光院士团队吕旭杰研究员课题组提出的 “压力陈化”（Pressure Aging）新概念发表于 《美国科学院院刊》PNAS。这一重要研究成果可以将高压下的材料结构及其增强性能部分锁定至常压条件，从而为解决上述问题提供了新的途径，将对高压化学和材料科学产生深远的影响。
