Reaching metastability via isothermal pressure pathways – Dr. Gang Liu
JULY 26, 2018
The phenomenon of metastability, in which a system is in a state that is stable but not at the lowest energy, is obtained most-commonly through fast cooling from high temperature. Now, a team of scientists co-led by Dr. Gang Liu from HPSTAR report attainable metastable states in 2D solar perovskites from compression-decompression paths. The metastable phases show much superior optical properties.
Metastability is common phenomenon in condensed matters and is widely observed in nature. For example, diamond is a metastable form of carbon.
Metastability is a desirable material property, where in some cases we need the unique properties of metastable states to achieve particular applications that cannot access by stable structures.
The results, as reported on July 23, 2018 in PNAS, could be important for developing new metastable materials with special performance.
Pressure is commonly used to change the phase of a material in addition to heating and cooling. The scientists led by Dr. Gang Liu of HPSTAR focused on structural and related optical property studies of organic-inorganic hybrid perovskites using high pressure techniques.
This time the scientists found new metastable states in 2D hybrid perovskites using compression-decompression processes under ambient temperature.
For organic materials, we couldn’t get a metastable state from heating-cooling method since the organic materials will decompose during such process. While pressure, is a safe way that can be used to achieve metastable states without degrading organic molecules.
What excites the scientists is the metastable phase shows 8.25% bandgap narrowing, indicating a significant improvement in light absorption. “This value is the largest pressure-driven change among all reported solar perovskites”, remarked by Dr. Gang Liu.
Their further x-ray diffraction experiments explain that the narrowed bandgap is the result of widened Pb-I-Pb bond angles from recrystallization during decompression.
“It’s possible that other energy materials could have similar metastable states with improved properties when they undergo compression-decompression processes,” said Dr. Gang Liu. “Our study may provide a new path for obtaining metastability with unexpected properties in known materials”.
Caption: Pressure induced bandgap evolution of (BA)2(MA)2PbI10 before compression and after decompression with 8.2% ambient bandgap narrowing and schematic illustration of Pb-I-Pb bonds in (BA)2PbI4 under compression.
亚稳态材料具有其稳定态没有的特殊性质，比如磁性合金的自旋玻璃态、不定形冰、过冷液态金属，因此其在科学研究和工程应用上受到广泛关注。迄今为止，材料的亚稳态通常需要借助升温-快速冷却等变温手段来达到。然而，这类依靠加热-冷却的方法在应用于受热易分解的有机物质上面临着很大的挑战。因此，发展出一套不借助热效应而能达到材料亚稳态的技术手段极为重要。北京高压科学研究中心刘罡研究员所在的国际研究团队，近日成功利用了在高压条件下物质非晶化和退压重结晶的现象，实现了二维结构有机-无机杂化钙钛矿的亚稳态。此类退压后的亚稳态二维杂化钙钛矿表现出了很大程度的光带隙缩小和在65%相对湿度条件下的长期稳定性，其中最大的带隙缩小达到了8.25%。结合第一性原理计算，该研究结果确认了压力可制备出其他热力学过程无法达到的亚稳态材料。更重要的是，这项研究理清了压力循环后钙钛矿材料性能得到提高的根源。优异的亚稳态性能也为应用在太阳能电池领域的杂化钙钛矿提供了进一步优化物质特性和提升光电转化效率的可能，同时也为高压材料研究开展了新的方向。