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 and Dr. Jue Gong of Northern Illinois University 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.

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