New perovskite materials challenge traditional notions of high-pressure chemistry

High pressure materials science has taken off in recent decades, with advances in previously difficult experimental techniques and technologies such as diamond anvils, which squeeze material samples between two diamonds at pressures up to millions of times greater than the surface of the Land.

The field uses these extreme conditions that reflect the deep interiors of planets to discover new materials, modify properties of known materials in potentially useful and even exotic ways, and test their concepts of how materials work or simulate what Earth is like inside .

Meanwhile, perovskite is the most abundant mineral in the Earth’s mantle (composed of calcium titanate, CaTiO₃) and the name of any material that has the same special crystalline structure as this mineral. Perovskite structures are of great interest to materials scientists because of the many interesting properties that are important in a range of microelectronics, telecommunications, and clean energy applications.

Using advanced high-pressure techniques, Professor Changqing Jin, who leads the research team at the Institute of Physics of the Chinese Academy of Sciences, also attached to the University of the Chinese Academy of Sciences (UCAS), has been manufacturing many new materials with perovskite structures and new functionality for some time. Recently, his lab has been synthesizing a new type of perovskite compound, called “double perovskites,” which has twice the “unit cell,” or smallest possible building block of a crystal, of regular perovskites.

The findings were published in the peer-reviewed journal. Angewandte Chemie.

The study details how the researchers exposed their latest double perovskite, made up of yttrium, cobalt, iridium and oxygen atoms (AND_twoCOIrO_{6 6}), at different levels of extreme pressure and what happened when they did.

For most materials, an increase in pressure allows an increase in the number of atoms that can immediately gather around a central atom in a crystal (called the coordination number.

But the new double perovskite, AND_twoCOIrO_{6 6}, did not adhere to traditional theories that the order of the crystal structure tends to increase with increasing pressure.

Instead, when synthesized at room pressure, AND_twoCOIrO_{6 6} it’s very neat, but surprisingly when synthesized at 6 gigapascals (GPa, or roughly 60,000 times standard atmospheric pressure), while the unit cell got smaller, there was now only a partial order.

Then at 15 GPa, the researchers found disorder. The increasing pressure had reversed the normal sequence of order to disorder that the researchers expected. Furthermore, the magnetic properties of the material changed

“Interestingly, 15 GPa is also the pressure that is in the boundary region between the upper and lower mantle on Earth,” said Zheng Deng, another team member. “This is precisely where many perovskite materials are formed.”

Gaining more insight into this unexpected transition from Pressure Dependent Order Disorder could help scientists better understand the properties of the minerals that make up the mantle and deepest interior of our planet.

“This violates our intuition about high pressure chemistry,” Deng continued. “It means we will have to completely reconsider the effects of pressure on the solid state sciences.”

The discovery could allow the design and synthesis of new useful materials at high pressures with attributes that would otherwise be difficult to achieve under normal conditions.