A reconstruction of a perovskite crystal (CaTiO3) grown on a similar perovskite substrate (NdGaO3), showing the electron density and oxygen octahedral tilt; (insert) artist's conception of the interface between the substrate and the perovskite film. Image: Yakun Yuan/Penn State.
A reconstruction of a perovskite crystal (CaTiO3) grown on a similar perovskite substrate (NdGaO3), showing the electron density and oxygen octahedral tilt; (insert) artist's conception of the interface between the substrate and the perovskite film. Image: Yakun Yuan/Penn State.

A team of materials scientists from Penn State, Cornell University and Argonne National Laboratory have, for the first time, visualized the three-dimensional (3D) atomic and electron density structure of the most complex perovskite crystal system decoded to date.

Perovskites are minerals that can acts as electrical insulators, semiconductors, metals or superconductors, depending on the arrangement of their atoms and electrons. Perovskite crystals have an unusual grouping of oxygen atoms that form an octahedron – an eight-sided polygon. This arrangement of oxygen atoms acts like a cage that can hold a large number of the elements in the periodic table. Additionally, other atoms can be fixed to the corners of a cube outside of the cage, at precise locations that can alter the material's properties, such as changing it from a metal to an insulator or from a non-magnet to a ferromagnet.

In their current work, the team grew the very first perovskite crystal to be discovered, called calcium titanate, on top of a series of other perovskite crystal substrates with similar but slightly different oxygen cages at their surfaces. Because the thin film perovskite on top wants to conform to the structure of the thicker substrate, it contorts its cages in a process known as tilt epitaxy.

The researchers found that the tilt epitaxy of calcium titanate caused a very ordinary material to become ferroelectric – a spontaneous polarization – and to remain ferroelectric up to 900K. They were also able to visualize the 3D electron density distribution in calcium titanate thin film for the first time.

"We have been able to see atoms for quite some time, but not map them and their electron distribution in space in a crystal in three dimensions," said Venkat Gopalan, professor of materials science and physics at Penn State. "If we can see not just where atomic nuclei are located in space, but also how their electron clouds are shared, that will tell us basically everything we need to know about the material in order to infer its properties."

That was the challenge the team set for itself over five years ago when Gopalan gave the project to his student Yakun Yuan, who is now lead author of a paper on this work in Nature Communications. Based on a rarely used x-ray visualization technique called COBRA, (coherent Bragg rod analysis) originally developed by a group in Israel, Yuan figured out how to expand and modify the technique to analyze one of the most complicated, least symmetrical material systems studied to date. This system is a strained 3D perovskite crystal with octahedral tilts in all directions, grown on another equally complex crystal structure.

"To reveal 3D structural details at the atomic level, we had to collect extensive datasets using the most brilliant synchrotron X-ray source available at Argonne National Labs and carefully analyze them with the COBRA analysis code modified for accommodating the complexity of such low symmetry," said Yuan.

According to Gopalan, very few perovskite oxygen cages are perfectly aligned throughout the material. Some rotate counter-clockwise in one layer of atoms and clockwise in the next; some cages are squeezed out of shape or tilt in directions that are in or out of plane to the substrate surface. From the interface of a film with the substrate it is grown on, all the way to its surface, each atomic layer may have unique structures and patterns. All of these distortions make a difference to the material properties, which the team can predict using a computational technique called density functional theory (DFT).

"The predictions from the DFT calculations provide insights that complement the experimental data and help explain the way that material properties change with the alignment or tilting of the perovskite oxygen cages," said Susan Sinnott, head and professor of materials science and engineering at Penn State, whose group performed the theoretical calculations.

The team also validated their advanced COBRA technique against multiple images of their material produced by the powerful Titan transmission electron microscope in the Materials Research Institute at Penn State. Since the electron microscope images extremely thin electron-transparent samples in a two-dimensional (2D) projection, not all of the three-dimensional (3D) image could be captured, even with the best microscope available today and with multiple sample orientations. This is where 3D dimensional imaging by the COBRA technique outperformed the electron microscopy in such complex structures.

The researchers believe their COBRA technique is applicable to the study of many other 3D, low-symmetry atomic crystals.

This story is adapted from material from Penn State, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.