Stanford Scientists Uncover Exotic Chemical State in New Material

Forms of gold that do not exist stably in nature are at the heart of new crystalline materials with interesting properties.

For the first time, researchers at Stanford University have discovered a way to create and stabilize an extremely rare form of gold (called Au) that has lost two negatively charged electrons.²⁺. The materials that stabilize this elusive version of this precious element are halide perovskites. This is a type of crystalline material that holds great promise for a variety of applications, including more efficient solar cells, light sources, and electronic components.

Surprisingly, Au²⁺ Perovskites can also be made quickly and easily at room temperature using commercially available materials.

“It was truly surprising that we were able to synthesize a stable material containing Au.”²⁺“At first I couldn’t believe it,” said Hemamala Karunadasa, associate professor of chemistry at Stanford University’s School of Humanities and Sciences and lead author of the recently published study. natural chemistry. “Development of Au, the first of its kind”²⁺ Perovskites are interesting. The gold atoms in perovskites have strong similarities to the copper atoms in high-temperature superconductors and to heavy atoms with unpaired electrons, such as Au.²⁺, exhibiting cool magnetic effects not seen in lighter atoms. ”

Structure of gold halide perovskite. The elongated gold chloride octahedron consists of gold (Au) surrounded by six adjacent chlorine (Cl) atoms, which are shaded in the structure. The burnt red octahedron represents Au2+ chloride and the gold octahedron represents Au3+ chloride. The blue-green spheres represent cesium (Cs) atoms, and the light green spheres represent chlorine (Cl) atoms. The inset shows the shortest gold-chloride bond. Credit Karunadasa et al. 2023.

“Halide perovskites have properties that make them very attractive for many everyday applications, so we’ve been looking to expand this family of materials,” says the researcher, a doctoral student at Stanford University. said Kurt Lindquist, lead author of the study and currently a postdoctoral fellow.inorganic chemistry scholar princeton university. “Unprecedented Au”²⁺ Perovskites could open up interesting new avenues. ”

heavy electrons in gold

Gold as an elemental metal has long been prized for its relative rarity and its unique malleability and chemical inertness. This means it doesn’t react with chemicals in the environment, won’t discolor over time, and can be easily formed into jewelry or coins. Another important reason for its value is the color from which gold gets its name. Perhaps no other metal has such a unique rich color in its pure state.

The fundamental physics behind gold’s prized appearance also explains why Au is chosen.²⁺ Karunadasa explained that this is very rare.

The fundamental reason is the relativistic effect originally postulated in Albert Einstein’s famous theory of relativity. “Einstein taught us that when an object moves very fast and its speed approaches a significant fraction of the speed of light, it becomes heavier,” Karunadasa said.

This phenomenon also applies to particles, and has significant effects on “giant” heavy elements such as gold, whose nuclei contain many protons. These particles collectively exert a huge positive charge, causing negatively charged electrons to swirl around the nucleus at breakneck speeds. As a result, the electrons become heavier and tightly surround the nucleus, blunting its charge and allowing the outer electrons to drift farther than in normal metals. This rearrangement of electrons and their energy levels causes gold to absorb blue light, making it appear yellow to our eyes.

Thanks to the theory of relativity, due to the arrangement of gold’s electrons, atom Naturally occurring as Au¹⁺ And au³⁺lose one or three electrons and release Au, respectively.²⁺. (The “2+” indicates a net positive charge due to the loss of two negatively charged electrons, and the chemical symbol for gold “Au” comes from the Latin word “aurum,” meaning gold.) .)

A squeeze of vitamin C

Au with appropriate molecular composition²⁺ Researchers at Stanford University found that it can be tolerated.Lindquist said he “stumbled across” the new Au.²⁺– Storing perovskites while working on a broader project centered around magnetism semiconductor For use with electronic equipment.

Lindquist mixed the salts cesium chloride and gold.³⁺– Combine chlorides in water and add hydrochloric acid acid “We threw a little vitamin C” into the solution, he said. In the subsequent reaction, vitamin C (acid) donates (negatively charged) electrons to the common Au.³⁺ Formation of Au²⁺. Interestingly, Au²⁺ It is stable in solid perovskite, but not in solution.

“We can make this material in the lab in about five minutes at room temperature using very simple ingredients,” Lindquist says. “The final powder is a very dark green, almost black, and is surprisingly heavy due to the gold content.”

Realizing that he may have been exposed to new chemistry, so to speak, Lindquist used spectroscopy and X-ray diffraction to investigate how perovskites absorbed light and characterize their crystal structure. We conducted numerous tests on perovskites, including .A physics and chemistry research group led by Young Lee, professor of applied physics at Stanford. photon Edward Solomon, Monroe E. Spahrt Professor of Chemistry and Professor of Photon Science, contributed further to the study of Au’s behavior.²⁺.

The experiment finally proved the existence of Au.²⁺ He researched perovskites, and in the process added a chapter to the 100-year saga of chemistry and physics involving Linus Pauling, winner of the Nobel Prize in Chemistry in 1954 and the Nobel Peace Prize in 1962.About gold perovskites containing common forms of Au¹⁺ And au³⁺. Coincidentally, Pauling later also studied the structure of vitamin C, one of the ingredients he needed to produce stable perovskites containing the elusive gold.²⁺.

“I love the connection between Linus Pauling and our work,” Karunadasa said. “This perovskite synthesis makes for a good story.”

Looking to the future, Karunadasa, Lindquist and colleagues plan to further study the new material and fine-tune its chemistry.Hope is to be Au²⁺ Perovskites can be used in applications that require magnetism and conductivity because the electrons jump out of the gold.²⁺ Au to³⁺ with perovskite.

“We are excited to explore what Au is like.²⁺ Perovskite could do it,” Karunadasa said.

Reference: “Stabilization of Au2+ in mixed-valence 3D halide perovskites” Kurt P. Lindquist, Armin Eghdami, Christina R. Deschene, Alexander J. Heyer, Jiajia Wen, Alexander G. Smith, Edward I. Solomon, Young S Lee, Jeffrey B. Neaton, Dominic H. Ryan, Hemamala I. Karunadasa, August 28, 2023. natural chemistry.
DOI: 10.1038/s41557-023-01305-y

Karnadasa too Senior Researcher Precourt Energy Research Institute Principal Investigator and Faculty Scientist at the Stanford Institute for Materials and Energy Sciences; SLAC National Accelerator Laboratory. Solomon is a professor of photon science at the Stanford Synchrotron Radiation Light Source (SLAC). Other Stanford co-authors are Christina R. Deschene and Alexander J. Heyer, both graduate students in the Department of Chemistry. and Jiajia Wen, staff scientist at SLAC. Other co-authors include Armin Eghdami and Alexander G. Smith, graduate students in the Department of Physics at the University of California, Berkeley. Jeffrey B. Neaton, Professor of Physics, University of California, Berkeley. Dominic H. Ryan, professor of physics at McGill University.

This research was funded in part by the U.S. National Science Foundation, the U.S. Department of Energy, the Quebec Institute of Natural Technology, and the Natural Sciences and Engineering Research Council of Canada.