An unstable form of gold found in nature is at the heart of a new crystalline material with interesting properties. For the first time, Stanford University researchers have found a way to create and stabilize an extremely rare type of gold that has lost two of its negatively charged electrons, called Au2+. The material that stabilizes this elusive valuable element is a halide perovskite, a 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 simply at room temperature using off-the-shelf ingredients.
"It's really surprising that we were able to synthesize a stable material containing Au2+—I didn't even believe it at first," said Hemamala Karunadasa, an associate professor of chemistry in Stanford's School of Humanities and Sciences and senior author of the paper, which was recently published in Nature Chemistry. "Creating this unprecedented Au2+ perovskite is exciting. The gold atoms in the perovskite are very similar to the copper atoms in high-temperature superconductors, and heavy atoms with unpaired electrons, such as Au2+, exhibit cold magnetic effects not seen in lighter atoms."
Structure of gold halide perovskites. The elongated gold chloride octahedrons, composed of gold (Au) surrounded by six adjacent chlorine (Cl) atoms, are shaded in the structure: the burnt red octahedron represents Au2+-chloride, and the gold octahedron represents Au3+-chloride. Turquoise spheres represent cesium (Cs) atoms, and light green spheres represent chlorine (Cl) atoms. The inset shows the shortest gold chloride bond. Credit Cardrunadasa et al. 2023.
"Halide perovskites have very attractive properties for many everyday applications, so we are always looking to expand this family of materials. Unprecedented Au2+ perovskites could open up some interesting new avenues," said Kurt Lindquist, the study's lead author, who conducted the research as a doctoral student at Stanford University and is now a postdoctoral scholar in inorganic chemistry at Princeton University.
Heavy electrons in gold
As an elemental metal, gold has long been valued for its relative scarcity, unparalleled ductility and chemical inertness, meaning it can be easily crafted into jewelry and coins, does not react with chemicals in the environment, and does not tarnish over time. Another key reason for its value is the gold's namesake color. Arguably, no other metal has such a uniquely rich hue in its pure state.
Karunadasa explains that the basic physics behind gold's much-lauded appearance also explains why Au2+ is so rare.
The underlying cause is a relativistic effect, originally proposed in Albert Einstein's famous theory of relativity. "Einstein told us that when an object moves very fast and its speed approaches a large fraction of the speed of light, the object becomes heavier," Karunadasa said.
This phenomenon also applies to particles, and has profound consequences for "large" heavy elements, such as gold, whose nuclei possess large numbers of protons. Together, these particles create a huge positive charge that forces negatively charged electrons to swirl around the nucleus at extremely fast speeds. As a result, the electrons become heavier and tightly surround the nucleus, weakening its charge and causing the outer electrons to drift farther than in typical metals. This rearrangement of electrons and their energy levels causes gold to absorb blue light and therefore appear yellow to our eyes.
Due to the arrangement of gold electrons, due to the theory of relativity, atoms naturally appear as Au1+ and Au3+, losing one or three electrons respectively, and abandoning Au2+. (The "2+" represents the net positive charge resulting from the loss of two negatively charged electrons, and gold's chemical symbol "Au" comes from "aurum," the Latin word for gold.)
Vitamin C Squeeze
Researchers at Stanford University have discovered that Au2+ can persist as long as the molecular structure is correct. Lindquist said he "accidentally discovered" the new Au2+-containing perovskite while working on a broader project centered on magnetic semiconductors for electronic devices.
Lindquist mixed a salt called cesium chloride and gold chloride in water and added hydrochloric acid to the solution, "and a small amount of vitamin C," he said. In a subsequent reaction, vitamin C (an acid) donates a (negatively charged) electron to the common Au3+, forming Au2+. Interestingly, Au2+ is stable in solid perovskites but unstable in solution.
"In the lab, we can make this material in about five minutes at room temperature using very simple ingredients," Lindquist said. "We ended up with a dark green, almost black powder that was surprisingly heavy because of the gold in it."
Recognizing that they might have found a new frontier in chemistry, so to speak, Lindquist conducted extensive tests on the perovskite, including spectroscopy and X-ray diffraction, to study how it absorbs light and characterize its crystal structure. A research team in physics and chemistry at Stanford led by Young Lee, Professor of Applied Physics and Photonic Sciences, and Edward Solomon, the Monroe E. Spaght Professor of Chemistry and Professor of Photonic Sciences, further contributed to the study of the behavior of Au2+.
These experiments ultimately confirmed the presence of Au2+ in perovskites, and in the process added a new chapter to the century-old story of chemistry and physics of Linus Pauling, who won the Nobel Prize in Chemistry in 1954 and the Nobel Peace Prize in 2017. 1962. Early in his career, he worked on gold perovskites containing the common forms Au1+ and Au3+. Coincidentally, Pauling later also studied the structure of vitamin C, one of the ingredients needed to create stable perovskites containing the elusive Au2+.
Going forward, Karunadasa, Lindquist and colleagues plan to further study this new material and tweak its chemical composition. It is hoped that Au2+ perovskites can be used in applications that require magnetism and conductivity because electrons jump from Au2+ to Au3+ in the perovskite.
"We are excited to explore the uses of Au2+ perovskites," said Karunadasa.