Striking rare gold: Researchers unveil new material infused with gold in an exotic chemical state

For the first time, scientists at Stanford University have discovered a means to produce and maintain Au2+, an incredibly rare type of gold that has lost two negatively charged electrons. Halide perovskite, a family of crystalline materials with considerable potential for use in light sources, electronics components, and more efficient solar cells, is the material stabilizing this elusive variety of the valuable element.

Remarkably, Au2+ perovskite may be prepared quickly and easily at room temperature with readily available materials.

Hemamala Karunadasa, an associate professor of chemistry at the Stanford School of Humanities and Sciences and the study's senior author, said, "I didn't even believe it at first that we were able to synthesize a stable material containing Au2+." The study was published on August 28 in Nature Chemistry. It's great to be able to create this unique Au2+ perovskite. Heavy atoms with unpaired electrons, like Au2+, exhibit cool magnetic phenomena not found in lighter atoms, and the gold atoms in the perovskite have striking similarities to the copper atoms in high-temperature superconductors."

Kurt Lindquist, the study's lead author and present postdoctoral scholar in inorganic chemistry at Princeton University, conducted the research while a doctoral student at Stanford University. "Halide perovskites possess really attractive properties for many everyday applications, so we've been looking to expand this family of materials," Lindquist said. "Some fascinating new avenues could be opened by an unprecedented Au2+ perovskite."

Gold's heavy electrons

As an elemental metal, gold has long been prized for its unparalleled malleability and chemical inertness, which allow it to be readily fashioned into coins and jewelry that won't tarnish over time or react with the environment. The eponymous color of gold is another important factor in its worth; few other pure metals have a color as rich and unique as gold's.

According to Karunadasa, the underlying physics of gold's celebrated look also explains why Au2+ is so scarce.

Relativistic effects, first proposed in Albert Einstein's renowned theory of relativity, are the main cause. Karunadasa stated, "Einstein taught us that objects get heavier when they move very fast and their velocity approaches a significant fraction of the speed of light."

This phenomena holds true for particles as well, and it has significant ramifications for "massive" heavy elements like gold, whose atomic nuclei have a lot of protons. Because of the enormous positive charge that these particles together exert, negatively charged electrons are forced to spin rapidly around the nucleus. Because of this, the nucleus's charge is blunted and outside electrons are able to float farther than in ordinary metals because the electrons around it become heavier and more closely packed. Due to the reorganization of electrons and their energy levels, gold absorbs blue light and appears yellow to the human sight.

Relativity's work on the arrangement of gold's electrons results in the atom occurring naturally as Au1+ and Au3+, which spurn Au2+ and lose one or three electrons, respectively. (The Latin term for gold, aurum, is the source of the "Au" chemical symbol for gold, which denotes a net positive charge from the loss of two negatively charged electrons.)

A quick vitamin C boost

The Stanford researchers discovered that Au2+ can persist with the correct chemical arrangement. Lindquist said that while working on a larger research based on magnetic semiconductors for application in electrical devices, he "stumbled upon" the novel Au2+-harboring perovskite.

Lindquist combined Au3+-chloride and a salt known as cesium chloride in water, then added hydrochloric acid to the mixture "with a little vitamin C thrown in," according to him. Vitamin C, an acid, contributes a negatively charged electron to the common Au3+ in the subsequent reaction, generating Au2+. Interestingly, Au2+ is not stable in solution but stable in solid perovskite.

"With very basic ingredients, we can make this material in the lab in about five minutes at room temperature," Lindquist stated. "We're left with a powder that's almost entirely black, extremely dark green, and surprisingly heavy due to the gold content."

Sensing they might have found pay dirt in chemistry, so to speak, Lindquist put the perovskite through a battery of experiments, including X-ray diffraction and spectroscopy, to see how it absorbed light and to define its crystal structure. The behavior of Au2+ was further studied by Stanford research groups in physics and chemistry under the direction of Edward Solomon, the Monroe E. Spaght Professor of Chemistry and professor of photon science, and Young Lee, a professor of applied physics and photon science.

In the end, the investigations confirmed the presence of Au2+ in a perovskite and, in doing so, opened a new chapter in the century-old history of chemistry and physics including Linus Pauling, the recipient of the Nobel Peace Prize in 1962 as well as the Nobel Prize in Chemistry in 1954. He worked on gold perovskites comprising the common types Au1+ and Au3+ early in his career. Interestingly, Pauling went on to study the structure of vitamin C, which is necessary to produce a stable perovskite that has the elusive Au2+.

Karunadasa remarked, "We adore Linus Pauling's connection to our work." "This perovskite's synthesis is an interesting story."

Karunadasa, Lindquist, and others intend to refine the chemistry of the novel substance and do more research on it in the future. As electrons go from Au2+ to Au3+ in the perovskite, it is hoped that an Au2+ perovskite would be useful in applications that call for conductivity and magnetism.

"We're eager to investigate the potential of an Au2+ perovskite," Karunadasa stated.

Provided by Stanford University