A form of gold that does not occur stably in nature is at the heart of a new crystalline material with intriguing properties.
For the first time, Stanford researchers have found a way to create and stabilize an extremely rare form of gold that has lost two negatively charged electrons, denoted Au2+. The material stabilizing this elusive version of the valued element is a halide perovskite—a class of crystalline materials that holds great promise for various applications including more-efficient solar cells, light sources, and electronics components.
Surprisingly, the Au2+ perovskite is also quick and simple to make using off-the-shelf ingredients at room temperature.
“It was a real surprise that we were able to synthesize a stable material containing Au2+—I didn’t even believe it at first,” said Hemamala Karunadasa, associate professor of chemistry at the Stanford School of Humanities and Sciences and senior author of the study published recently in Nature Chemistry. “Creating this first-of-its-kind Au2+ perovskite is exciting. The gold atoms in the perovskite bear strong similarities to the copper atoms in high-temperature superconductors, and heavy atoms with unpaired electrons, like Au2+, show cool magnetic effects not seen in lighter atoms.”
“Halide perovskites possess really attractive properties for many everyday applications, so we’ve been looking to expand this family of materials,” said Kurt Lindquist, the lead author of the study who conducted the research as a Stanford doctoral student and is now a postdoctoral scholar in inorganic chemistry at
This phenomenon applies to particles, too, and has profound consequences for “massive” heavy elements, such as gold, whose atomic nuclei boast a large number of protons. These particles collectively exert immense positive charge, forcing negatively charged electrons to whirl around the nucleus at breakneck speeds. As a consequence, the electrons grow heavy and tightly surround the nucleus, blunting its charge and allowing outer electrons to drift farther than in typical metals. This rearrangement of electrons and their energy levels leads to gold absorbing blue light and therefore appearing yellow to our eye.
Because of the arrangement of gold’s electrons, thanks to relativity, the
“In the lab, we can make this material using very simple ingredients in about five minutes at room temperature,” said Lindquist. “We end up with a powder that’s very dark green, nearly black, and is surprisingly heavy because of the gold it contains.”
Recognizing that they may have hit new chemistry paydirt, so to speak, Lindquist performed numerous tests on the perovskite, including spectroscopy and X-ray diffraction, to investigate how it absorbs light and to characterize its crystal structure. Stanford research groups in physics and chemistry led by Young Lee, professor of applied physics and of
Looking ahead, Karunadasa, Lindquist, and colleagues plan to study the new material further and tweak its chemistry. The hope is that an Au2+ perovskite can be used in applications that require magnetism and conductivity as electrons hop from Au2+ to Au3+ in the perovskite.
“We’re excited to explore what an Au2+ perovskite could do,” Karunadasa said.
Reference: “Stabilizing Au2+ in a mixed-valence 3D halide perovskite” by 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 and Hemamala I. Karunadasa, 28 August 2023, Nature Chemistry.
DOI: 10.1038/s41557-023-01305-y
Karunadasa is also a senior fellow at the Precourt Institute for Energy and a 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 Stanford Synchrotron Radiation Lightsource, SLAC. Additional Stanford co-authors are Christina R. Deschene and Alexander J. Heyer, graduate students in the Department of Chemistry; and Jiajia Wen, a staff scientist at SLAC. Additional co-authors include Armin Eghdami and Alexander G. Smith, graduate students in the Department of Physics, University of California-Berkeley, and Jeffrey B. Neaton, professor of physics at the University of California-Berkeley; and Dominic H. Ryan, professor of physics at McGill University.
The research was funded in part by the U.S. National Science Foundation, the U.S. Department of Energy, the Fonds de recherche du Québec–Nature et technologies, and the Natural Sciences and Engineering Research Council Canada.