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
