Researchers create first-of-its-kind Au2+ perovskite

Stanford researchers have achieved a remarkable breakthrough by creating and stabilizing an exceedingly rare form of gold, Au2+, which has lost two negatively charged electrons. They accomplished this feat using a halide perovskite, a class of crystalline materials known for their potential in applications like more efficient solar cells, lighting systems, and electronic components. What’s astonishing is that this Au2+ perovskite can be produced easily with readily available ingredients at room temperature.

Hemamala Karunadasa, an associate professor of chemistry at Stanford, and the senior author of the study, expressed her initial disbelief at this achievement. She described the creation of this unique Au2+ perovskite as thrilling. The gold atoms within the perovskite exhibit intriguing similarities to copper atoms found in high-temperature superconductors. Furthermore, heavy atoms with unpaired electrons, such as Au2+, display fascinating magnetic properties not observed in lighter atoms.

Kurt Lindquist, the lead author of the study who conducted this research as a Stanford doctoral student and is now a postdoctoral scholar in inorganic chemistry at Princeton University, highlighted the promising potential of this unprecedented Au2+ perovskite in expanding the family of halide perovskite materials. This discovery opens exciting new possibilities in various practical applications.

Heavy electrons in gold

Gold, renowned for its scarcity, remarkable malleability, and resistance to chemical reactions that cause tarnishing, has always held a special place. Its distinctive, rich color, unlike any other metal in its pure form, adds to its allure.

The reason behind gold’s unique characteristics, and the rarity of Au2+, lies in the realm of physics, specifically, Albert Einstein’s theory of relativity. Einstein’s theory tells us that as objects approach significant fractions of the speed of light, they become heavier. This phenomenon applies to particles, even heavy elements like gold, which have numerous protons in their atomic nuclei. The collective positive charge of these protons causes electrons to whirl around the nucleus at incredibly high speeds. Consequently, the electrons gain weight and envelop the nucleus closely, neutralizing its charge and allowing outer electrons to move farther from the nucleus. This rearrangement of electrons and their energy levels results in gold absorbing blue light and exhibiting the distinctive yellow color we associate with it.

Due to the unique electron arrangement influenced by relativity, gold naturally exists as Au1+ and Au3+, where it loses one or three electrons, respectively, but Au2+ is avoided. The “2+” signifies a net positive charge resulting from the loss of two negatively charged electrons, and the chemical symbol “Au” for gold traces its origins to “aurum,” the Latin word for this precious metal.

A squeeze of vitamin C

In their research, Stanford scientists made a serendipitous discovery when Kurt Lindquist, while working on a project involving magnetic semiconductors for electronic devices, stumbled upon the Au2+-containing perovskite.

To create this unique material, Lindquist combined cesium chloride and Au3+-chloride in water, adding hydrochloric acid and a touch of vitamin C. In this reaction, vitamin C, acting as an acid, donated an electron to transform the common Au3+ into the elusive Au2+. Intriguingly, while Au2+ remained stable within the solid perovskite structure, it proved less so in solution.

Remarkably, this material could be synthesized in a simple five-minute procedure at room temperature, resulting in a dark green, almost black powder due to its gold content.

The team conducted a series of tests, including spectroscopy and X-ray diffraction, to understand the perovskite’s light absorption and crystal structure. Other Stanford research groups, led by Young Lee and Edward Solomon, contributed to the study of Au2+ behavior.

These experiments confirmed the presence of Au2+ in the perovskite and added a new chapter to a century-old story in chemistry and physics, connected to Linus Pauling, who studied gold perovskites containing Au1+ and Au3+ early in his career and coincidentally, later, investigated the structure of vitamin C—a key ingredient in stabilizing the elusive Au2+.

The researchers are now eager to explore the potential applications of Au2+ perovskite, particularly in scenarios requiring magnetism and conductivity, where electrons transition between Au2+ and Au3+ within the perovskite structure. The future holds exciting prospects for this newfound material.

Source: Stanford University

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