Metal Researchers Create A Novel Gold Compound

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Gold is considered to be chemically very inert. However, researchers at European XFEL have now demonstrated that the most noble of all metals becomes chemically active under extreme conditions. They produced a completely new compound of gold and hydrogen: solid gold hydride.

An international research team has, for the first time, created a solid chemical compound of gold and hydrogen. Under extremely high pressure and temperatures, a so-called gold hydride formed in experiments at the European XFEL. This discovery contradicts the previous assumption that gold hardly reacts with hydrogen and opens up new perspectives for chemistry under extreme conditions.

Gold is considered one of the least reactive metals. However, under pressures like those found in the Earth's interior, the noble metal reveals different qualities. Researchers at the High-Energy-Density (HED) experimental station at European XFEL compressed gold to over 40 gigapascals (GPa) using a diamond anvil cell and heated it to around 4,532 degrees Fahrenheit with ultrashort flashes from the European XFEL X-ray laser. Under these conditions, the gold reacted with hydrogen released from embedded hydrocarbons, forming a novel gold-hydrogen compound.

Remarkable: Superionic State of the New Compound

"We were able to demonstrate that the lattice of gold atoms forms a hexagonally close-packed arrangement above 40 GPa, with disordered hydrogen atoms in the interstitial spaces," explains Mungo Frost from the SLAC National Accelerator Laboratory. The compound, known as gold hydride, has the chemical formula Au₂Hx, with the hydrogen content x increasing with pressure and ranging from zero to nearly one.

Particularly remarkable: The hydrogen in the newly discovered compound moves very easily through the solid lattice of gold atoms. Science refers to this state as the superionic state. This property has so far been primarily observed in hydrogen-rich materials, but in this form, it is completely new for gold-containing compounds.

When cooled, the gold hydride decomposes back into normal, face-centered cubic structured gold. Therefore, the researchers suspect that gold hydride remains stable only under high-temperature conditions. Because samples in previous experiments were cooled back to room temperature before analysis, the compound was evidently overlooked in past studies.

Further Unexpected Compounds Possible

"Our results show that even elements as inert as gold can exhibit entirely new chemical properties under extreme pressure and temperature conditions," says Ulf Zastrau, head of the HED experimental station at European XFEL, where the experiment was conducted. The team sees this as an indication that many more unexpected compounds could exist, which have so far remained undiscovered due to a lack of suitable methods.

This discovery not only represents a novelty in gold chemistry but also has far-reaching implications for high-pressure experiments, where gold has so far been used as a completely inert material. Future research aims to determine whether similar effects occur with other noble metals as well.

The investigations were conducted at the European XFEL in Schenefeld near Hamburg (Germany), one of the most powerful X-ray lasers in the world.

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