An international team of researchers, led by scientists at SLAC National Accelerator Laboratory in the United States and involving the European XFEL in Germany, has accidentally synthesized a previously unknown compound: gold hydride. This breakthrough, achieved under extreme laboratory conditions, is expected to significantly deepen the understanding of chemistry under immense pressure and temperature, providing new perspectives on the conditions found within giant planets and the processes occurring in stars.
The discovery emerged from experiments initially focused on how hydrocarbons transform into diamonds when subjected to extreme pressure and heat. During these experiments at the European XFEL, hydrocarbon samples were prepared with a thin gold foil. This foil was intended solely to absorb X-rays and conduct heat to the hydrocarbon samples. However, the researchers were surprised to observe the formation of gold hydride alongside the expected diamond structures.
"It was completely unexpected because gold is typically very 'boring' chemically—almost unreactive. That's precisely why we chose it as an X-ray absorber," stated Mungo Frost, a SLAC researcher and lead author of the study. The experiments involved compressing hydrocarbon samples using a diamond anvil cell, reaching pressures far exceeding those found beneath Earth's mantle, and heating them to over 1,900°C (3,452°F) using powerful X-ray bursts.
The newly formed gold hydride revealed that hydrogen atoms, under these extreme conditions, entered a superionic state, allowing them to move freely within the rigid gold lattice. This phenomenon not only enhances the conductivity of the gold hydride but also provides a novel method for studying the behavior of dense hydrogen. "We can use the gold lattice as a 'witness' to see what the hydrogen is doing," explained Frost, highlighting the compound's utility in observing hydrogen under conditions that are otherwise difficult to access experimentally.
Siegfried Glenzer, Director of the High Energy Density Division at SLAC and a principal investigator, commented on the broader significance of the findings: "It is important for us to be able to experimentally produce and model these states of matter. These simulation tools can also be applied to model the exotic properties of other materials under extreme conditions."
Published in Angewandte Chemie International Edition, the findings demonstrate that chemical behavior can be dramatically altered under extreme environments, challenging the conventional understanding of gold's inert nature. While the gold hydride compound was found to be stable only under these intense conditions and separated upon cooling, simulations suggest that higher pressures could lead to greater integration of hydrogen within the gold lattice. This accidental discovery not only expands the frontier of chemical knowledge but also provides a unique lens through which to view the complex and often surprising processes occurring in the universe's most extreme environments.