Researchers Find Diamond Melting Point Is Lower Than Previously Measured

Researchers zapping synthetic diamond plates with a powerful ultraviolet laser discovered the mineral’s melting point is more than 1,300 degrees Fahrenheit lower than previously measured. The findings, published in Nature Physics on August 13, align experimental data with theoretical models and offer new clues about the icy interiors of Uranus and Neptune.

Laser Experiments Reveal a Major Discrepancy in Diamond Melting

It sounds counterintuitive to melt diamond, the hardest natural material on Earth. Yet when researchers subject synthetic plates of the mineral to extreme conditions using powerful ultraviolet lasers, the diamond transitions rapidly from transparent to mirror-like under the resulting shock waves. That sharp spike in reflectivity provides a clear signal that melting is underway. Understanding how diamond responds to shock waves from lasers is an important part of developing nuclear fusion, the process that powers stars. Nuclear fusion is also a potential energy source for the future, so researchers have put a lot of effort into developing models that describe and predict how diamond behaves.

Researchers Find Diamond Melting Point Is Lower Than Previously Measured

For two decades, scientists struggled to reconcile a persistent 20% gap between experimental melting measurements and the temperatures predicted by theoretical models. The difference reached roughly 1,244 degrees Celsius, or 2,240 degrees Fahrenheit. The extreme conditions diamond melts at are so extreme that it’s extraordinarily difficult to measure it in labs on Earth.

By tracking the brightness of the glowing samples alongside the reflectivity changes, researchers finally mapped the melting point with high precision. The team determined that previous experiments overestimated the melting temperature by more than 700 degrees Celsius, or over 1,300 degrees Fahrenheit, bringing experimental data into agreement with theoretical models.

“We were able to take tiny diamond samples and shock compress them to temperatures hotter than the surface of the sun and to pressures higher than the center of Neptune and Uranus — and still measure atomic structure, temperature, density and optical reflectivity,” study co-author Marius Millot, a research scientist at Lawrence Livermore National Laboratory in California, said in a statement.

What Happens to Carbon Under Extreme Pressure

Beyond pinpointing the melting point, the team examined the atomic structure of the samples using X-ray diffraction during the compression process. They discovered that the diamond did not transition to a different kind of solid carbon before melting, possibly because the energy required to rearrange the atoms was too large, the researchers wrote.

However, they also hypothesized that multiple shocks could be powerful enough for this transition to occur and that the way the shocks are applied to the diamond might affect how it changes phase. Under pressures of approximately 660 to 1,060 gigapascals and a temperature of about 6,727 degrees Celsius (12,140 F), diamond exists as solid chunks floating in liquid carbon.

Unlike most forms carbon takes on Earth — like coal, graphite and diamond — liquid carbon is metallic, so it conducts electricity. It’s also denser than diamond. So hypothetically, if you somehow were to put liquid carbon in a cup without instantly vaporizing it, a chunk of solid diamond could happily bob around in it like an ice cube in a glass of water.

Implications for Nuclear Fusion and Ice Giants

Understanding how diamond behaves under such extreme conditions is also important for nuclear fusion research, as certain types of experiments involve lasers melting and crushing a diamond capsule to put the capsule’s contents, solid deuterium and tritium, under more than 30 petapascals of pressure and temperatures higher than 180 million F (100 million C), the requisite conditions for a fusion chain reaction to occur.

Knowing how diamond behaves under these extreme conditions is also important for understanding the ice giant planets Uranus and Neptune. Based on measurements from the Voyager 2 spacecraft in the late 1980s and lab experiments on Earth, scientists think it literally rains huge chunks of diamond inside these planets and that their mantles may have liquid carbon oceans with diamonds floating around like icebergs.