Researchers using a powerful ultraviolet laser have melted synthetic diamond under extreme pressure, revealing that previous experimental estimates miscalculated diamond’s melting point by more than 1,300 degrees Fahrenheit. The findings, published August 13 in Nature Physics, align laboratory data with theoretical predictions and shed light on planetary interiors.
Scientists have long struggled to map out how the hardest natural material on Earth behaves under immense heat and pressure. While theoretical models and laboratory data usually align, a stubborn discrepancy persisted regarding diamond. Previous experimental data and model-predicted melting temperatures of diamond differed by roughly 20%, or about 2,240 degrees Fahrenheit (1,244 degrees Celsius). Reconciling this gap proved extraordinarily difficult because the required melting conditions are notoriously hard to measure inside a terrestrial laboratory.
A team of researchers finally bridged that gap by targeting tiny plates of synthetic diamond with an ultraviolet laser. The resulting shock waves were powerful enough to force a striking transformation as they passed through the samples, turning the diamond from transparent to mirror-like. That sharp increase in optical reflectivity signaled that the material had melted.
Laser Shock Waves and Precise Temperature Tracking
By tracking how brightly the diamond plates glowed during the ultraviolet laser bombardment, the research team could measure the melting temperature with great precision. The experiments subjected tiny diamond samples to conditions rivaling the solar system’s most violent environments.
“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.
That shock compression revealed that the actual melting point of diamond was more than 1,300 degrees Fahrenheit lower than previously thought, according to the team. This drop places the physical measurements in line with theoretical predictions, solving a two-decade-old physics puzzle.
Atomic Structure and Metallic Liquid Carbon
Using X-ray diffraction to examine the atomic structure of the samples during the process, the researchers discovered that the diamond did not transition to a different kind of solid carbon before melting. The team suggested that the energy required to rearrange the atoms was too large, though they hypothesized that multiple shocks could be powerful enough for this transition to occur.
Under extreme pressures ranging from approximately 660 to 1,060 gigapascals and temperatures near 12,140 degrees Fahrenheit (6,727 degrees Celsius), the experiments showed that diamond exists as solid chunks floating in liquid carbon. This liquid form shares little in common with familiar carbon allotropes like coal, graphite, and diamond. Unlike most forms carbon takes on Earth, liquid carbon is metallic, so it conducts electricity, and it is also denser than diamond.
These extreme-pressure characteristics validate theoretical scenarios where solid chunks of diamond could happily bob around in it like an ice cube in a glass of water, a situation the new experiment proves is possible deep within other planets.
Implications for Nuclear Fusion and Ice Giant Planets
Understanding how diamond behaves under extreme stress carries direct practical applications for nuclear fusion research, the process that powers stars and a potential energy source for the future. Researchers have put a lot of effort into developing models that describe and predict how diamond behaves, including experiments that 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 degrees Fahrenheit (100 million Celsius), the requisite conditions for a fusion chain reaction to occur.
Beyond terrestrial energy technology, the findings refine models describing the internal architecture of ice giant planets like 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.
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