Researchers using powerful ultraviolet lasers have revealed that previous estimates of diamond’s melting point were overestimated by more than 1,300 degrees Fahrenheit (700 degrees Celsius). Published on August 13 in the journal Nature Physics, the new findings finally resolve a decades-old discrepancy between laboratory measurements and theoretical models of the hardest natural material on Earth.
It sounds counterintuitive to imagine diamond — nature’s hardest substance — melting into a pool of liquid. Understanding how diamond responds to shock waves from lasers is an important part of developing nuclear fusion, the process that powers stars, which is also a potential energy source for the future. Yet, when subjected to extreme conditions under ultraviolet lasers, the material undergoes a radical transformation, shifting from transparent to mirror-like as strong shock waves tear through the atomic lattice. The strong increase in reflectivity is one indication the diamond melted.
For two decades, physicists have grappled with a stubborn puzzle: theoretical models and experimental data simply refused to align most of the time. The gap between what computer models predicted and what physical experiments showed was roughly 20%, creating a perplexing 1,244 C (2,240 F) discrepancy that left researchers searching for answers. Researchers have struggled to explain these discrepancies because the conditions diamond melts at are so extreme that it’s extraordinarily difficult to measure it in labs on Earth. There’s also been some debate about whether diamond reorganizes its atoms into a different kind of solid carbon before turning into a liquid at the end of the melting process.
Laser Shocks and High-Pressure Experiments
To finally settle the debate, scientists zapped tiny plates of synthetic diamond with an ultraviolet laser, creating shock waves that were so powerful that as they passed through the samples, the diamond changed from transparent to mirror-like. In a study published Aug. 13 in the journal Nature Physics, scientists created shock waves with these ultraviolet lasers.
“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,” said study co-author Marius Millot, a research scientist at Lawrence Livermore National Laboratory in California, in a statement.
By combining this change with measurements of how brightly the diamonds glowed while being zapped, the researchers mapped the melting temperature with great precision. Their findings demonstrated that the actual melting point sits more than 1,300 degrees Fahrenheit lower than earlier experimental estimates suggested, putting the melting point in line with theoretical predictions and finally explaining the long-held discrepancy.
Unlocking the Strange Behavior of Liquid Carbon
The team also measured the samples’ atomic structure with X-ray diffraction and saw that the diamond didn’t 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.
The researchers found that between about 660 and 1,060 gigapascals of pressure and at around 12,140 F (6,727 C), diamond exists as solid chunks floating in liquid carbon. As the pressure increases, more diamond transitions into liquid carbon, which is thought to be a very strange material.
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. In these extreme states, a solid diamond fragment can happily bob around in it like an ice cube in a glass of water, a situation proven possible deep within other planets.
Implications for Nuclear Fusion and Ice Giant Planets
Pinpointing how carbon behaves under crushing pressures does more than solve a fundamental physics puzzle. Understanding this is 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 such 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.
