Researchers have determined that the melting point of diamond is more than 1,300 degrees Fahrenheit (700 degrees Celsius) lower than previously established, a discovery made possible by a new laser-based experiment. The findings, published on August 13 in the journal Nature Physics, resolve a significant long-standing disagreement between theoretical models and prior experimental data.
While diamond is renowned as the hardest natural substance on Earth, it does melt under extreme conditions, such as those created by powerful lasers. Accurately modeling this behavior is crucial for advancements in nuclear fusion, the process that fuels stars and represents a potential future energy source. For years, scientists grappled with a discrepancy where experimental data suggested diamond melted at approximately 2,240 F (1,244 C)—roughly 20% higher than theoretical predictions. Additionally, there was debate over whether diamond transforms into a different solid carbon structure before becoming liquid.
To address these issues, the team utilized an ultraviolet laser to strike tiny plates of synthetic diamond, generating shock waves intense enough to render the material mirror-like. This dramatic increase in reflectivity served as a key indicator of melting. By analyzing both the reflectivity changes and the glow intensity of the samples, the researchers mapped the melting temperature with unprecedented precision.
“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 Marius Millot, a research scientist at Lawrence Livermore National Laboratory and co-author of the study.
The results indicated that the actual melting temperature is significantly lower than earlier estimates, aligning closely with theoretical predictions. Using X-ray diffraction to examine atomic structure, the team also found no evidence that diamond transitions into another solid carbon form before melting, likely because the energy required for such rearrangement is too great. However, they hypothesized that multiple shock waves applied in specific ways might still induce this transition.
The study also revealed that between 660 and 1,060 gigapascals of pressure and at temperatures around 12,140 F (6,727 C), diamond exists as solid fragments suspended in liquid carbon. Unlike the coal, graphite, or diamond forms common on Earth, liquid carbon is metallic, electrically conductive, and denser than its solid counterpart. This means solid diamond could theoretically float in liquid carbon similarly to how ice floats in water.
These insights have profound implications for planetary science, particularly regarding ice giants like Uranus and Neptune. Data from the Voyager 2 spacecraft and previous lab experiments suggest these planets may experience diamond rain and possess mantles containing oceans of liquid carbon with floating diamond chunks. With the new melting data, scientists can now refine their models of planetary interiors and carbon cycles.
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