A team of scientists from two universities in China synthesized a material harder than diamond in the laboratory.
When carbon is subjected to extreme heat and pressure inside the Earth, it can crystallize into diamonds; the hardest (though not necessarily the hardest) natural mineral on the planet. In diamond, each carbon atom is bonded to four other carbon atoms in a tetrahedral arrangement, but under the right circumstances, the carbon can be arranged hexagonally, forming a harder material known as lonsdaleite.
We know about the existence of lonsdaleite thanks to the impact of the meteorite that formed it.
In 1891, while examining a meteorite in Diablo Canyon, Arizona, scientists reported finding “hard particles” in it. Later, in 1939, the hard particles were confirmed to be a mixture of diamonds, graphite and a new substance never seen before, now called lonsdaleite after the crystallographer Professor Dame Kathleen Lonsdale.
At first, scientists expected that the unusual material would be a diamond with a hexagonal structure, and not the classic cubic diamonds we are used to. However, studying meteorite samples in 2022, the team discovered that they were composed of nanostructured hexagonal and cubic diamonds with graphene-like outgrowths in between. The sample was technically a diaphyte, where two minerals grow at the same time, resulting in a less ordered crystal structure full of stacking “mistakes.”
Besides being pretty amazing that meteorites could hit with such force to create hexagonal diamonds, it opened up the possibility that scientists could synthesize them.
“With its potentially superior mechanical properties and intriguing structure, lonsdaleite has also received intense research interest in materials science. Theoretical calculations suggest that HD [hexagonal diamond] could even surpass cubic diamond (CD), the hardest and least compressible material known in nature today,” the new team explains in their paper.
Using shock compression techniques aimed at mimicking the natural production of these “super diamonds,” researchers have been able to produce them in the past. However, they ended up with a lot of graphite and diamond in the process. In the new paper, the team used pressure and temperature to produce lonsdaleite, optimizing the conditions for production.
“Theoretical calculations suggest that the total energy of CD is slightly lower than that of HD and that, starting from a graphite precursor, the energy barrier for the direct transition of graphite to CD is slightly lower than that for the transformation of graphite to HD, and therefore CD is usually the dominant product,” the team explained in their study.
“To overcome such unfavorable factors for HD growth, we synthesized HD from graphite via intermediate post-graphite phases in which interlayer bonding can lock the stacking close to AB in compressed graphite and prevent further layer sliding during high-temperature stimulation, favoring HD formation. Both our experiments and simulations show that, in addition to the formation of the post-graphite phase, the presence of a temperature gradient is also critical for HD synthesis,” it states.
In addition to potentially opening the way for new superconductors and being able to withstand about 58 percent more stress than diamonds, producing the material in the laboratory could tell us about the formation of natural lonsdaleite.
“For example, natural lonsdaleite is rarely found on Earth because the planetary interior can rarely provide suitable […] conditions. More importantly, the excellent thermal stability and ultrahigh hardness of HD suggest its great potential for industrial applications, providing opportunities for this remarkable material,” the study states, writes IFLScience.
Photo: Noraydesign
