Is Lonsdaleite harder than diamond

Theoretical world record: Significantly harder than diamond

Shanghai (China) / Las Vegas (USA) - If you press two materials together at a point, it used to be the diamond that left its mark on the other material. Two years ago, however, a composite showed a comparable hardness, a compound with parts of the mineral wurtzite boronide. And now theoretical models by Chinese-American researchers show that pure wurtzite boronide is likely to be 18 percent harder than diamond - if it is changed slightly under pressure and its crystal structure shifts. According to the calculations, the mineral Lonsdaleite is even 58 percent harder than diamond. The researchers explain that the structural shift could already take place in a typical pressure test. The new world record will only apply if the material can confirm these predictions in a practical test. The materials would be of interest for a wide range of applications, as they can withstand more heat than diamonds: for example, as coatings for drilling and cutting tools, for spaceships or electronic components.

"Here we reveal the deformation mechanism in two stages, which is responsible for the fact that wurtzite boronide shows a surprising hardness," writes the team led by Zicheng Pan and Hong Sun in the journal "Physical Review Letters". "We are also showing that the same mechanism also works in Lonsdaleit and is setting a new record," said the researchers from Shanghai Jiao Tong University and colleagues from the University of Nevada. Diamond gets its hardness from a strong lattice structure of its carbon atoms, which are arranged in an octahedron shape. Lonsdaleit is also called "hexagonal diamond" because of its natural hardness and crystal structure. Wurtzite, a compound of zinc and sulfur, also crystallizes in a hexagonal structure. According to the calculations, under high pressure there is a phase transition in which chemical bonds flip over, but the volume remains unchanged. The new crystal structure provides the unexpected hardness.

In nature, the two materials occur only in extremely small quantities, which are insufficient to test the theory, let alone for industrial use as a diamond substitute. Wurtzite boronide is formed under the extreme pressures and temperatures of volcanic eruptions, Lonsdaleit is found after meteorite impacts, for example in the Siberian Tunguska crater or in the southern German Ries crater. However, the researchers hope to use new production methods - including in the nano range - to produce serious quantities of the minerals in the not too distant future and to test their calculations.

source: "Harder than Diamond: Superior Indentation Strength of Wurtzite BN and Lonsdaleite", Zicheng Pan, Hong Sun et al .; Physical Review Letters, vol 102, 055503 (2009)
DOI: 10.1103 / PhysRevLett.102.055503