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It is well known that hardness and Young's modulus can change as a function of pressure. Are there any chemical substances that under sufficiently high pressure exceed the hardness of diamonds at the same pressure?

P.S. Cross-posted on Physics.SE

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    $\begingroup$ You know cross-posting is frowned upon? BTW there's a few thing that are arguably harder than "diamond", even without pressure. $\endgroup$
    – Mithoron
    Apr 20 at 12:19
  • $\begingroup$ Yes I’m aware at normal pressures nothing meaningful is harder than Diamond. But we also know that at normal pressures nothing is denser than Osmium. At high pressures materials like Iridium are denser. Similarly it’s natural to wonder if at high pressures there can be materials harder than Diamond. $\endgroup$ Apr 20 at 12:26
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    $\begingroup$ The diamond cubic structure is fairly open, and carbon, like silicon and germanium, assumes a different, denser crystal structure at high pressures. It would not surprise me if a close packed crystal ultimately showed a smaller compressibility than diamond at some high pressure. $\endgroup$
    – Jon Custer
    Apr 22 at 17:48
  • $\begingroup$ Please don't cross-post $\endgroup$ Apr 26 at 4:52

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Diamonds are graded as a 10 on the Mohs scale of mineral hardness, which ranges from 1 to 10. However, some materials are tougher than diamonds. One such material is wurtzite boron nitride (w-BN). Using a typical hardness test that indents the material with a small pyramid and measures the force required, researchers recently discovered that w-BN easily topped diamond's hardness rating of 114 Gpa, compared to diamond's 97 Gpa. The shape of the hardness indenter ensures that the stress it applies to the material is not limited to a single direction relative to a crystal plane in the material. This causes more intricate stresses, allowing the bonds to reorganise themselves into a somewhat different structure; in the case of w-BN, this reordering significantly boosts the material strength.

Lonsdaleite, which is an allotrope of carbon, has a similar structure to w-BN. When hardness indentions are done on Lonsdaleite, the same bond-flipping process will take place, surpassing both diamond and w-BN, with a strength of 152 GPa, which is 58% greater than ordinary diamond.

Reference: Pan, Z., Sun, H., Zhang, Y., & Chen, C. (2009). Harder than Diamond: Superior Indentation Strength of Wurtzite BN and Lonsdaleite. Physical Review Letters, 102(5), 055503. https://doi.org/10.1103/PhysRevLett.102.055503

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