The cotunnite-type phase was claimed to be the hardest known oxide wit... - dofaq.co
titanium dioxide

The cotunnite-type phase was claimed to be the hardest known oxide wit...

wikipedia - 17 Jul 2022
The cotunnite-type phase was claimed to be the hardest known oxide with the Vickers hardness of 38 GPa and the bulk modulus of 431 GPa (i.e. close to diamond's value of 446 GPa) at atmospheric pressure.[22] However, later studies came to different conclusions with much lower values for both the hardness (7–20 GPa, which makes it softer than common oxides like corundum Al2O3 and rutile TiO2)[23] and bulk modulus (~300 GPa).[24][25]

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