Cotunnite-structured titanium dioxide: the hardest known oxide

TL;DR

This study reports the discovery of a cotunnite-structured titanium dioxide polymorph that is the hardest known oxide, with exceptional hardness and low compressibility, synthesized under high pressure and temperature conditions.

Contribution

The paper introduces a new cotunnite-structured TiO2 polymorph with record-breaking hardness and low compressibility, combining theoretical predictions and experimental validation.

Findings

Hardness of 38 GPa measured for cotunnite TiO2

Bulk modulus of 431 GPa indicating low compressibility

Synthesized at pressures above 60 GPa and temperatures above 1000 K

Abstract

Despite great technological importance and many investigations, a material with measured hardness comparable to that of diamond or cubic boron nitride has yet to be identified. Combined theoretical and experimental investigations led to the discovery of a new polymorph of titanium dioxide with titanium nine-coordinated to oxygen in the cotunnite (PbCl2) structure. Hardness measurements on the cotunnite-structured TiO2 synthesized at pressures above 60 GPa and temperatures above 1000 K reveal that this material is the hardest oxide yet discovered. Furthermore, it is one of the least compressible (with a measured bulk modulus of 431 GPa) and hardest (with a microhardness of 38 GPa) polycrystalline materials studied thus far.