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The Discovery of a New Ultra Hard Phase of Titania

Computer simulation is very well established in the role of helping us to understand detailed observations of complex systems. In recent years the reliability and scale of first principles simulations has reached a level which allows predictive simulations to be performed on rather realistic models of materials and thus simulation has begun to have a significant impact on the design of new materials.

Ultra hard materials are of enormous technological importance and the aim of much research has been the development of materials with a measured hardness comparable to diamond or boron nitride. A combined theoretical and experimental study lead to the discovery of a new polymorph of titanium dioxide where titanium is nine-coordinated to oxygen in the cotunnite (PbCl2) structure. Hardness measurements on this phase, 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 so far.

Ab initio Hartree-Fock calculations of the bulk phase stability of TiO2 in a number of phases were performed with the CRYSTAL code. The results for the rutile, anatase, columbite, baddeleyite (P21/c), pyrite (Pa), fluorite (Fm3m), and cotunnite phases are displayed in the figure below.

The computed equation of state for a variety of TiO2 phases

This suggests predict that cotunnite structured TiO2 is stable above 50 GPa (Fig. 1) and that is will have a remarkably high bulk modulus of around 380 GPa.

Experimentally it was found that rutile and anatase transform to the baddeleyite structure at 12 GPa. At pressures above 45 GPa the quality of the diffraction pattern decreased drastically and at about 60 GPa, after heating at 1600-1800 K the material transformed to the cottunite phase. The bulk modulus and hardness of the cottunite phase were also measured and it was found to be the hardest oxide material yet discovered.

Hardness of polycrystalline materials

Material Bulk Modulus (GPa) Hardness GPa
B4C 200 30
SiC 248 29
Al2O3 252 20
SiO2, Stishovite 291 32
WC 421 30
Cubic BN 369 32
Cotunnite TiO2 431 38
Sintered Diamond 444 50
This work was performed in collaboration with coleagues in Upsala University, CSIRO Australia and the Royal Institute of Technology in Stockholm. The following publications may be of interest.

  • L. S. Dubrovinsky, N. A. Dubrovinskaia, V. Swamy, J. Muscat, N. M. Harrison, R. Ahuja, B. Holm, B. Johansson The Hardest Known Oxide Nature 410 p.653 (2001)
  • J. Muscat, V Swamy, NM Harrison First Principles Calculations of the Phase Stability of TiO2 Phys Rev B 65 p.224112 (2002)

 
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