Highlights

Radiation Damage Modelling of TiO₂ – Rutile, Anatase and Brookite

Scientific Achievement

TiO₂ provides a means by which the stability of materials to radiation damage can be easily modelled.  The three different structures, rutile, anatase, and brookite are found naturally, and with the exception of brookite can be easily prepared in the laboratory (brookite forms naturally in high pressure aqueous environments.  The major advantage of TiO₂ in this context is the change of structure with no change in composition. This allows the effect of structure alone to be studied.  The three polymorphs have differing structures with differing atomic co-ordination, bond lengths, shared edges between the TiO₆ octahedra and polyhedral distortion (the degree by which the titanium-oxide octahedron differs from the ‘ideal octahedron’ structure).

Multiple grains of each sample were irradiated at 50K using the IVEM-TANDEM to a point where all the spots in the TEM patterns had disappeared, this point was then used to define when the sample went amorphous.  The fluence of ions required to transform the anatase was 2.3x10¹⁴ ions cm^-2, and the brookite 8.1x10¹⁴ ion cm^-2.  However, the rutile was found to be stable up to 5x10¹⁵ ions cm^-2.

The results when compared with the structural properties show that the tolerance is correlated with the number of shared edges, polyhedral distortion and the crystalline-amorphous volume change.

Simulations using DL_POLY have shown qualitative agreement, and have shown that the tolerance of rutile to irradiation is based on there being low energy pathways for recovery.  In anatase however, there are not similar pathways coupled with significant energy barriers to recombination.

Significance

This work has utilised the unique capabilities of the IVEM-TANDEM and the polymorphs of TiO₂ to being to understand the effects of structure on the radiation tolerance of materials.  The use of simulation to correlate with experiment has enhanced both, and allows the application of simulation to be improved.  This work has been published in Phys. Rev. B 77, 214201 (2008), and Nucl. Inst and Methods B 266 (12-13), 2665-2670 (2008).  The work has been presented at the Scientific Basis for Nuclear Waste Management XXXII in Boston 2008, and has been submitted for the conference proceedings.

The future of this work is to complete the collection of data to determine the stability of these polymorphs at different temperatures, and to determine the relative stability of the three polymorphs

Performers

G. R. Lumpkin, K. L. Smith, M. G. Blackford (Institiute of Materials Engineering, ANTSO, Australia); N. J. Zaluzec (Argonne-MSD)