Technetium and ruthenium incorporation into rutile TiO2

The incorporation of TcO2 into the rutile phase of TiO2 has been theoretically examined to understand the potential of the system for use as a wasteform. The effects of defect clustering and the transmutation of Tc into Ru were considered. Results suggest that as a single defective species, Tc has a temperature-dependent, moderate solubility into rutile TiO2, which increases when clustering is considered. The {2TcTi} cluster that preferentially forms was found to have a Tc-Tc bond length similar to the cation distance in TcO2. The transmutation of Tc to Ru has two effects: first, the preferred binary cluster morphology changes from that of first nearest neighbour for {2TcTi} to that of second nearest neighbour for {2RuTi}. Second, the defect solubility is lower for single RuTi defects and Ru-containing defect clusters. This will likely result in the formation of a secondary phase as transmutation proceeds, if Tc is added to rutile TiO2 to its solution limit. The varying solubility of RuO2 in TiO2 with temperature was compared with previously published experimental data with encouraging results. The use of two different DFT codes (one plane-wave and one local-orbital) was used to find the correct magnetic ordering in the defective lattices, which increased the confidence in the methods and therefore the results.