Ion irradiation damage in geikielite (MgTiO3)

A series of ion-irradiation experiments were conducted to explore the radiation response of the ilmenite-group oxide geikielite (MgTiO3)). In these experiments, oriented single crystals were irradiated with either 200 keV Ar2+ or 400 keV Xe2+ and Rutherford back-scattering spectrometry combined with ion channelling (RBS/C) was used to characterize consequent radiation damage. In the 200 keV Ar2+ experiments, the sample was held at 100 K and a buried amorphous layer 55 nm thick formed underneath a defective crystalline layer 90 nm thick after exposure to a fluence of 2 1015 ions cm 2. More detailed experiments with 400 keV Xe2+ employed incremental ion irradiation followed by RBS/C to determine the extent and rate of radiation damage at temperatures of 170, 300 and 470 K. These irradiations show that there is a strong temperature dependence for damage accumulation and that critical amorphization fluences increase from 2 1015 Xe2+ cm 2 (170 K) to 6 1015 -Xe2+ cm 2 (300 K) to greater than 2.5 1016 Xe2+ cm 2 (470 K). Damage appears to accumulate in several stages, with a rapid initial growth that levels at an intermediate stage. This is followed by an increase in and, eventually, saturation in the damage rate. At 170 and 300 K the damage fraction saturates at 100%, whereas saturation occurs at about 80% at 470 K. RBS/C data suggest the possible formation of a radiation-induced metastable phase in the damaged region, which may be analogous to pressure- or temperature-induced phase transformations in other ilmenite-group oxides. In particular, these materials transform to either the lithium niobate or the orthorhombic perovskite structure at high pressures and temperatures. The results presented in this study and similar investigations on the olivine system suggest that ionicity, composition and melting temperature may play important roles in the radiation response of ceramics, and particularly in predicting the relative radiation tolerance of materials within a solid-solution series.