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 eith er 200 keV Ar2+ or 400 keV Xe2+ and Rutherford back-scattering spectro metry combined with ion channelling (RBS/C) was used to characterize c onsequent radiation damage. In the 200 keV Ar2+ experiments, the sampl e was held at 100 K and a buried amorphous layer 55 nm thick formed un derneath a defective crystalline layer 90 nm thick after exposure to a fluence of 2 x 10(15) ions cm(-2). More detailed experiments with 400 keV Xe2+ employed incremental ion irradiation followed by RBS/C to de termine the extent and rate of radiation damage at temperatures of 170 , 300 and 470 K. These irradiations show that there is a strong temper ature dependence for damage accumulation and that critical amorphizati on fluences increase from 2 x 10(15) Xe2+ cm(-2) (170 K) to 6 x 10(15) Xe2+ cm(-2) (300 K) to greater than 2.5 x 10(16) 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 in crease in and, eventually, saturation in the damage rare. At 170 and 3 00 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 i n other ilmenite-group oxides. In particular, these materials transfor m to either the lithium niobate or the orthorhombic perovskite structu re at high pressures and temperatures. The results presented in this s tudy and similar investigations on the olivine system suggest that ion icity, composition and melting temperature may play important roles in the radiation response of ceramics, and particularly in predicting th e relative radiation tolerance of materials within a solid-solution se ries.