DISPLACIVE RADIATION EFFECTS IN THE MONAZITE-STRUCTURE AND ZIRCON-STRUCTURE ORTHOPHOSPHATES

Monazite-structure orthophosphates, including LaPO4, PrpO(4), NdPO4, S mPO4, EuPo4, GdPO4, and natural monazite, and their zircon-structure a nalogs, including ScPO4, YPO4, TbPO4, TmPO4, YbPO4, and LuPO4, were ir radiated by 800 keV Kr2+ ions in the temperature range of 20 to 600 K. The critical amorphization dose was determined in situ as a function of temperature using selected-area electron diffraction. Amorphization doses were in the range of 10(14) to 10(16) ions/cm(2), depending on the temperature. Materials with the zircon structure were amorphized a t higher temperatures than those with the monazite structure. The crit ical amorphization temperature ranged from 350 to 485 K for orthophosp hates with the monazite structure. The critical amorphization temperat ure ranged from 350 to 485 K for orthophosphates with the monazite str ucture and from 480 to 580 K for those with the zircon structure. Howe ver, natural zircon (ZrSiO4) can be amorphized at over 1000 K. Within each structure type, the critical temperature of amorphization increas ed with the atomic number of the lanthanide cation. Structural topolog y models are consistent with the observed differences between the two structure types, but not predict the relative amorphization doses for different compositions. The ratio of electronic-to-nuclear stopping co rrelates well with the observed of susceptibility to amorphization wit hin each structure type, consistent with previous results that electro nic-energy losses enhance defect recombination in the orthophosphates.