Title: Annealing of alpha-decay damage in zircon: a Raman spectroscopic study
Abstract: Recrystallization and structural recovery in -decay damage in zircon samples have been studied using Raman spectroscopy. Fifteen zircon samples with different degrees of radiation damage have been thermally annealed between 600 K and 1800 K for up to 28 days and 8 hours. The experimental results from this study reveal that recrystallization in the damaged zircon samples is a multi-stage process that depends on the degree of initial damage of the samples. In partially damaged samples the lattice recovery of damaged crystalline ZrSiO4takes place at temperatures as low as about 700 K, as shown by a remarkable band-sharpening and a significant increase in the frequencies of 1and 3Si-O stretching vibrations together with the external band near 357 cm-1with increasing temperature. A dramatic increase of Raman scattering intensity of ZrSiO4occurs in partially damaged samples near 1000 K due to a recrystallization process involving epitaxial growth. Heavily damaged samples tend to decompose into ZrO2and SiO2at high temperatures. Tetragonal ZrO2has been observed under annealing between 1125 K and about 1600 K in heavily damaged samples while monoclinic ZrO2appears above 1600 K. Weak signals from ZrSiO4were detected at 1125 K in highly metamict zircon although the main recrystallization appears to occur near 1500 K accompanied by a decrease of the volumes of ZrO2as well as SiO2 . This suggests that this recrystallization is associated with the reaction of ZrO2with SiO2to form ZrSiO4 .