A Raman spectroscopic study of high-uranium zircon from the Chernobyl "lava"

We have studied technogenic, high-uranium zircon, which crystallised from melt formed during the accident at the Chernobyl Nuclear Power Plant in 1986, by confocal Raman spectroscopy, electron microprobe, and backscattered electron imaging. The correlation between Raman and electron microprobe measurements allowed studying mode behaviour as a function of U content of the strongly zoned zircon crystals. The USiO4 content in the crystals ranges between 0.6 and 11.6 mol. %, corresponding to 0.8 and 15.8 wt. % UO2 respectively. The frequency of the nu(1)(SiO4) symmetrical and nu(1)(SiO4) anti-symmetrical stretching mode decreases by 0.67(3) and 0.75(3) cm(-1) per mol. % USiO4, respectively, which is a result of an increasing Si-O bond length with increasing U content. The lattice modes show a comparable shift to lower frequencies, whereas the internal nu(2)(SiO4) and nu(4)(SiO4) bending modes exhibit no or only a small shift to lower frequencies (< 0.12 cm(-1) per mol. % U). Only the frequency of the lowest energy band (E-g) near 202 cm-1 increases slightly with increasing U content, which is unexpected and indicates that the cation-(SiO4)(4-) potentials, the electron orbitals, and/or the cation radius have a strong effect on this mode. The line broadening, reflecting the range of local distortions (i.e., microscopic strain), is most pronounced for the lattice modes, in agreement with the large size difference of both cations. We found that the E-g lattice modes, involving the movement of the SiO4 tetrahedron and the cation within the a(b) plane, show significantly larger line broadening with increasing U concentration than the B g modes, involving lattice vibrations along the c axis. This suggests that the microscopic strain is significant larger in the a(b) plane than along the c axis, which can be explained by the structural properties of zircon.