IRRADIATION DAMAGE IN MONAZITE-(Ce): AN EXAMPLE TO ESTABLISH THE LIMITS OF RAMAN CONFOCALITY AND DEPTH RESOLUTION

Crystals of synthetic monazite-(Ce), and foils of the same material 1 mm thick, were irradiated with 1, 3.5, and 7 MeV Au ions, with total fluences in the range 0.6-51 x 10(13) ions/cm(2). The triple irradiation resulted in surficial structural damage extending up to similar to 1.5 mu m into the samples. The irradiation damage generated was studied using Raman microspectroscopy. At low doses of irradiation, internal stretching bands of the PO4 groups show significant broadening (reflecting a decrease in short-range order and increase in strain in the remnant crystalline volume-fraction of the radiation-damaged solid), which is accompanied by notable losses in intensity and shifts of bands toward lower wavenumbers. At the highest dose of irradiation, the material has become amorphous, and the crystalline PO4 Raman modes disappeared. These observations, however, were only made in the case of the thin lamella prepared using a focused ion-beam system. The Raman spectra of the analogously irradiated, unprepared crystals are dominated by bands of apparently undamaged, crystalline CePO4. We assign the latter to the host crystal underneath the surficial, damaged region, which contributes strongly to the spectrum obtained because of the insufficient depth-resolution of the confocal spectrometer. Limits in confocality, and the related potential misinterpretation of spectroscopic results, must be considered.