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Tracing the extent of radiation damage in monazite combining XRD and Raman spectroscopy

Monazite is a relatively common accessory mineral which is used for age dating owing to substantial amounts of U and Th in its crystal structure. Although it sometimes contains a significant amount of radioactive elements, it never occurs naturally metamict. This is attributed to its crystal structure properties, especially to the P-O bonds which are shorter and stronger than Si-O bonds in zircon, and thus more resistant to radiation damage. Also, it has been shown that the critical temperature of amorphization ranges between 150 and 200°C, thus promoting the healing of the radiation damage in the structure. The monazite-(Ce) from Eptevann, Norway, contains more than 10% of radionuclides on oxide basis, mostly as ThO2, and does not exhibit properties of a metamict mineral. X-ray data for the thermally untreated mineral show characteristic monazite diffraction lines. The unit cell parameters are as follows: a = 6.473(1) Å , b = 6.986(1) Å , c = 8.178(1) Å ,  = 126.511(8)°. However, diffraction data differ slightly after annealing at 200, 500 and 1000°C: unit cell parameters decrease, the intensity of diffraction maxima increases whereas their width decreases as the temperature rises. Crystallite size and strain analyses show a decrease in crystal strain and increase in crystallite size. Thus, thermally untreated monazite has crystal size of  300 nm which increases to  1000 nm for monazite heated at 1000°C. Nevertheless, there is no significant change in both size and strain when the mineral is heated up to 200, i.e. in the range of critical amorphization temperature. The increase in heating temperature causes an increase of intensity for the vibration band around 970 cm-1 (symmetrical stretching) observed by Raman spectroscopy, while the band width decreases with the increase of annealing temperature. The changes in this internal mode indicate the shifting of slightly displaced atoms in PO4 tetrahedra and annealing of possibly present Frenkel’ s defects with the increase of temperature. Although, in general, the mineral does not appear metamict, all changes observed during heating experiments confirm the presence of the radiation damage in the structure of monazite, as suggested in previous studies. The changes in atom distances during healing of the radiation-damaged monazite are to be compared with the structural data observed for the monazite samples which do not contain radionuclides. These minor changes of structural parameters are expected to influence the shift of the Raman band position for the observed stretching mode, its intensity and width during heating experiments.

monazite ; radiation damage ; XRD ; Raman spectroscopy

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