Published in:
Zeitschrift für Kristallographie - Crystalline Materials, 232 (2017) 1-3, Seite 25-38
Language:
English
DOI:
10.1515/zkri-2016-1965
ISSN:
2196-7105;
2194-4946
Origination:
Footnote:
Description:
<jats:title>Abstract</jats:title>
<jats:p>Radiation damage in minerals is caused by the α-decay of incorporated radionuclides, such as U and Th and their decay products. The effect of thermal annealing (400–1000 K) on radiation-damaged pyrochlores has been investigated by Raman scattering, X-ray powder diffraction (XRD), and combined differential scanning calorimetry/thermogravimetry (DSC/TG). The analysis of three natural radiation-damaged pyrochlore samples from Miass/Russia [6.4 wt% Th, 23.1·10<jats:sup>18</jats:sup> α-decay events per gram (dpg)], Panda Hill/Tanzania (1.6 wt% Th, 1.6·10<jats:sup>18</jats:sup> dpg), and Blue River/Canada (10.5 wt% U, 115.4·10<jats:sup>18</jats:sup> dpg), are compared with a crystalline reference pyrochlore from Schelingen (Germany). The type of structural recovery depends on the initial degree of radiation damage (Panda Hill 28%, Blue River 85% and Miass 100% according to XRD), as the recrystallization temperature increases with increasing degree of amorphization. Raman spectra indicate reordering on the local scale during annealing-induced recrystallization. As Raman modes around 800 cm<jats:sup>−1</jats:sup> are sensitive to radiation damage (M. T. Vandenborre, E. Husson, Comparison of the force field in various pyrochlore families. I. The A<jats:sub>2</jats:sub>B<jats:sub>2</jats:sub>O<jats:sub>7</jats:sub> oxides. <jats:italic>J. Solid State Chem.</jats:italic>
<jats:bold>1983</jats:bold>, <jats:italic>50</jats:italic>, 362, S. Moll, G. Sattonnay, L. Thomé, J. Jagielski, C. Decorse, P. Simon, I. Monnet, W. J. Weber, Irradiation damage in Gd<jats:sub>2</jats:sub>Ti<jats:sub>2</jats:sub>O<jats:sub>7</jats:sub> single crystals: Ballistic versus ionization processes. <jats:italic>Phys. Rev.</jats:italic>
<jats:bold>2011</jats:bold>, <jats:italic>84</jats:italic>, 64115.), the degree of local order was deduced from the ratio of the integrated intensities of the sum of the Raman bands between 605 and 680 cm<jats:sup>−1</jats:sup> divided by the sum of the integrated intensities of the bands between 810 and 860 cm<jats:sup>−1</jats:sup>. The most radiation damaged pyrochlore (Miass) shows an abrupt recovery of both, its short- (Raman) and long-range order (X-ray) between 800 and 850 K, while the weakly damaged pyrochlore (Panda Hill) begins to recover at considerably lower temperatures (near 500 K), extending over a temperature range of ca. 300 K, up to 800 K (Raman). The pyrochlore from Blue River shows in its initial state an amorphous X-ray diffraction pattern superimposed by weak Bragg-maxima that indicates the existence of ordered regions in a damaged matrix. In contrast to the other studied pyrochlores, Raman spectra of the Blue River sample show the appearance of local modes above 560 K between 700 and 800 cm<jats:sup>−1</jats:sup> resulting from its high content of U and Ta impurities. DSC measurements confirmed the observed structural recovery upon annealing. While the annealing-induced ordering of Panda Hill begins at a lower temperature (ca. 500 K) the recovery of the highly-damaged pyrochlore from Miass occurs at 800 K. The Blue-River pyrochlore shows a multi-step recovery which is similarly seen by XRD. Thermogravimetry showed a continuous mass loss on heating for all radiation-damaged pyrochlores (Panda Hill ca. 1%, Blue River ca. 1.5%, Miass ca. 2.9%).</jats:p>