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 18 α-decay events per gram (dpg)], Panda Hill/Tanzania (1.6 wt% Th, 1.6 · 10 18 dpg), and Blue River/Canada (10.5 wt% U, 115.4 · 10 18 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 −1 are sensitive to radiation damage . 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 −1 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 %).
The structural changes associated with the R-3m to C2/c ferroelastic phase transition in the solid solution Pb3(P1–x As x O4)2 have been followed by Raman spectroscopy on samples with x = 0.00, 0.08, 0.37, 0.57, 0.65, 0.80, 1.00 over the temperature range 80–850 K. The high-temperature structural state of four intermediate samples with x = 0.37, 0.57, 0.65 and 0.80 was also studied by synchrotron single-crystal X-ray diffraction. The new results, in combination with previous data, reveal that Pb3(P1–x As x O4)2 undergoes a complex sequence of transformation processes with several characteristic temperatures that can be clearly identified in addition to that of the ferroelastic phase transition at T c. These structural changes are mainly driven by two phonon modes: vibrations of the Pb2 atoms (stereochemically active in the C2/c phase perpendicular to the layers characteristic of the palmierite-type structure), and rotational modes of the PO4 and AsO4 units within the plane of the layers. The former ensures coupling of the atomic displacements both within and between the layers, while the latter favours the coupling of the monoclinic species within the layers. With decreasing temperature, at T 4 ≫ T c the Pb2 atoms are displaced from their special positions on the triad axis of the R-3m structure. At T 3 the size of the nanoclusters composed of coupled displacements of Pb2 atoms on the monoclinic pattern is sufficient to produce both macroscopic effects in all compositions (diffuse super-lattice intensities in the diffraction patterns, non-symmetry-breaking strain in the unit-cell parameters, a step in the C p curve) and microscopic effects in the broadening of the vibrational modes of the Pb2 atoms. The temperature interval between T 3 and T 4 is considerably smaller for the intermediate compositions than for the end-members indicating that the chemical disorder on the tetrahedral site facilitates the condensation of monoclinic species, probably by smearing the potential barrier between different monoclinic orientational states. At T 2 < T 3, but still above T c, the AsO4 tetrahedra undergo monoclinic deformation in order to better coordinate the adjacent displaced Pb2 atoms, while the geometry of the PO4 tetrahedra remains unchanged down to T c (and for As-rich compositions, slightly below T c). At T c the ordered Pb2 displacements and rotated tetrahedra drive a displacement of the Pb1 atoms from their trigonal (paraelastic) positions and long-range-ordered ferroelastic domains are formed. However, for all compounds, residual disorder of the Pb2 displacements persists below T c to a further characteristic temperature T 1 at which the displacements of all types of atoms become completely ordered on all length scales.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.