29Si\mathrm{^{29}Si} MAS NMR spectroscopy and synchrotron XRD study of metamict Cardiff titanite

Metamict titanite from the Cardiff uranium mine(M28696) in Ontario, Canada, has been analyzed using29Si magic angle spinning nuclear magnetic resonancespectroscopy (MAS NMR). A broad Gaussian shaped NMRsignal at –81 ppm occurs at room temperature resultingfrom the mainly locally ordered metamict structural state.NMR signals were obtained at room temperature andafter annealing at 600, 950, 1220 and 1470 K. Because ofincreasing crystallinity the full width at half maximum(FWHM) decreased from 24 ppm to 20 ppm, respectivelyusing a pseudo-Voigt fit. For comparison highly-crystallinetitanite from Rauris showed an NMR signal at –79.3 ppmwith FWHM of 4.1 ppm and an almost Lorentzian profilebecause of its good long range order. Integrating synchrotronX-ray diffraction (XRD) signals of Cardiff titaniteshow an increase of the crystallographic long range orderat annealing temperatures considerably lower than thelocal ordering seen by NMR

Mechanical and structural properties of radiation-damaged allanite-(Ce) and the effects of thermal annealing

The onset of thermally induced, heterogeneous structural reorganization of highly radiation-damaged allanite-(Ce) begins at temperatures below 700 K. Three strongly disordered allanite samples (S74 20414: ~ 0.55 wt% ThO2, 22.1 wt% REE oxides, and maximum radiation dose 3.5 × 1018 α-decay/g; LB-1: ~1.18 wt% ThO2, 19.4 wt% REE oxides, and maximum radiation dose 2.0 × 1019 α-decay/g; R1: ~ 1.6 wt% ThO2, 19.7 wt% REE oxides, and maximum radiation dose 2.6 × 1018 α-decay/g) were step-wise annealed to 1000 K in air. Using orientation-dependent nanoindentation, synchrotron single-crystal X-ray diffraction (synchrotron XRD), X-ray powder diffraction (powder XRD), differential scanning calorimetry and thermogravimetric analysis (DSC/TG), mass spectrometry (MS), 57Fe Mössbauer spectroscopy and high-resolution transmission electron microscopy (HRTEM), a comprehensive understanding of the structural processes involved in the annealing was obtained. As a result of the overall increasing structural order, a general increase of hardness (pristine samples: 8.2–9.3 GPa, after annealing at 1000 K: 10.2–12 GPa) and elastic modulus (pristine samples: 115–127 GPa, after annealing at 1000 K: 126–137 GPa) occurred. The initially heterogeneous recrystallization process is accompanied by oxidation of iron, the related loss of hydrogen and induced stress fields in the bulk material, which cause internal and surface cracking after stepwise annealing from 800 to 1000 K. HRTEM imaging of the pristine material shows preserved nanometer-sized crystalline domains embedded in the amorphous matrix, despite the high degree of structural damage. The results show that hardness and elastic modulus are sensitive indicators for the structural reorganization proces