Alpha-decay damage in titanite

Abstract

Titanite can incorporate minor amounts of radioactive impurity components (particularly U and Th) that affect the crystal structure by ɑ- and β-decay events; in particular, ɑ-decay gives rise to significant knock-on structural damage by causing atomic displacements. We have examined a series of chemically and structurally well-characterized titanite samples by a variety of techniques to follow the progress of the metamictization process at low radiation doses. Ten titanite samples were characterized by electron microprobe analysis, powder X-ray diffraction, and powder infrared (IR) spectroscopy. The X-ray diffraction patterns vary from sharp and well resolved to almost totally degraded, reflecting the various degrees of ɑ-decay damage. The powder IR spectra show a similar variation, and the order of increasing pattern degradation is almost the same as that for the X-ray diffraction patterns, indicating that both features reflect the same physical property of the material. However, the annealing behavior is different: powder X-ray diffraction patterns become sharp and well resolved; powder IR spectra sharpen slightly, but do not recover to anywhere near the same extent. Four representative samples were selected for further work. The crystal structures were refined using MoKɑ X-ray single-crystal diffraction data. The crystals were then annealed at 1090 °C under Ar and the intensity data were again measured. For small degrees of ɑ-decay damage, the structure seems to be completely restored on annealing; this is not the case for titanite with the largest amount of ɑ-decay damage. Polarized single-crystal IR spectra of undamaged titanite show a single sharp (OH) stretching band at ~3490 cm^(-1) with a little fine structure reflecting local cation disorder around the OH. With increasing ɑ-decay damage, the sharpness of the absorption band decreases and a wide wing appears on the low-energy side of the sharp (OH) band. Mössbauer spectroscopy shows only Fe^(3+) to be present in undamaged titanite; as ɑ-decay damage increases, the amount of Fe^(2+) increases, suggesting that radioactive decay causes reduction as well as atomic displacement. Fe^(2+) is easily oxidized on heating. The ^(29)Si MAS-NMR peak width is strongly correlated with increasing radiation damage and increasing Fe content, and no signal was observed from the most damaged titanite. With increasing ɑ-decay damage, single-crystal electron diffraction patterns develop diffuse halos indicative of a mean atomic spacing of 3.6 Å. In bright field, undamaged material shows continuous lattice fringes. Small amounts of damage are characterized by mottled diffraction contrast superimposed on largely continuous lattice fringes. The most damaged titanite shows mottled diffraction contrast with coexisting crystalline and aperiodic domains produced by overlapping ɑ-recoil tracks, corresponding to damage on the order of 30-50% of that required to render the structure fully aperiodic. Ti XANES spectra show intensification of the principal pre-edge feature (ls → 3d transition) with increasing damage, indicative of increasing local asymmetry around the Ti position. Loss of resolution in the EXAFS spectra also indicates increasing disorder around Ti with increasing damage. There is no sign of any ^([4])Ti in even the most damaged samples, although it was detected in a glass of titanite composition. The metamictization process begins by the formation of isolated ɑ-recoil and ɑ-particle tracks. With increasing dose, the ɑ-tracks begin to overlap, producing aperiodic domains; in the most damaged titanite examined, there were approximately equal amounts of coexisting crystalline and aperiodic material. At this stage, the crystalline domains still retain their original orientation, except where affected by low-temperature annealing. As undamaged titanite does not (usually) contain significant Fe^(2+), it seems that the damage process is accompanied by reduction of Fe^(3+) → Fe^(2+), which resides in the aperiodic domains. These domains incorporate much more hydrogen (as OH) than is contained in crystalline titanite, presumably a result of postdamage diffusion of H into the structure. All information is consistent with the Ewing model for metamict materials, an aperiodic random network structure; HRTEM images show patterns of random contrast consistent with the random network model, with no evidence to support any microcrystalline model of the metamict state. High-temperature annealing only partly restores the structure, the apparent degree of recovery being dependent on the coherence length of the experimental technique used to characterize the material. The degree of recovery is also dependent on the amount and pattern of damage. We suggest that the original structure is recovered when the ratio of surface area to volume for the damaged material is high, and the interface can exert a strong memory effect on the amorphous material; when large equant aperiodic domains form, they are annealed to a more defect-free and relatively stable aperiodic network structure.