Diffuse reflectance spectroscopy of Pu ions in zirconolite and perovskite
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Diffuse reflectance spectroscopy of Pu ions in zirconolite and perovskite E. R. Vance, K. S. Finnie, Y. Zhang and B. D. Begg, Materials and Engineering Science, ANSTO, Menai, NSW 2234, Australia [email protected]
ABSTRACT Diffuse reflectance spectroscopy measurements at ambient temperatures have been made over the near infrared-visible range (5000-25000 cm-1) on polycrystalline ceramic zirconolite (CaZrTi2O7) and perovskite (CaTiO3) samples doped with Pu4+. The Pu concentrations were varied between 0.001 and 0.1 formula units. The Pu ions gave rise to a number of unresolved intraconfigurational f-f electronic absorption bands of a few hundred cm-1 bandwidth. Pu ions were targeted to substitute in the Ca sites as either trivalent or tetravalent species and as tetravalent species in the Zr site of zirconolite by the appropriate choice of charge compensation and firing atmosphere. There was approximate agreement of the Kubelka-Munk absorption intensities with Beer's Law for the different Pu4+ substitution schemes, apart from some "new" bands, attributed to impurities, observed in the most dilute zirconolite sample. No clear spectral differences were evident when Pu4+ was targeted to Ca or Zr sites in zirconolite. Samples prepared in reducing atmospheres with a view to producing Pu3+ were strongly absorbing, leading to suppression of Pu transitions.
INTRODUCTION Plutonium-bearing wastes have high priorities for geological disposal. Titanate-based ceramics have been shown to be appropriate for immobilizing surplus Pu [1]. For this purpose, the aqueous dissolution of zirconolite and pyrochlore-structured material have been studied under standard laboratory conditions [1,2], but information on the dependence of the aqueous dissolution rates on redox conditions and Pu valence in the solid phase is limited. Previous X-ray absorption spectroscopy, electron microscope and X-ray diffraction studies have shown that Pu3+ and Pu4+ can be substituted in the Ca sites of perovskite (CaTiO3) and zirconolite (CaZrTi2O7); a limited amount only of Pu3+ can be substituted in the Zr site of zirconolite [3]. Pu4+ forms in air-fired samples and it is necessary to use a reducing atmosphere to achieve the Pu3+ state. Perovskite is stable under reducing atmospheres above ~1400oC, but zirconolite is difficult to form as a single phase in such conditions [4], although it is stable in high-purity argon. Transmission optical spectroscopy at low temperatures can be used to study the electronic structure of Pu ions in solid host materials. However this necessitates the use of transparent single crystals. In the absence of such crystals we have used diffuse reflectance spectroscopy (DRS) over the 5000-25000 cm-1 range on perovskite and zirconolite samples containing 0.001 0.1 formula units (f.u.) of substitutional Pu to assist definition of Pu valences in these materials, with the eventual aim of the work being to try to understand Pu valence effects on aqueous dissolution. Electronic absorption is expected in this spectral region for both
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