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Abstract: The local structure and chemical speciation of the mixed valence, fluorite-based oxides UO2+ x (0.00⩽x⩽0.20) and PuO2+ x /PuO2+ x−y (OH)2 y ·zH2O have been determined by U/Pu LIII XAFS spectroscopy. The U spectra indicate (1) that the O atoms are incorporated as oxo groups at short (1.75Å) U–O distances consistent with U(VI) concomitant with a large range of U displacements that reduce the apparent number of U neighbors and (2) that the UO2 fraction remains intact implying that these O defects interact to form clusters and give the heterogeneous structure consistent with the diffraction patterns. The PuO2+ x system, which does not show a separate phase at its endpoint, also displays (1) oxo groups at longer 1.9Å distances consistent with Pu(V+δ), (2) a multisite Pu–O distribution even when is close to zero indicative of the formation of stable species with H2O and its hydrolysis products with O2−, and (3) a highly disordered, spectroscopically invisible Pu–Pu component. The structure and bonding in AnO2+ x are therefore more complicated than have previously been assumed and show both similarities but also distinct differences among the different elements. [Copyright &y& Elsevier]

Pu L[sub 3] XAFS measurements show that the excess oxygen in single phase PuO[sub 2+x] occurs as oxo groups with Pu-O distances of 1.831.91 Å. This distance and the energy of the edge (via comparison with a large number of related compounds) are more consistent with a Pu(IV/V) than a Pu(IV/VI) mixture. Analogous to Pu(IV) colloids, although the Pu-Pu pair distribution remains single site even when it shows substantial disorder, the Pu-O distribution can display a number of additional shells at specific distances up to 3.4 Å even in high fired materials when no oxo groups are present, implying intrinsic H[sup +]/OH[sup -](/H[sub 2]O). The number of oxo atoms increases when samples are equilibrated with humid air at ambient temperature, indicating that the Pu reactivity in this solid system differs notably from that of isolated complexes and demonstrating the importance of nanoscale cooperative phenomena and total free energy in determining its chemical properties. [ABSTRACT FROM AUTHOR]

Abstract: Diffuse reflectance spectra at room temperature have been collected on polycrystalline perovskite (CaTiO3), zirconolite (CaZrTi2O7) and thorutite (ThTi2O6) samples doped with Np. The Np species give rise to a number of broad, unresolved intraconfigurational f–f electronic transition bands in the near-infrared and visible spectral regions. Both Np3+ and Np4+ can substitute in the Ca site of perovskite by the appropriate choice of charge compensation and sintering atmosphere. No detectable spectral differences were observed when Np4+ was targeted towards either the Ca or Zr sites in zirconolite. Np4+ but not Np5+ can be incorporated in the Th site of thorutite by sintering in air. Even in the most dilute [≤0.03 formula units (f.u.)] Np4+ samples of the different titanates studied, there was only very approximate agreement with the Kubelka–Munk law. [Copyright &y& Elsevier]

Reports on the Pu L[sub3] x-ray near edge absorption spectra for Plutonium. Coverage for chalcogenides, chlorides, hydrates, hydroxides, nitrates, carbonates and other compounds as solids and in solution; Substitution in zirconolite, perovskite and borosilicate glass; Extension of the correlations database between the energy and shape of the spectra from the XANES with valence and site symmetry to higher order chemical effects.

Trivalent Pu can be incorporated in the silicate apatite structure to form Ca[sub 2]Pu[sub 8](SiO[sub 4])[sub 6]O[sub 2] by sintering under reducing conditions, while the incorporation of tetravalent Pu in the Ca/rare earth sites in oxidizing or neutral conditions is limited to only 0.6 formula units (f.u.). The d-spacings and intensities of the X-ray pattern of hexagonally structured Ca[sub 2]Pus(SiO[sub 4])[sub 6]O[sub 2] after firing at 1250°C are given, and the a and c lattice parameters are 0.9561[sub 1] and 0.7028[sub 1] nm, respectively. The respective solid solubility limits of U and Hf in Ca[sub 2]Gd[sub 8-x](U/Hf)[sub x](SiO[sub 4])[sub 6]O[sub 2] apatite samples were 0.3 and 0.2 f.u. [ABSTRACT FROM AUTHOR]

Highlights: [•] The incorporation of neptunium (Np) and plutonium (Pu) in thorutite (ThTi2O6) has been studied. [•] The effect of Np/Pu doping on the unit cell parameter changes has been discussed from the structure point of view. [•] The effect of Y as charge compensator to encourage the formation of higher valences of Np and Pu has been explored. [•] The resulting Np/Pu doped thorutite samples have been characterised by using XRD, SEM and DRS. [ABSTRACT FROM AUTHOR]

Studies the behavior of uranium (U) in zirconolite, CaZrTi2O7, which is an important constituent of synroc. Study of U in a range of atmospheres; Related pyrochlore-structured titanate phase; Processing atmosphere of solid materials to immobilize multivalent waste ions such as actinides.