Chemical ordering as a precursor to formation of orderedδ-UZr2phase: a theoretical and experimental study.

A combination of special quasi-random structure (SQS) analysis, density functional theory (DFT) based simulations and experimental techniques are employed in determining the transformation pathway for the disorderedγ-(U, Zr) phase (bcc structure) to transform into the chemically orderedδ-UZr2phase (C32, AlB2type structure). A novel Monte-Carlo based strategy is developed to generate SQS structures to study theß→ωdisplacive phase transformation in A1-xBxbinary random alloy. Structures generated with this strategy and using DFT calculations, it is determined that (222)bccplane collapse mechanism is energetically unfavorable in chemically disordered environment at UZr2composition. A mechanically and dynamically stable 24 atom SQS structure is derived which serves as a structural model of chemically orderedδ-UZr2structure. Finally, a thermodynamic basis for the mechanism of theγtoδtransformation has been established which ensures chemical ordering is a precursor to the subsequent displacive transformation to form chemically orderedδ-UZr2structure.

Characterization of Sb-doped Bi(2)UO(6) solid solutions by X-ray diffraction and X-ray absorption spectroscopy.

The preparation and characterization of Sb-doped Bi(2)UO(6) solid solutions, in a limited composition range, is reported for the first time. The solid solutions were prepared by solid-state reactions of Bi(2)O(3), Sb(2)O(3) and U(3)O(8) in the required stoichiometry. The reaction products were characterized by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) measurements at the Bi and U L(3) edges. The XRD patterns indicate the precipitation of additional phases in the samples when Sb doping exceeds 4 at%. The chemical shifts of the Bi absorption edges in the samples, determined from the XANES spectra, show a systematic variation only up to 4 at% of Sb doping and support the results of XRD measurements. These observations are further supported by the local structure parameters obtained by analysis of the EXAFS spectra. The local structure of U is found to remain unchanged upon Sb doping indicating that Sb(+3) ions replace Bi(+3) during the doping of Bi(2)UO(6) by Sb.