Structure studies on lanthanide technetium pyrochlores as prospective host phases to immobilize 99technetium and fission lanthanides from effluents of reprocessed used nuclear fuels

We report here results of a systematic investigation regarding the incorporation of 99Tc into pyrochlore oxide structures, Ln2Tc2O7, where Ln represents trivalent lanthanide Ln3+ cations, while 99Tc is a tetravalent, Tc4+, metal cation. In this study, we used the following Ln cations: Pr, Nd, Sm, Gd and Lu. The goal in this preliminary study was to characterize and quantify the range of stability of the lanthanum technetium pyrochlore oxide phase. Powder X-ray diffraction (XRD) and Rietveld analysis was used to characterize the crystalline phase content, while scanning electron microscopy (SEM) was used to characterize the microstructure and homogeneity of the Ln-Tc pyrochlore specimens. All of the pyrochlore samples exhibited good crystallinity and their lattice parameters could be refined with remarkable accuracy. Low refinement residuals (RBragg) of 1.1-3.1% were achieved. The refined, cubic lattice parameters ranged from 1.0447156(83) nm for Pr2Tc2O7 to 1.013777(22) nm for Lu2Tc2O7, with a linear trend relative to the Ln3+ ionic radius. We also demonstrated here the successful synthesis of Nd2Tc2O7, using a simple, scalable synthesis route.