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.