Miscibility Gap in the U–Nd–O Phase Diagram: a New Approach of Nuclear Oxides in the Environment?

To some extent, rare-earth-doped UO₂ is representative of an irradiated nuclear fuel. The two phases we observed previously in neodymium-doped UO₂ are now interpreted as the existence of a miscibility gap in the U–Nd–O phase diagram using new results obtained with Raman spectroscopy. Extrapolating the miscibility gap in the U–Nd–O phase diagram to irradiated UO₂ opens the path to a new understanding of nuclear oxides in the environment

Room-Temperature Synthesis, Hydrothermal Recrystallization, and Properties of Metastable Stoichiometric FeSe

Room-temperature precipitation from aqueous solutions yields the hitherto unknown metastable stoichiometric iron selenide (ms-FeSe) with tetragonal anti-PbO type structure. Samples with improved crystallinity are obtained by diffusion-controlled precipitation or hydrothermal recrystallization. The relations of ms-FeSe to superconducting β-FeSe_1–<i>x</i> and other neighbor phases of the iron–selenium system are established by high-temperature X-ray diffraction, DSC/TG/MS (differential scanning calorimetry/thermogravimetry/mass spectroscopy), ⁵⁷Fe Mössbauer spectroscopy, magnetization measurements, and transmission electron microscopy. Above 300 °C, ms-FeSe decomposes irreversibly to β-FeSe_1–<i>x</i> and Fe₇Se₈. The structural parameters of ms-FeSe (<i>P</i>4/<i>nmm</i>, <i>a</i> = 377.90(1) pm, <i>c</i> = 551.11(3) pm, <i>Z</i> = 2), obtained by Rietveld refinement, differ significantly from literature data for β-FeSe_1–<i>x</i>. The Mössbauer spectrum rules out interstitial iron atoms or additional phases. Magnetization data suggest canted antiferromagnetism below <i>T</i>_N = 50 K. Stoichiometric non-superconducting ms-FeSe can be regarded as the true “parent” compound for the “11” iron-chalcogenide superconductors and may serve as starting point for new chemical modifications