The synthesis, structure and thermal stability of the periodate double perovskites A2NaIO6 (A= Ba, Sr, Ca) was investigated in the context of potential application for the immobilisation of radioiodine. Ab initio, thorough structure determinations are presented, revising the previously accepted space groups using as yet unreported neutron diffraction and DFT simulation characterisation alongside X-ray diffraction and Raman spectroscopy.The materials were found to exhibit rock-salt ordering of Na and I on the perovskite B-site; Ba2NaIO6 was found to adopt the Fm-3m aristotype structure, whereas Sr2NaIO6 and Ca2NaIO6 adopt the P21/n hettotype, characterised by co-operative octahedral tilting. DFT simulations determined the Fm-3m and P21/n structures of Ba2NaIO6 to be energetically degenerate at room temperature, whereas diffraction and spectroscopy data evidence only the presence of the Fm-3m phase at room temperature, which may imply an incipient phase transition for this compound. The periodate double perovskites were found to exhibit remarkable thermal stability, with Ba2NaIO6 only decomposing above 1050 °C in air, which is apparently the highest recorded decomposition temperature so far recorded for any iodine bearing compound.

A range of stoichiometric and mixed A-site cation brannerite glass-ceramics have been synthesised and characterised. The formation of UTi2O6 in glass is reliant on ensuring all uranium remains tetravalent by processing in an inert atmosphere. ThTi2O6 forms in glass under both inert and oxidising atmospheres due to the lack of other easily available oxidation states. CeTi2O6 could not be made to form within this glass system. The formation of A0.5B0.5Ti2O6 phases depends strongly on the oxidation states of the A and B cations available in the process atmosphere, with the most successful compositions having an average final oxidation state of (A,B)4+. Mixed cation brannerite compositions that formed in argon include U0.75Th0.25Ti2O6 and U0.71Ce0.29Ti2O6. Those forming in air include U0.23Th0.77Ti2O6, Th0.37Ce0.63Ti2O6, and U0.41Ce0.59Ti2O6.

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Detailed chemical analysis of the solidified molten fuel still residing in the stricken Chernobyl reactor unit 4 are inferred using multi-modal micro-focus x-ray analysis of a low-radioactivity proxy. A fascinating...

Aliquots of well-characterized Ce-brannerite were annealed at different temperatures under N 2 and synthetic air atmospheres. The autoreduction of cerium at temperature was observed using thermogravimetry to monitor the mass lost as O 2 was evolved. It has been shown that the brannerite structure is stable with a small fraction of Ce 3+ , charge-balanced by O vacancies. The range of stability was determined to be Ce 4+ 0.975 Ti 2 O 5.95 , the fully oxidized end-member, to Ce 3.87+ 0.975 Ti 2 O 5.886 , as reduced by annealing under N 2 at 1075 °C. Higher temperatures under N 2 led to further reduction of Ce and collapse of the brannerite structure. Cebrannerite remained stable on heating to 1300 °C in synthetic air, with multiple steps of oxidation and reduction corresponding to changes in the average Ce oxidation state. We propose that the autoreduction of Ce at temperature is an important factor in the overall thermodynamic stability of Ce-brannerite at temperature and has a large impact on the energetics of formation of Ce-brannerite.