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The thermal stability of the synthetic iodine analogue of vanadinite, Pb5(VO4)3I, in air has been investigated by in situ high temperature X-ray powder diffraction between 300 and 1070 K. Rietveld refinement of phase fractions shows that breakdown to lead orthovanadate, Pb3(VO4)2, begins at temperatures above 540 K, with complete loss of iodine above 680 K. More than 50 K below the onset of breakdown, the unit-cell parameters of Pb5(VO4)3I show anomalous contraction in the crystallographic x–y plane (reduction of a) which we associate with movement of iodine within the [0001] channels of the structure. The implications of these results for immobilization of 129I in potential apatite-related crystalline radioactive waste forms are discussed.

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h i g h l i g h t sDense iodide sodalite prepared by HIP of hydrothermally synthesised powders. Sodalite was free from leachable secondary phases. Leach tests indicate self-arresting congruent dissolution. a b s t r a c tAn iodide sodalite wasteform has been prepared by Hot Isostatic Pressing of powder produced by hydrothermal synthesis. The wasteform was free of leachable secondary phases which can mask leaching mechanisms. Leaching is by congruent dissolution and leach rates decrease as Si and Al accumulate in the leachate. Differential normalised leach rates are 0.005-0.01 g m À2 d À1 during the 7-14 day period. This indicates that sodalite dissolution in natural groundwater, already saturated in these elements, will be very low. Crown

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