Structural modifications induced by AgI in silver aluminophosphate glasses and their consequence on thermal properties

Silver phosphate glass-forming systems are potential host matrices for the conditioning of radioactive iodine. Large amounts of iodine can be incorporated in these glasses without significant volatilisation and their phosphate network can be strengthened thanks to its crosslinking by aluminum ions. Three series of glasses, corresponding to Ag₂O/P₂O₅ molar ratios of 1, 1.66 and 2, with and without alumina (up to 3 mol percent), have been studied for different iodine contents (0 to 1.2 vol percent). The influence of AgI content was assessed by ³¹P and ²⁷Al NMR characterisations. In the series with Ag₂O/P₂O₅=2, the formation of a glassy fraction is observed for an iodine content beyond 0.4 vol percent whereas these materials were entirely crystallised without AgI. For this glassy fraction, we confirm that AgI is inserted in the aluminophosphate glassy network. Progressive insertion of AgI in glassy compositions eventually induces a crystallisation phenomenon at the highest proportions corresponding to an exclusion of alumina from the network and the formation of aluminophosphate phases such as Al(PO₃)₃ or AlPO₄. This phenomenon involves structural and thermal modifications, which enable to propose an optimum iodine content (around 0.6 vol percent) that can be inserted under the form of AgI in these glasses for iodine conditioning. (authors)