Triclinic-cubic phase transition in actinide diphosphates AnP2O7 (An=Th, U, Np, Pu). Structural investigation by synchrotron radiation and solid-state NMR

Actinide-bearing phosphate compounds can be encountered in mineralogy, biochemistry or issued from anthropogenic activities like nuclear fuel reprocessing. They have also been envisioned and investigated as potential hosts for actinide waste disposal. In the present studies we focussed on the diphosphate family of general formula AnP2O7 (An=actinide). The structural behaviour of four actinide-bearing diphosphates has been investigated (An=Th, U, Np, Pu). We found that their room temperature structure is not cubic as previously reported [1] but triclinic (distorted cubic) and transforms into a cubic form around 150°C. The phase transition has been characterized by various experimental techniques and the room temperature structure determined from synchrotron radiation data. It was also found that all four compounds exhibit negative thermal expansion above 673K.JRC.E.3-Materials researc