Résumé
β-Thorium phosphate diphosphate (
β-TPD), considered as a very promising radwaste storage material, was obtained from thorium phosphate hydrogenphosphate hydrate (TPHPH) precursor through dehydration and hydrogen phosphate condensation. The structures of TPHPH, intermediate
α-thorium phosphate diphosphate (
α-TPD) and its hydrate (
α-TPDH) have been resolved ab initio by Rietveld analysis of their synchrotron diffraction patterns. All were found orthorhombic (space group
Cmcm) and similarly composed of [ThPO
4]
4
4+ slabs alternating with disordered layers hosting either [HPO
4·H
2O]
2
4− (TPHPH), [P
2O
7·2H
2O]
4− (
α-TPDH), or [P
2O
7]
4− (
α-TPD), unlike the 3D structure of
β-TPD. The diphosphate groups of
α-TPD and
α-TPDH are strongly bent. The irreversible transition to the final
β-TPD consists in a shearing of the slabs and a reduction of the interslabs cavities that explains the stability of this high-temperature form.
The 2D structure of the TPHPH precursor, that sustains first the HPO
4 condensation and dehydration in the interslabs, then a shearing of the slabs leading to stable
β-TPD, a 3D framework suitable for long-term storage of actinide elements.