From thorium phosphate hydrogenphosphate hydrate to β-thorium phosphate diphosphate: Structural evolution to a radwaste storage ceramic

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 [ThPO4]44+ slabs alternating with disordered layers hosting either [HPO4.H2O]24- (TPHPH), [P2O7.2H2O]4- (α-TPDH), or [P2O7]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. - Graphical abstract: The 2D structure of the TPHPH precursor, that sustains first the HPO4 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