Abstract
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.
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