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

摘要

beta-Thorium phosphate diphosphate (beta-TPD), considered as a very promising radwaste storage material, was obtained from thorium phosphate hydrogen phosphate hydrate (TPHPH) precursor through dehydration and hydrogen phosphate condensation. The structures of TPHPH, intermediate alpha-thorium phosphate diphosphate (alpha-TPD) and its hydrate (alpha-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](4)(4+) [ThPO](4)(4+) slabs alternating with disordered layers hosting either [HPO4. H2O](2)(4-) (TPHPH), [P2O7 (.) 2H(2)O](4-) (alpha-TPDH), or [P2O7](4-) (alpha-TPD), unlike the 3D structure of beta-TPD. The diphosphate groups of alpha-TPD and alpha-TPDH are strongly bent. The irreversible transition to the final beta-TPD consists in a shearing of the slabs and a reduction of the interslabs cavities that explains the stability of this high-temperature form. (c) 2006 Elsevier Inc. All rights reserved.