BaTh(PO4)2 and BaNp(PO4)2 appear as the prototypes of a new family of double phosphates, which differ from cheralite by the ordering of the cations and from yavapaiite by their higher coordination numbers (14 for Ba and 8 for Th/Np).
Gilles Wallez, UPMC Univiversity of Paris, France, and colleagues have synthesized BaTh(PO4)2 by a wet chemistry route and the new isotype BaNp(PO4)2 by solid-state reaction. Although the former has been the subject of several studies since the early 1980s, the crystal structures of both compounds are reported by Wallez and his team for the first time.
Rietveld analysis shows Ba layers alternating with An(PO4)2 slabs along the a axis. Although those compounds are isostructural to RbEu(SO4)2, these phosphates can be seen as modified yavapaiite derivatives with increased coordination numbers for Ba (XIV) and Th/Np (VIII). Raman and electron absorption spectra highlight the similarities and differences with chemically related compounds.
Further experiments will be carried out on the uranium and plutonium counterparts to ascertain the structural homogeneity of the BaAnIV(PO4)2 family, which could be of interest for the synthesis of solid solutions such as host matrices for minor actinides. BTP and BNP also exhibit good thermal stability, which encourages the measurement of some thermochemical parameters associated to durability aspects, such as solubility.
From a purely theoretical point of view, the search for possible phase transitions between BTP-type compounds and yavapaiites also appears as an interesting topic.
- BaAnIV(PO4)2 (AnIV = Th, Np) – A New Family of Layered Double Phosphates,
G. Wallez, D. Bregiroux, K. Popa, P. E. Raison, C. Apostolidis, P. Lindqvist-Reis, R. J. M. Konings, A. F. Popa,
Eur. J. Inorg. Chem. 2011.