Triclinic-Cubic Phase Transition and Negative Expansion in the Actinide IV (Th, U, Np, Pu) Diphosphates

The AnP2O7 diphosphates (An = Th, U, Np, Pu) have been synthesized by various routes depending on the stability of the AnIV cation and its suitability for the unusual octahedral environment. Synchrotron and X-ray diffraction, thermal analysis, Raman spectroscopy and 31P nuclear magnetic resonance reveal them as a new family of diphosphates which probably includes the recently studied CeP2O7. Although they adopt at high temperature the same cubic archetypal cell as the other known MP2O7 diphosphates, they differ by a very faint triclinic distortion at room temperature that results from an ordering of the P2O7 units, as shown using highresolution synchrotron diffraction for UP2O7. The uncommon triclinic-cubic phase transition is first-order, and its temperature is very sensitive to the ionic radius of AnIV. The conflicting effects which control the thermal variations of the P-O-P angle are responsible for a strong expansion of the cell followed by a contraction at higher temperature. This inversion of expansion occurs at a temperature significantly higher than the phase transition, at variance with the parent compounds with smaller MIV cations in which the two phenomena coincide. As shown by various approaches, the P-Ob-P linkage remains bent in the cubic form.

WALLEZ G;  RAISON Philippe;  DACHEUX Nicolas;  CLAVIER Nicolas;  BYKOV Denis;  DELEVOYE Laurent;  POPA Karin;  BREGIROUX D.;  FITCH Andrew;  KONINGS Rudy; 

2013-01-08

AMER CHEMICAL SOC

JRC67988

0020-1669,   

http://pubs.acs.org/doi/abs/10.1021/ic300036y,    https://publications.jrc.ec.europa.eu/repository/handle/JRC67988,   

10.1021/ic300036y,