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

The <i>An</i>P₂O₇ diphosphates (<i>An</i> = Th, U, Np, Pu) have been synthesized by various routes depending on the stability of the <i>An</i>^IV 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 high-resolution synchrotron diffraction for UP₂O₇. The uncommon triclinic–cubic phase transition is first order, and its temperature is very sensitive to the ionic radius of <i>An</i>^IV. 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 M^IV cations in which the two phenomena coincide. As shown by various approaches, the P–O_b–P linkage remains bent in the cubic form