Triclinic–Cubic
Phase Transition and Negative
Expansion in the Actinide IV (Th, U, Np, Pu) Diphosphates
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Abstract
The <i>An</i>P<sub>2</sub>O<sub>7</sub> diphosphates
(<i>An</i> = Th, U, Np, Pu) have been synthesized by various
routes depending on the stability of the <i>An</i><sup>IV</sup> cation and its suitability for the unusual octahedral environment.
Synchrotron and X-ray diffraction, thermal analysis, Raman spectroscopy,
and <sup>31</sup>P nuclear magnetic resonance reveal them as a new
family of diphosphates which probably includes the recently studied
CeP<sub>2</sub>O<sub>7</sub>. Although they adopt at high temperature
the same cubic archetypal cell as the other known MP<sub>2</sub>O<sub>7</sub> diphosphates, they differ by a very faint triclinic distortion
at room temperature that results from an ordering of the P<sub>2</sub>O<sub>7</sub> units, as shown using high-resolution synchrotron diffraction
for UP<sub>2</sub>O<sub>7</sub>. The uncommon triclinic–cubic
phase transition is first order, and its temperature is very sensitive
to the ionic radius of <i>An</i><sup>IV</sup>. 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<sup>IV</sup> cations
in which the two phenomena coincide. As shown by various approaches,
the P–O<sub>b</sub>–P linkage remains bent in the cubic
form