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

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• 作者：Gilles Wallez ; Philippe E. Raison ; Nicolas Dacheux ; Nicolas Clavier ; Denis Bykov ; Laurent Delevoye ; Karin Popa ; Damien Bregiroux ; Andrew N. Fitch ; Rudy J. M. Konings
• 刊名：Inorganic Chemistry
• 出版年：2012
• 出版时间：April 2, 2012
• 年：2012
• 卷：51
• 期：7
• 页码：4314-4322
• 全文大小：433K
• 年卷期：v.51,no.7(April 2, 2012)
• ISSN：1520-510X

文摘

The AnP₂O₇ diphosphates (An = Th, U, Np, Pu) have been synthesized by various routes depending on the stability of the An^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鈥揷ubic phase transition is first order, and its temperature is very sensitive to the ionic radius of An^IV. The conflicting effects which control the thermal variations of the P鈥揙鈥揚 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鈥揙_b鈥揚 linkage remains bent in the cubic form.