title: Triclinic-cubic phase transition and negative expansion in the actinide IV (Th, U, Np, Pu) diphosphates.
abstract: The AnP(2)O(7) 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 (31)P nuclear magnetic resonance reveal them as a new family of diphosphates which probably includes the recently studied CeP(2)O(7). Although they adopt at high temperature the same cubic archetypal cell as the other known MP(2)O(7) diphosphates, they differ by a very faint triclinic distortion at room temperature that results from an ordering of the P(2)O(7) units, as shown using high-resolution synchrotron diffraction for UP(2)O(7). The uncommon triclinic-cubic 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-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.
authors.affiliation: Chimie ParisTech, UPMC Univ. Paris 06, CNRS-UMR 7574, Laboratoire de Chimie de la Matière Condensée de Paris (LCMCP), 75005 Paris, France. gilles.wallez@upmc.fr
authors.forename: Gilles
authors.initials: G
authors.lastname: Wallez
authors.forename: Philippe E
authors.initials: PE
authors.lastname: Raison
authors.forename: Nicolas
authors.initials: N
authors.lastname: Dacheux
authors.forename: Nicolas
authors.initials: N
authors.lastname: Clavier
authors.forename: Denis
authors.initials: D
authors.lastname: Bykov
authors.forename: Laurent
authors.initials: L
authors.lastname: Delevoye
authors.forename: Karin
authors.initials: K
authors.lastname: Popa
authors.forename: Damien
authors.initials: D
authors.lastname: Bregiroux
authors.forename: Andrew N
authors.initials: AN
authors.lastname: Fitch
authors.forename: Rudy J M
authors.initials: RJ
authors.lastname: Konings