Cationic ordering in tysonite type structures: III: Crystal structure of Ba5U4F26, the first fully ordered superstructure of anion-deficient tysonite

Abstract

Ba5U4F26, the first fully ordered superstructure deriving from tysonite type with anionic deficit is prepared by annealing, below 983 K, the high temperature Ba1+xU1-xF6-2x tysonite solid solution for x = 0.11. It is structurally characterized in the monoclinic C2/c space group [a = 22.420(10) Å; b = 12.898(6) Å; c = 16.871(7) Å; β = 117.36(5)°].This new phase presents a full cationic and anionic ordering with 5 Ba and 4 U cationic sites. The BaF10 and BaF11 polyhedra are close to Edshammar polyhedra whereas U4+cations are 8-fold and 9-fold coordinated. Ba5U4F26 has structural features analogous to the BaUF6 ordered varieties and particularly to β-BaUF6. This description extends to the anion-deficient Ba1+xU1-xF6-2x and more generally to the M(II)1+x M’(IV)1-xF6-2x solid solutions the short range order features proposed for all the disordered tysonite M(II)M’(IV)F6 phases: decrease of the coordination number of M’(IV) cations and of the connectivity of M(II) and M’(IV) polyhedra from face-sharing to edge-sharing and from edge-sharing to corner-sharing, with anionic vacancies located in the neighbourhood of U cations.