B-cation arrangements in double perovskites

Two new mixed-valent triple perovskites, Ba3MRu2O9 (M = Li, Na), were grown from reactive hydroxide fluxes. They crystallize in the hexagonal space group P63/mmc, where Ru(V) and Ru(VI) are disordered on only one crystallographic site. Upon cooling, single crystals of Ba3NaRu2O9 undergo a complex symmetry-breaking structural transition at ca. 225 K from room-temperature hexagonal symmetry to a low-temperature orthorhombic symmetry, space group Cmcm. Accompanying this structural transition is a rather abrupt decrease in the magnetic susceptibility at 210 K followed by a steady decrease in the susceptibility with decreasing temperature. Interestingly, the lithium analogue does not display any structural transition down to 100 K. The structural transition in Ba3NaRu2O9 generates three crystallographically unique Ru sites in the low-temperature structure as compared to only one distinct site in the room-temperature structure. On the basis of an analysis of the Ru−Ru distances in the face-sharing bi-octahedra,...

Novel Mixed-Valent (V/VI) Triple Perovskite Ruthenates: Observation of a Complex Low-Temperature Structural and Magnetic Transition

Crystal Structure of 4H BaRuO3: High Pressure Phase Prepared at Ambient Pressure☆

A detailed study of the layered manganite La1+xSr2-xMn2O7 has been performed, establishing that within the composition range 0.1 less than or equal to x less than or equal to 0.45 the phases crystallize in the I4/mmm space group. The evolution of structural parameters with x: in this composition range has been followed using a novel application of an existing program for the Rietveld analysis of powder diffraction data. The structure, a familiar intergrowth of rock-salt (La,Sr)O slabs and double perovskite (La,Sr)(2)Mn2O6 units, is characterized by a reluctance to deform the latter. This manifests as a ''pumping'' of the larger Sr-II ion into the 12-coordinate site of the structure as x is increased. We report these features of the structure as well as electrical transport and magnetic properties, in light of recent observations of giant, negative magnetoresistance in these systems.

S.H. Kim

Papers

Structural and electrical studies of mixed copper/ruthenium oxides and related compounds of zinc and antimony

The crystal structure and electronic properties of Ba4Ru3MO12 (M = Li, Na, Mg, Zn