Crystal growth and structural investigation of A2BReO6 (A = Sr, Ba; B = Li, Na)

Single crystals of the double perovskite rhenates A(2)BReO(6) (A = Sr, Ba; B = Li, Na) were grown out of molten hydroxide fluxes. Single crystals of orange/yellow Ba2LiReO6, Ba2NaReO6 and Sr2LiReO6 were solved in the cubic, Fm-3m space group with a = 8.1214(11)angstrom, 8.2975(3)angstrom, and 7.9071(15)angstrom, respectively, while Sr2NaReO6 was determined to be monoclinic P2(1)/n with a = 5.6737(6)angstrom, b = 5.7988(6)angstrom, c = 8.0431(8)angstrom, and beta = 90.02(6). The cubic structure consists of a rock salt lattice of corner-shared ReO6 and MO6 (M = Li, Na) octahedra which, in the monoclinic structure, are both tilted and rotated. A discrepancy exists between the symmetry of Sr2LiReO6,, indicated by the single-crystal refinement of flux-grown crystals (Cubic, Fm-3m) and the symmetry indicated by the powder-diffraction data collected on polycrystalline samples prepared by the ceramic method (tetragonal, 14/m). It is possible that the cubic crystals are a kinetic product that forms in small quantities at low temperatures, while the powder represents the more stable polymorph that forms at higher reaction temperature. (C) 2008 Elsevier Inc. All rights reserved.