Crystal structure, vibrational and thermal behavior of Ba(NH4)[Co(CN)6]•4H2O: A new precursor for the synthesis of hexagonal BaCoO3

The crystal structure of Ba(NH4)[Co(CN)(6)]center dot 4H(2)O complex was determined by X-ray diffraction methods. It crystallizes in the hexagonal P6(3)/m space group with a = b = 7.6882(2) angstrom, c = 14.4764(4) angstrom, and Z= 2 molecules per unit cell. The structure was solved from 475 reflections with I > 2 sigma(I) and refined to an agreement R1-factor of 0.0214. The cobaltocyanide anion has an octahedral shape with its Co(III) ion sited at a crystallographic special position of point symmetry (3) over bar (S-6) [d(Co-C) = 1.886(3) angstrom and d(C-N) = 1.143(4) angstrom]. The barium ion is in a 9-fold environment at a crystal special position (6) over bar (C-3h), coordinated to cyanide N-atoms of neighboring complexes [d(Ba-N) = 2.862(3) angstrom] and to one type of water molecule laying onto an m mirror plane [d(Ba-Ow) = 2.924(4) angstrom]. The ammonium ion and the other type of water molecule share four symmetry-related C-3 lattice sites with equal occupancy. Under the P6(3) subspace group these four sites split into two independent pairs of C-3 sites, one occupied by two symmetry-related ammonium ions, the other by two (disordered) water molecules. The complex was characterized by UV-Vis, IR and Raman spectroscopy and the thermal decomposition was studied to investigate the formation of BaCoO3-delta. This compound, in its hexagonal form, could be synthesized at temperatures as low as 675 degrees C. At 750 degrees C a new orthorhombic poly-type is obtained in a mixture with the hexagonal form which is associated with the appearance of oxygen vacancies. These temperatures of synthesis and reaction times are lower than those reported by ceramic and sol-gel synthesis. These soft treatments gave rise to homogeneous particles of small grain size. (C) 2014 Elsevier Ltd. All rights reserved.