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

D. M. GIL; A. DI SANTO; F. POMIRO; G. E. ECHEVERRIA; O. E. PIRO; R. E. CARBONIO; A. BEN ALTABEF

POLYHEDRON

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2014 vol. 73 p. 51 - 58

0277-5387

The crystal structure of the complex Ba(NH4)[Co(CN)6].4H2O was determined by X-ray diffraction methods. It crystallizes in the hexagonal P63/m space group with a = b = 7.6882(2) Å, c = 14.4764(4) Å, and Z = 2 molecules per unit cell. The structure was solved from 475 reflections with I>2(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-bar (S6) [d(Co-C) = 1.886(3) Å and d(C-N) = 1.143(4) Å]. The barium ion is in a 9-fold environment at a crystal special position 6-bar (C3h), coordinated to cyanide N-atoms of neighboring complexes [d(Ba-N) = 2.862(3) Å] and to one type of water molecule laying onto an m mirror plane [d(Ba-Ow) = 2.924(4) Å]. The ammonium ion and the other type of water molecule share four symmetry-related C3 lattice sites with equal occupancy. Under the P63 subspace group these four sites split into two independent pairs of C3 sites, one occupied by two symmetry-related ammonium ions, the other by two (disordered) water molecules. The complex was characterized by UV-Visible, IR and Raman spectroscopy and the thermal decomposition was studied in order to investigate the formation of BaCoO3-δ. This compound, in its hexagonal form could be synthesized at temperatures as low as 675o C. At 750o C a new orthorhombic polytype is obtained in a mixture with the hexagonal form, this 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.