Crystal Structure, Vibrational and Thermal Behavior of Ba(NH4)[Co(CN)6]·4H2O: A New Precursor for The Synthesis of Hexagonal BaCoO3

DIEGO MAURICIO GIL; ALEJANDRO DI SANTO; FERNANDO POMIRO; GUSTAVO A. ECHEVERRIA; OSCAR E. PIRO; RAÚL ERNESTO CARBONIO; AIDA BEN ALTABEF

POLYHEDRON

PERGAMON-ELSEVIER SCIENCE LTD

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

0277-5387

The crystal structure of Ba(NH4)[Co(CN)6].4H2O complex 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>2s(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 (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 (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 to investigate the formation of BaCoO3-
. This compound, in its hexagonal form, could be synthesized at temperatures as low as 675o C. At 750o 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.