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

Abstract

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>2σ(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–Vis, IR and Raman spectroscopy and the thermal decomposition was studied to investigate the formation of BaCoO3−δ. This compound, in its hexagonal form, could be synthesized at temperatures as low as 675°C. At 750°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.