The local structure and composition of Ba4Nb2O9-based oxycarbonates

X-ray powder-diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), electron diffraction (ED), infrared spectroscopy (IR), thermogravimetry (TG) and mass spectroscopy (MS) were performed to investigate the composition and the crystal structure of tetra-barium di-niobate (V) Ba4Nb2O9. The TG, MS and IR studies revealed that the compound is a hydrated oxycarbonate. Assuming that the carbonate stoichiometrically replaces oxygen, the composition of the low-temperature α-modification, obtained by slow cooling from 1100 °C, corresponds to Ba4Nb2O8.8(CO3)0.2·0.1H2O, while the quenched high-temperature γ-modification has the Ba4Nb2O8.42(CO3)0.58·0.38H2O composition. The α-phase has a composite incommensurately modulated structure consisting of two mutually interacting [Ba]∞ and the [(Nb,□)O3]∞ subsystems. The composite modulated crystal structure of the α-phase can be described with the lattice parameters a =10.2688(1) Å, c =2.82426(8) Å, q=0.66774(2)c* and a superspace group R View the MathML source3¯m (00γ)0s . The HRTEM analysis demonstrates the nanoscale twinning of the trigonal domains parallel to the {1 0 0} crystallographic planes. The twinning introduces a one-dimensional disorder into the [(Nb,□)O3]∞ subsystem, which results in an average P View the MathML source6¯2c crystal structure of the α-phase. Possible places for the carbonate group in the structure are discussed using a comparison with other hexagonal perovskite-based oxycarbonates.