The local structure and composition of Ba{sub 4}Nb{sub 2}O{sub 9}-based oxycarbonates

• Published in: Journal of solid state chemistry 2010-08, Vol.183 (8)
• Main Authors: Bezjak, Jana, Abakumov, Artem M., Recnik, Aleksander, Macek Krzmanc, Marjeta, Jancar, Bostjan, Suvorov, Danilo
• Format: Article
• Language: English
• Subjects: ABSORPTION SPECTROSCOPY; ALKALINE EARTH METAL COMPOUNDS; BARIUM COMPOUNDS; CARBON COMPOUNDS; CARBONATES; CHEMICAL ANALYSIS; COHERENT SCATTERING; COOLING; CRYSTAL LATTICES; CRYSTAL STRUCTURE; CRYSTALLOGRAPHY; DIFFRACTION; ELECTRON DIFFRACTION; ELECTRON MICROSCOPY; GRAVIMETRIC ANALYSIS; INFRARED SPECTRA; LATTICE PARAMETERS; MASS SPECTROSCOPY; MATERIALS SCIENCE; MICROSCOPY; MINERALS; NANOSTRUCTURES; NIOBATES; NIOBIUM COMPOUNDS; OXIDE MINERALS; OXYGEN COMPOUNDS; PEROVSKITE; PEROVSKITES; QUANTITATIVE CHEMICAL ANALYSIS; REFRACTORY METAL COMPOUNDS; SCATTERING; SPECTRA; SPECTROSCOPY; TEMPERATURE RANGE; TEMPERATURE RANGE 1000-4000 K; THERMAL ANALYSIS; THERMAL GRAVIMETRIC ANALYSIS; TRANSITION ELEMENT COMPOUNDS; TRANSMISSION ELECTRON MICROSCOPY; TRIGONAL LATTICES; TWINNING; X-RAY DIFFRACTION
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2010.06.003

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) Ba{sub 4}Nb{sub 2}O{sub 9}. 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 {alpha}-modification, obtained by slow cooling from 1100 {sup o}C, corresponds to Ba{sub 4}Nb{sub 2}O{sub 8.8}(CO{sub 3}){sub 0.2}.0.1H{sub 2}O, while the quenched high-temperature {gamma}-modification has the Ba{sub 4}Nb{sub 2}O{sub 8.42}(CO{sub 3}){sub 0.58}.0.38H{sub 2}O composition. The {alpha}-phase has a composite incommensurately modulated structure consisting of two mutually interacting [Ba]{sub {infinity}} and the [(Nb,{open_square})O{sub 3}]{sub {infinity}} subsystems. The composite modulated crystal structure of the {alpha}-phase can be described with the lattice parameters a=10.2688(1) A, c=2.82426(8) A, q=0.66774(2)c* and a superspace group R3{sup -bar }m(00{gamma})0s. The HRTEM analysis demonstrates the nanoscale twinning of the trigonal domains parallel to the {l_brace}1 0 0{r_brace} crystallographic planes. The twinning introduces a one-dimensional disorder into the [(Nb,{open_square})O{sub 3}]{sub {infinity}} subsystem, which results in an average P6{sup -bar }2c crystal structure of the {alpha}-phase. Possible places for the carbonate group in the structure are discussed using a comparison with other hexagonal perovskite-based oxycarbonates. - Graphical abstract: The composition and the local crystal structure of generally known {alpha}-Ba{sub 4}Nb{sub 2}O{sub 9} was studied. The compound is hydrated oxycarbonate with the composition of Ba{sub 4}Nb{sub 2}O{sub 8.8}(CO{sub 3}){sub 0.2}.0.1 H{sub 2}O. It has a composite incommensurately modulated structure consisting of two mutually interacting subsystems, i.e., [Ba]{infinity} and the [(Nb,)O{sub 3}]{infinity} subsystem.