Structure, microstructure and physicochemical properties of BaW 1−x Nb x O 4−δ materials

Nb-doped BaWO4 with the assumed formula BaW1−xNbxO4−δ (x = 0, 0.005, 0.01, 0.02 and 0.05) were prepared by solid-state reaction method. Crystal structure and phase composition were determined by X-ray diffraction method. Scanning electron microscopy (SEM) coupled with energy-dispersive spectrometry (EDS) was used to describe microstructure and chemical composition of synthesised materials. It was found that solubility limit of niobium in the BaWO4 structure is the range 0.5–1 mol%, as formation of second phase—Ba5Nb4O15—was observed for samples with higher dopant content. For evaluation of the chemical stability of synthesized materials, the comparative CO2/H2O exposure test was performed. Samples were exposed to carbon dioxide- and water vapour-rich atmosphere (7% CO2 in air, 100% RH) at 298 K for 700 h. During this exposition, the chemical reactions between the samples and the surrounding gaseous atmosphere resulting in formation of barium hydroxide and/or barium carbonate can process. Thermogravimetry (TG) method was used for chemical stability evaluation. The comparison of samples before and after the CO2/H2O exposure test was performed. To support the interpretation of TG results, the analysis of gaseous products evolved during thermal treatment of the samples was done using mass spectrometer. The effect of dopant on the BaWO4 chemical stability improvement was observed. In order to determine the electrical properties of obtained materials, the DC resistance measurements in synthetic air atmosphere were taken. It was shown that niobium doping and the presence of second phase—Ba5Nb4O15—leads to an increase in the total conductivity of synthesised materials.