Phase relations in a barium-rich high-temperature region (25-45 mol% CuO, 900-1100 deg. C) of the BaO-CuOx system

The phase relations have been studied in the BaO-CuOx system in the range of 25.0-45.0 mol% CuO at 900-1100 deg. C at P(O2)=21 kPa (air) by visual polythermal analysis (VPA), powder X-ray diffraction (XRD), electron diffraction (ED) with simultaneous energy-dispersive X-ray (EDX) elemental analysis in a transmission electron microscope (TEM), and iodometric chemical analysis. The discrete deviations 2.02 (101:50), 2.04 (102:50), 2.10 (105:50) of Ba/Cu (Ba:Cu) composition from the stoichiometric ratio 2:1 have been found for the known Ba2CuO3+δ oxides in the subsolidus region 900-970 deg. C. Unit cell parameters of the 101:50 orthorhombic oxide, 102:50 tetragonal one, 105:50 orthorhombic one are, respectively, a=4.049, b=3.899, c=13.034 A; a=3.985, c=12.968 A; a=4.087, b=3.897 and c=12.950 A. ED patterns of the 101:50, 102:50, 105:50 oxides show characteristic supercell reflections with the respective vector 1/60[5 4 0], ∼2/11<1 1 0> and 1/6[2 0 0]. Oxides of the 2:1, 7:4, 5:3 and 23:20 compositions have been found in the crystallization region 970-1050 deg. C. Unit cell parameters of the 2:1 orthorhombic oxide are a=4.095, b=3.795, c=13.165 A. Interplanar spacings and X-ray characteristic peak intensities of the 7:4, 5:3 and 23:20 oxides are given. Oxides 2:1 and 7:4 melt pseudocongruently at 1020 and 1005 deg. C, oxides 5:3 and 23:20 melt incongruently at 995 and 980 deg. C, respectively. A diagram of the phase relations in the studied region of the BaO-CuOx system has been constructed, whose structure is considered as the total projection of phase states of the system existing for different x. - Graphical Abstract: A diagram of phase relations in the BaO-CuOx system in the range of 30.0-45.0 mol% CuO at 900-1050 deg. C at P(O2)=21 kPa (air) constructed on the data obtained by visual polythermal analysis (the liquidus line), XRD and ED with elemental analysis. Highlights: → A combination of physical-chemical and structural methods was used. → New discrete oxides have been found. → The diagram reflects the total projection of phase states at different x.