Structural and electrical properties of Mg–Cu- and Mg–Cu–Li-doped bismuth niobate semiconductors with the pyrochlore structure

This work studies a series of synthesized Bi$_{1.6}$Mg$_{0.8-x}$Cu$_x$Nb$_{1.6}$O$_{7-\delta}$ ($x$ = 0.2, 0.4) semiconductors and their Lidoped compositions. A detailed structure investigation combining high-resolution neutron-, synchrotron-, and X-ray diffraction methods, as well as DFT calculations, revealed the preferential location of Cu and Li atoms at the Bi sites and Mg atoms at the Nb ones. According to high-temperature X-ray diffraction data, a structural modification caused by the activation of oxygen transport occurs at 200°C. The linear thermal expansion coefficient was found to be 3.6–4.6⋅10$^{−6}$ K$^{−1}$ (50–400°C). Magnetic susceptibility measurements allowed us to determine weak antiferromagnetic exchange interactions. The direct band gap was predicted using the DFTHSE03 hybrid functional calculation, and the optical direct band gap was estimated at 2.3–2.4 eV. Impedance spectroscopy and a dc four-probe technique were also employed to examine the samples$^,$ electrical properties. The high mixed electronic-ionic conductivity of the pyrochlores was detected, while the vacancies created by Lidoping in Bi$_{1.5-y}$Li$_y$Mg$_{0,375}$Cu$_{0,375}$Nb$_{1.5}$O$_{7-\delta}$ have been found not to affect the conductivity. Besides, the pyrochlores are chemically compatible with the La$_{0.7}$Sr$_{0.3}$MnO$_3$ perovskite (up to 800°C). These make us believe that the studied Mg–Cu- and Mg–Cu–Li-doped bismuth niobate semiconductors can become the basis for composite electrodes to boost their oxygen conductivity

Effect of Li and Li-RE co-doping on structure, stability, optical and electrical properties of bismuth magnesium niobate pyrochlore

New Bi$_{1.5}$Mg$_{0.9-x}$LixNb$_{1.5}$O$_{7–δ}$ (x = 0.25; 0.40) and Bi$_{1.4}$RE$_{0.1}$Mg$_{0.5}$Li$_{0.4}$Nb$_{1.5}$O$_{7–δ}$ (RE – Eu, Ho, Yb) compounds with the pyrochlore structure were synthesized. The displacements of the A-site atoms (96g) and O' ones (32e) as well as the Li and RE atoms distribution in the A-sites were determined. The dopant distribution was proven by ab initio calculations. The most preferable (Bi$_{1.5}$Li$_{0.5}$)(Nb$_{1.5}$Mg$_{0.5}$)O$_7$ model was predicted with a direct band gap of 3.18 eV corresponding to the experimental Eg for Bi$_{1.5}$Mg$_{0.5}$Li$_{0.4}$Nb$_{1.5}$O$_{7–δ}$. The thermal stability of the compounds in air up to 1100–1220 °C and the reducing atmosphere up to 400 °C was determined. The charge disbalance in the A$_2$O' sublattice and the oxygen vacancies predetermine the dielectric behavior of the ceramics up to 200 °C, the mixed conductivity at high temperatures (T > 200 °C), and the proton transport up to 400 °C