Photoelectron Spectroscopy Study of the Optical and Electrical Properties of Cr/Cu/Mn Tri-Doped Bismuth Niobate Pyrochlore

The multielement pyrochlore of the composition Bi1.57Mn1/3Cr1/3Cu1/3Nb2O9−Δ (sp. gr. Fd-3m:2, 10.4724 Å) containing transition element atoms—chromium, manganese and copper in equimolar amounts—was synthesized for the first time using the solid-phase reaction method. The microstructure of the ceramics is grainless and has low porosity. The sample is characterized by reflection in the red (705 nm) color region. The band gap for the direct allowed transition in the sample is 1.68 eV. The parameters of the Bi5d, Nb3d, Сr2p, Mn2p, and Cu2p X-ray photoelectron spectroscopy (XPS) spectra for the mixed pyrochlore are compared with the parameters of transition element oxides. For the complex pyrochlore, a characteristic shift in the Bi4f and Nb3d spectra to the region of lower energies by 0.15 and 0.60 eV, respectively, is observed. According to the XPS Cu2p and Mn2p spectra of pyrochlore, copper, and manganese cations are in a mixed charge state; they mainly have an effective charge of +2/+3, and the Cr2p spectrum is a superposition of the spectra of chromium ions in the charge state of +3, +4, +6. At 24 °С, the permittivity of the sample in the frequency range (104–106 Hz) weakly depends on the frequency and is equal to ~100, the dielectric loss tangent is 0.017. The activation energy of conductivity is equal to 0.41 eV. The specific electrical conductivity of Bi1.57Cr1/3Cu1/3Mn1/3Nb2O9−Δ increases with the temperature increasing from 1.8 × 10−5 Ohm−1·m−1 (24 °С) to 0.1 Ohm−1·m−1 (330 °С). Nyquist curves for the sample are modeled by equivalent electrical circuits.