Optical and dielectric properties of nanosized ceramics Bi2Mg1-xСrxTa2O9.5-Δ with pyrochlore structure

Chromium-containing pyrochlores Bi2Mg1-xСrxTa2O9.5-Δ with a variable content of magnesium / chromium ions were obtained by solid-phase synthesis. The synthesized samples were characterized by a loose nanosized microstructure formed by partially intergrown ceramic grains. They were bright green. With an increase in the chromium content in the samples, the unit cell parameter of the solid solutions decreased from 10.4890 to 10.4467 Å, obeying Vegard’s rule. The absorption spectra of chromium pyrochlores contained several features in the range below 400 nm, a shoulder in the range of 450 – 520 nm, a broad absorption in the range of 600 – 700 nm with a maximum at 650 nm, and a narrow peak at 724 nm. The band gap of the obtained materials for direct allowed most intense electronic transitions was in the range of 2.14 – 2.29 eV. At 23°С, the permittivity of the samples in a wide frequency range (102 –106 Hz) was practically independent of frequency and decreased with increasing content of chromium ions in solid solutions from 19.5 to 16.5 at 106 Hz. Above 105 Hz, the dielectric loss tangent for all samples slightly depended on frequency and took low values of 0.004 (106 Hz). The conduction activation energy for the samples varied in the range 1.39 ÷1.48 eV.