TY - JOUR

T1 - Cr and Mg codoped bismuth tantalate pyrochlores: Thermal expansion and stability, crystal structure, electrical and optical properties, NEXAFS and XPS study

AU - Zhuk, N.A.

AU - Krzhizhanovskaya, M.G.

AU - Koroleva, A.V.

AU - Selyutin, A.A.

AU - Sekushin, N.A.

AU - Nekipelov, S.V.

AU - Sivkov, D.V.

AU - Kharton, V.V.

PY - 2023/7/1

Y1 - 2023/7/1

N2 - A series of Bi2Cr1-xMgxTa2O9+Δ solid solutions with the pyrochlore structure was synthesized by the method of solid-phase synthesis. Structural parameters of the disordered pyrochlore were determined by the Rietveld method (space group Fd-3m:2 (No. 227), Z = 8). As the magnesium content increases, the unit cell parameter rises continuously from 10.4596 ± 0.0001 Å (x = 0.3) to 10.5463 ± 0.0001 (x = 1.0) according to the Vegard rule. The grain size varies in the range of 0.5–2 μm, regardless of the magnesium/chromium(III) ratio. Chromium ions increase the range of thermal stability of ceramics up to 1110 °C. The thermal expansion coefficient in the range of 30–1110 °C increases continuously from 4.1 to 8.6 × 10−6 °C−1 (for x = 0.5). The oxidation states of ions were determined by X-ray spectroscopy (XPS and NEXAFS). It was shown that bismuth and magnesium ions in the pyrochlore are in the Bi(+3), Mg(+2) charge states, and for chromium the states Cr(+3), Cr(+6) are most probable. Based on the presence of an energy shift in the region of Ta4f, Ta5p and Ta5d absorption edges by ΔЕ∼0.7 eV and ∼1 eV, respectively, it was concluded that the oxidation degree of the Ta(+5-δ) ion decreased. The band gap for the samples in the case of direct allowed electronic transitions is in the range of 2.14–2.29 eV. At RT and in the frequency range (102–106 Hz), the dielectric permittivity of the samples is practically constant and it decreases with increasing amount of chromium from 20 (x = 0.7) to 16 (x = 0.3) at 1 MHz. In the high-frequency region (≥105 Hz), the dielectric loss tangent for all samples is independent on frequency and takes low values of ∼0.004 (1 MHz). The activation energy of conductivity for the samples varies in the range of 1.39–1.48 eV. The ability to adsorb water was found for chromium pyrochlores.

AB - A series of Bi2Cr1-xMgxTa2O9+Δ solid solutions with the pyrochlore structure was synthesized by the method of solid-phase synthesis. Structural parameters of the disordered pyrochlore were determined by the Rietveld method (space group Fd-3m:2 (No. 227), Z = 8). As the magnesium content increases, the unit cell parameter rises continuously from 10.4596 ± 0.0001 Å (x = 0.3) to 10.5463 ± 0.0001 (x = 1.0) according to the Vegard rule. The grain size varies in the range of 0.5–2 μm, regardless of the magnesium/chromium(III) ratio. Chromium ions increase the range of thermal stability of ceramics up to 1110 °C. The thermal expansion coefficient in the range of 30–1110 °C increases continuously from 4.1 to 8.6 × 10−6 °C−1 (for x = 0.5). The oxidation states of ions were determined by X-ray spectroscopy (XPS and NEXAFS). It was shown that bismuth and magnesium ions in the pyrochlore are in the Bi(+3), Mg(+2) charge states, and for chromium the states Cr(+3), Cr(+6) are most probable. Based on the presence of an energy shift in the region of Ta4f, Ta5p and Ta5d absorption edges by ΔЕ∼0.7 eV and ∼1 eV, respectively, it was concluded that the oxidation degree of the Ta(+5-δ) ion decreased. The band gap for the samples in the case of direct allowed electronic transitions is in the range of 2.14–2.29 eV. At RT and in the frequency range (102–106 Hz), the dielectric permittivity of the samples is practically constant and it decreases with increasing amount of chromium from 20 (x = 0.7) to 16 (x = 0.3) at 1 MHz. In the high-frequency region (≥105 Hz), the dielectric loss tangent for all samples is independent on frequency and takes low values of ∼0.004 (1 MHz). The activation energy of conductivity for the samples varies in the range of 1.39–1.48 eV. The ability to adsorb water was found for chromium pyrochlores.

KW - Pyrochlore

KW - Codoping

KW - XPS and NEXAFS spectroscopy

KW - Thermal stability

KW - Dielectric properties

UR - https://www.mendeley.com/catalogue/62566724-a2da-3c68-bf9b-6fee6c96a667/

U2 - 10.1016/j.jssc.2023.124074

DO - 10.1016/j.jssc.2023.124074

M3 - Article

VL - 323

JO - Journal of Solid State Chemistry

JF - Journal of Solid State Chemistry

SN - 0022-4596

M1 - 124074

ER -