Abstract

The solid state system Bi₂O₃:ZrO₂ has been investigated up to 28 mol% ZrO₂ using X-ray and neutron powder diffraction, ac impedance spectroscopy and differential thermal analysis. X-Ray powder diffraction has shown that a solid solution is formed with general composition Bi_2 − xZr_xO_3 + x/2 (0.05 ≤ x ≤ 0.17) for samples calcined at 850 °C. Members of the solid solution adopt a new β-Bi₂O₃ type structure termed β_III, which is closely related to Pb₂F₂O. The defect structure of a sample of composition x = 0.15 has been determined by combined high-resolution neutron and X-ray powder diffraction. High temperature powder X-ray diffraction has been used to confirm the structure of the high temperature polymorph, which is a cubic δ-Bi₂O₃ analogue. In the β_III-phase, solid solution formation proceeds through an anion interstitial mechanism with respect to the hypothetical end member β_III-Bi₂O₃, with interstitial oxide ions located in channels parallel to the c-axis. The defect structure involves a pair of Zr atoms coordinated to interstitial ions in the channels. A possible conduction mechanism is proposed. Differential thermal analysis and ac impedance measurements indicate a complex phase transition to a highly conducting polymorph (σ₇₀₀ = 1.02 S cm⁻¹) at temperatures above ca. 690 °C on heating. Crystal parameters: T = 298 K, Bi_1.85Zr_0.15O_3.075, M = 449.49, tetragonal, P4₂/nmc, a = 7.7206(8), c = 5.6370(6) Å, Z = 4, U = 336.0(1) Å³, D_c = 8.889(3) g cm⁻³; T = 1023 K, cubic, Fmm, a = 5.6277(4) Å, U = 178.24(4) Å³, Z = 2, D_c = 8.379(2) g cm⁻³.