Stabilisation and characterisation of a new beta(III)-phase in Zr-doped Bi2O3

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 d ifferential thermal analysis. X-Ray powder diffraction has shown that a sol id solution is formed with general composition Bi2-xZrxO3+x/2 (0.05 less th an or equal to x less than or equal to 0.17) for samples calcined at 850 de greesC. Members of the solid solution adopt a new beta -Bi2O3 type structur e termed beta (III), which is closely related to Pb2F2O. The defect structu re 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 tempera ture polymorph, which is a cubic delta -Bi2O3 analogue. In the beta (III)-p hase, solid solution formation proceeds through an anion interstitial mecha nism with respect to the hypothetical end member beta (III)-Bi2O3, with int erstitial 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 the rmal analysis and ac impedance measurements indicate a complex phase transi tion to a highly conducting polymorph (sigma (700) = 1.02 S cm(-1)) at temp eratures above ca. 690 degreesC on heating. Crystal parameters: T = 298 K, Bi1.85Zr0.15O3.075, M = 449.49, tetragonal, P4(2)/nmc, a = 7.7206(8), c = 5 .6370(6) Angstrom, Z = 4, U = 336.0(1) Angstrom (3), D-c = 8.889(3) g cm(-3 ); T = 1023 K, cubic, Fm (3) over barm, a = 5.6277(4) Angstrom, U = 178.24( 4) Angstrom (3), Z = 2, D-c = 8.379(2) g cm(-3).