EFFECT OF ALIOVALENT DOPANTS ON THE KINETICS OF PHASE-TRANSFORMATION AND ORDERING IN RE2O3-BI2O3 (RE = YB, ER, Y, OR DY) SOLID-SOLUTIONS

Bi2O3-based cubic solid solutions containing 20% ErO3 undergo transformation to a rhombohedral phase when annealed at temperatures less-than-or-equal-to 635-degrees-C. This transformation is generally very sluggish and is accompanied by a decrease in conductivity. The kinetics of this transformation were enhanced by the addition of CaO and suppressed by the addition of ZrO2. The time constants for transformation kinetics at 600-degrees-C for CaO-doped and undoped samples were approximately 55 and approximately 330 h, respectively, and the incubation periods ranged between approximately 20 and greater-than-or-equal-to 2000 h depending upon the dopant type, its concentration, and the temperature. This result is rationalized on the premise that cation interstitials are more mobile compared to cation vacancies. The same samples, originally cubic of CaF2 type, when annealed below about 600-degrees-C for a short period of time (a few hours) undergo a degradation in conductivity without a change in XRD patterns. The kinetics of this conductivity decay were observed to be significantly faster than the kinetics of cubic --> rhombohedral phase transformation. A similar degradation of conductivity was also observed in samples containing other stabilizers such as Y2O3, Dy2O3, and Yb2O3. The kinetics of conductivity decay without the formation of the rhombohedral phase were also found to depend upon the presence of aliovalent dopants. Specifically, ZrO2 suppressed this decay. Electron diffraction showed the formation of a superstructure in samples annealed at temperatures <600-degrees-C. Analysis of the diffraction patterns suggests that the structure corresponds to a doubling of the unit cell with the ordering of cations responsible for the origin of the superstructure. The decay in conductivity in the ordered state is attributed to the expected differences in bonding between oxygen ions and the two different cations, Bi and RE.