Sossina M. Haile

The influence of cation non-stoichiometry on the properties of undoped and gadolinia-doped barium cerate

The influence of A:B cation non-stoichiometry on the structural and electrical properties of BaCeO_3 and of 15% Gd-doped BaCeO_3 has been investigated. X-ray powder diffraction and water uptake measurements suggest the occupation of Ba sites by Gd in barium-deficient, doped materials. This phenomenon is exacerbated by high temperature processing, which causes additional loss of barium. The relationship between defect chemistry, stoichiometry and conductivity, as measured by a.c. impedance spectroscopy, is discussed.

The Kinetics of Ordering in Gadolinium Zirconate: an Unusual Oxygen Ion Conductor

Gadolinium zirconate, Gd{sub 2}Zr{sub 2}O{sub 7}, undergoes an order-disorder transition at {approximately} 1,550 C, transforming from a defect fluorite structure (Fm3m) to a pyrochlorine structure (Fd3m). Both cations and anions are ordered in the low-temperature, pyrochlore structure. In order to understand the interplay between anion and cation order parameters and ordering rates, the transformation kinetics of Gd{sub 2}Zr{sub 2}O{sub 7} have been examined via X-ray diffraction. Gadolinium zirconate is of particular interest because the oxygen ion conductivity of the ordered phase is significantly greater than that of the disordered phase, in contrast to virtually every other known solid electrolyte. This difference in conductivity has provided a second technique for characterizing the transformation kinetics: in situ A.C. impedance spectroscopy. Results of the X-ray diffraction showed the growth of superstructure peak intensity to follow an apparent (time){sup 1/2} dependence, rather than that expected from a nucleation and growth model. The impedance spectroscopy measurements, on the other hand, showed the conductivity to increase linearly with time. These results suggest the transition is second order in nature.

Preparation and characterization of polycrystalline and single crystal Gd doped barium cerium oxide

Polycrystalline samples of BaCe{sub x}Gd{sub 1{minus}x}O{sub 3} have been prepared with 0.10 < x < 0.20. X-ray powder diffraction indicates the samples contain only one crystalline phase for doping levels < 0.15. Transmission electron microscopy analyses on the 10% doped sample revealed no glassy phases in the intergranular regions. Single crystal fibers were prepared by Laser Heated Pedestal Growth (LHPG) from the polycrystalline samples. Fiber growth was non-conservative thus the single crystal quality was limited. Preliminary conductivity measurements on the polycrystalline samples using AC impedance spectroscopy are compared with literature data.