Oxygen ion and mixed conductivity in compounds with the fluorite and pyrochlore structure

Abstract

The effect of pyrochlore order in solid solutions with the defect fluorite structure was studied. The amount of order was changed by various heat treatments or by varying the cation composition. Examples of short range order and microdomainformation have been observed in Tb2Zr207+x and Gd2Zr207. Pyrochlore order results in a decrease of the activation enthalpy and pre-exponential factor for oxygen ion conduction. The results are explained by a model based on the occurrence of a preferential diffusion path with a low strain energy component in AH. Optimal conductivity is achieved in materials which are not completely ordered. Mixed conductivity in Ln2Zr207 with P structure can be realized by partial replacement of Ln by Tb. This results in good ionic as well as electronic conductivities.1.

Electrical and catalytic properties of some oxides with the fluorite or pyrochlore structure

“…For Nd2Zr207 the deviation of t i = 1 at temperatures above 00 C has been attributed prevlously to the onset of a p-type electronic contribution [22]. This behaviour is suppressed by introducing a few percent of cerium The value of AH for x = 0.00 agrees with values previously reported in literature [12,13,23]. Variations of AH-values for nominally equal compositions can be attributed to differences in sample preparation and small differences in stoichiometry.…”

“…In general disorder in the pyrochlore structure (due to deviation from stoichiometry, heat treatment or choice of cations) leads to higher activation enthalpies as has been explained in ref. [13]. The same phenomenon accounts for the investigated zirconates.…”

“…The two types of lattice oxygen with different binding energy, present in the structure, may lead to an improved selectivity for particular redox reactions [163. These characteristics of pyrochlores and the features of the electrode and electrocatalytic reaction process, mentioned in the previous Section, have led to the following selection of materials for preliminary-investigations: -Bismuth doped rare earth titanates: [13]. Nd2Zr^O_ was found to have a small p-type electronic conductivity ~n addition £o an ionic conductivity with an activation enthalpy of about 70-80 kJ/mol.…”

“…The pyrochlore structure can be considered as a fluorite super structure with doubled unit cell in which cations, anions and vacancies are all ordered. Several oxides were found to be reasonably good oxygen ion conductors with a rather low activation enthalpy [11, 13,14]. Various substitutions of cations are possible within this pyrochlore structure.…”

“…Then an activation enthalpy for ionic conductivity is found of Ii0 (+ 6) kJ/mol. The substitution of Zr by Ce increases the average radius of the B-cation and thus increases the amount of disorder [13]. 1 7 Impedances of specimens (Bi Y.…”