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.

No abstract

The ionic conductivity of ceramic Gd,Zrl -,02 I2 and Nd,02 solid solutions is measured as a function of composition. Both solid solutions are of the defect fluorite type and the fluoriterelated ordered pyrochlore phase is present around the stoichiometric compositions Gd,Zr,O, and Nd,Zrz07. As a function of composition the conductivity goes through a maximum for stoichiometric Gd?Zr,O, whereas the conductivity is minimal for Nd,Zr,O,. It appears that a t the stoichiometric pyrochlore composition both, activation enthalpy A H and pre-exponential factor u,, are minimal and that the absolute magnitude of the pre-exponential factor accounts for the difference in the net conductivity effect of the systems studied. The results obtained from both, single crystalline and ceramic samples can be correlated with the defect structure and ordering in the anion sublattice and with deviations of Vegard's law in the fluorite subcell dimensions. No discontinuity of uo or AH is observed at the fluorite-pyrochlore phase boundary. This suggests the existence of a hybrid phase around the fluorite-pyrochlore phase boundary in this type of system. Die Ionenleitfahigkeit von keramischen Gd,Zrl -,02z/2 und Nd,Zrl -,Oe -,p-Mischkristallen wird 81s Funktion der Zusammensetzung gemessen. Beide Mischkristalle sind vom Fluorit-Defekttyp und die fluoritverkniipfte geordnete Pyrochlorphase esistiert um den Bereich der stochiometrischen Zusammensetzungen Gd,Zr,O, und Nd,Zr,O,. I n Abhingigkeit von der Zusammensetzung durchliiuft die Leitfiihigkeit ein Maximum far stijchiornetrisches Gd,Zr,O,, wiihrend die Leitfiihigkeit fur Nd,ZrzO, minimal ist. Es zeigt sich, dal3 bei der stochiometrischen Pyrochlorzusammensetzungsowohl die Aktivierungsenthalpie AHals auch der prii-exponentielle Faktor a , minimal sind und daI3 die absolute Gro8e des prii-exponentiellen Faktors die Differenz in der Nettoleitfahigkeit der untersuchten Systeme erkliirt. Die sowohl von einkristallinen als auch von keramischen Proben erhaltenen Ergebnisse lassen sich mit der Defektstruktur und der Ordnung im Anionenuntergitter und mit den Abweichungen der Abmessungen der Fluorit-Unterzellen von der Vegardschen Regel korrelieren. Es wird keine Diskontinuitiit von u, oder A H an der Fluorit-Pyrochlor-Phasengrenze beobachtet. Dies weist auf die Existenz einer Hybridphase in der Niihe der Fluorit-Pyrochlor-Phasengrenze in diesem Systemtyp hin. l )

The mixed (oxygen ionic-electronic) conductor bismuth vanadate (BiVO4) was studied with respect to its electrochemical properties. The ionic transference numbers, measured by the concentration cell method in the temperature range of 800 to 1000 K, vary from 0.7 to 0.3. The total conductivity of this ceramic material is one order of magnitude lower than found for cubic yttria-stabilized zirconia (YSZ). The activation enthalpy for the electronic conductivity is high ( 193 kJ/mol) compared to the ionic conductivity (71 kJ/mol). The Po2 dependency of the conductivity data in combination with the Seebeck measurements showed electrons to be the majority charge carriers, indicating that BiVO4 is an n-type mixed conductor.

The surface oxygen exchange coefficient, ks, has been measured for the solid solution (Bi203)o75(ErzO3)~25 and (Bi203)o 6 (Yb203)o 4 (abbreviated BE25 and BT40), using gas-phase ~80 exchange techniques. The activation enthalpy of ks amounts to AE= 110 kJ/mol for BT40 and AE= 130 kJ/mol for BE25. The magnitude ofk~ for the purely ionic conducting BE25 is comparable with values obtained from electrode polarization (I-V) measurements (AE= 140 kJ/mol). The comparatively high k~ values show a (P~,)" dependence on oxygen pressure with values ofn close to 0.5, indicating surface control in the oxygen transport process. Bismuth oxide containing solid solutions show a large activity in the oxygen exchange reaction with the gas phase.