Electrical conductivity of fluorite and pyrochlore LnxZr1–xO2–x/2 (Ln = Gd, Nd) Solid Solutions

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 )

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.…”

“…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. [12,14,22]. In thetitanate series it is tried to introduce electronic conductivity with terbium and to obtain an enhanced adsorption behaviour with bismuth; cerium is expected to introduce an electronic contribution to the conductivity of Nd2Zr_O_.…”

“…Ionic transference numbers were determined by means of the e.m.f.-method using air and pure oxygen as reference gases. The experimental setup has been described elsewhere [12].…”

“…it [12,13] [133 [24] this work ii [133 In table IV total conductivities are collected for all compounds investigated, together with some values for other materials previously investigated. From these values it can be seen that the ionic conductivities of the materials investigated in the current study are relatively poor.…”

Cation size effects in oxygen ion dynamics of highly disordered pyrochlore-type ionic conductors

“…As shown, the computer simulation value for Gd 2 Zr 2 O 7 is quite similar to our result while that for Y 2 Zr 2 O 7 is somewhat lower than the value we obtained. However, it should be recalled that yttrium zirconate Y 2 Zr 2 O 7 does not exist as an ordered pyrochlore, as assumed in the computer simulations, 8 but as an anion deficient fluorite, and as pointed out by Van Dijk et al, 28 activation energies for oxygen migration are significantly higher in the latter, which would explain the difference of activation energies from simulation and experiment for Y 2 Zr 2 O 7 .…”