Voltage-dependent low-field resistivity of CaTiO3:Zn ceramics

The bulk and grain boundary conductivities of Ca(Ti1-xZnx)O3-x ceramics increase with time under a dc-bias voltage of <= 100Vcm(-1) and at temperatures in the range 250 to 600 degrees C. This low field effect, which is not observed in undoped CaTiO3, is attributed to the nature of a defect structure that contains Zn located on a Ti site, giving rise to underbonded surrounding oxide ions that are readily ionised on application of a dc bias. The conduction is predominantly p-type since holes, located on oxygen as O- ions, are more mobile than both the ionised electrons which are trapped, probably at the sample surface, and oxygen vacancies which form as charge compensation for the Zn2+ acceptor dopant. The conductivity increase is reversible on removal of the dc bias. The electrical properties of the ceramics were modelled successfully using an equivalent electrical circuit consisting of a parallel combination of a resistor, R, capacitor, C, and constant phase element, CPE, to model the bulk response, in series with a similar circuit to model the grain boundary response. At the highest temperatures, an additional parallel RC element was added in series to model the sample-electrode response.