Anomaly Negative Resistance Phenomena in Highly Epitaxial PrBa0.7Ca0.3Co2O5+delta Thin Films Induced from Superfast Redox Reactions

Thin films of Ca-doped double perovskite, PrBa0.7Ca0.3Co2O5+delta (PBCC), were epitaxially grown on (001) SrTiO3, and their redox reactions under a switching flow of H-2 and O-2 gases were examined at various temperatures by measuring the resistance R(t) of the films as a function of the gas flow time t. In the temperature range between 350 and 725 degrees C, these thin films are reduced and oxidized in an ultrafast manner under the flow of H-2 and O-2 gases, respectively, suggesting that PBCC thin films are promising candidates for developing ultra-sensitive oxygen sensors or SOFC cathodes at intermediate or high temperatures. When the gas flow is switched to O-2, the reduced PBCC thin films exhibit a negative resistance at temperatures above 600 degrees C but a positive resistance at temperatures below 600 degrees C. The probable cause for these anomalous transport properties is the diffusion of the H atoms from the cathode to the anode in the PBCC film, which provides a current opposite to that resulting from the external voltage.