Electrical transport studies and temperature-programmed oxygen evolution of PrO1.83

Abstract

The phases in the homologous series PrnO2n−2 (n=11, 10, 9 and 7) have been studied by temperature-programmed reduction (TPR), electrical conductivity and Seebeck coefficient measurements using PrO1.83 as the starting material. TPR measurements in helium indicate that oxygen evolution from PrO1.83 occurs in three distinct steps and quantification of the oxygen evolution shows the formation of the phases PrnO2n−2 (n=10, 9 and 7). Temperature-dependent electrical conductivity measurements for p(O2)=0 show breaks in the conductivity which occur at 635, 714 and 797 K, whereas in air the breaks occur only at 720 and 953 K. These correspond to the compositionally controlled phase transitions. The Arrhenius conductivity expression has been used to calculate the activation energies and pre-exponential factors in the stoichiometric regions. Results from TPR and conductivity experiments indicate that PrO1.83 and PrO1.71 have easily established temperature ranges of composition while PrO1.80 and PrO1.78 have stability ranges which are very much smaller. Seebeck coefficient measurements (thermopower) as a function of oxygen partial pressure and temperature indicate that the conduction changes from n to p type for the composition PrO1.71. The approximate independence of the Seebeck coefficient with temperature fits the Heikes theory for a hopping conductor. The discrepancies in the earlier reports on the conductivity of PrO1.83 are attributed to the variations in the p(O2) employed, rate of heating and also to a certain extent the partial hydroxylation and carbonation of the samples used. The results of the present experiments point out these aspects and clarify the discrepancies between previously published data. © 1998 Kluwer Academic Publishers