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

The phases in the homologous series PrnO2n-2 (n=11 10, 9 and 7) have been s tudied by temperature-programmed reduction (TPR), electrical conductivity a nd Seebeck coefficient measurements using PrO1.83 as the starting material. TPR measurements in helium indicate that oxygen evolution from PrO1.83 occ urs in three distinct steps and quantification of the oxygen evolution show s the formation of the phases PrnO2n-2 (n = 10, 9 and 7). Temperature-depen dent electrical conductivity measurements for p(O-2)=O show breaks in the c onductivity which occur at 635, 714 and 797 K, whereas in air the breaks oc cur only at 720 and 953 K. These correspond to the compositionally controll ed phase transitions. The Arrhenius conductivity expression has been used t o calculate the activation energies and pre-exponential factors in the stoi chiometric regions. Results from TPR and conductivity experiments indicate that PrO1.83 and PrO1.71 have easily established temperature ranges of comp osition while PrO1.80 and PrO1.78 have stability ranges which are very much smaller. Seebeck coefficient measurements (thermopower) as a function of o xygen 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 ho pping conductor. The discrepancies in the earlier reports on the conductivi ty of PrO1.83 are attributed to the variations in the p(O-2) employed, rate of heating and also to a certain extent the partial hydroxylation and carb onation of the samples used. The results of the present experiments point o ut these aspects and ciarify the discrepancies between previously published data. (C) 1998 Kluwer Academic Publishers.