Structure and Electrical -Transport Relations in Ba(Zr,Pr)O3-delta Perovskites


Members of the perovskite solid solution BaZr1-xPrxO3-delta (0.2 <= x <= 0.8) with potential high-temperature electrochemical applications were synthesized via mechanical activation and high-temperature annealing at 1250 degrees C. Structural properties were examined by Rietveld analysis of neutron powder diffraction and Raman spectroscopy at room temperature, indicating rhombohedral symmetry (space group R (3) over barc) for members x = 0.2 and 0.4 and orthorhombic symmetry (Imma) for x = 0.6 and 0.8. The sequence of phase transitions for the complete solid solution from BaZrO3 to BaPrO3 is Pm (3) over barm -> R (3) over barc -> Imma -> Pnma. The structural data indicate that Pr principally exists as Pr4+ on the B site and that oxygen content increases with higher Pr content. Electrical conductivity measurements in the temperature range of 250-900 degrees C in dry and humidified (pH(2)O approximate to 0.03 atm) N-2 and O-2, atmospheres revealed an increase of total conductivity by over 2 orders of magnitude in dry conditions from x = 0.2 to x = 0.8 (sigma approximate to 0.08 S cm(-1) at 920 degrees C in dry O-2 for x = 0.8). The conductivity for Pr contents x > 0.2 is attributable to positively charged electronic carriers, whereas for x = 0.2 transport in dry conditions is n-type. The change in conduction mechanism with composition is proposed to arise from the compensation regime for minor amounts of BaO loss changing from predominantly partitioning of Pr on the A site to vacancy formation with increasing Pr content. Conductivity is lower in wet conditions for x > 0.2 indicating that the positive defects are, to a large extent, charge compensated by less mobile protonic species. In contrast, the transport mechanism of the Zr-rich composition (x = 0.2), with much lower electronic conductivity, is essentially independent of moisture content.






Antunes, I; Amador, U; Alves, A; Correia, MR; Ritter, C; Frade, JR; Perez-Coll, D; Mather, GC; Fagg, DP

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This work was supported by the FCT, Portuguese Foundation for Science and Technology, under Project No. PTDC/CTM-ENE/6319/2014 and FEDER funds through the COMPETE 2020 Programme. D.P.-C. also acknowledges the FCT for financial support through a BPD grant (SFRH/BPD/112282/2015). The authors further acknowledge the financial support of MINECO, Spain (ENE2015-66183-R) and the CSIC, Spain (i-link0743). Access to the neutron facilities at the Institute Laue Langevin (Grenoble, France) under Grant No. 5-24-582 (D2B) is gratefully acknowledged.

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