Redox behavior and transport properties of La_(0.5-2x)Ce_xSr_(0.5+x)FeO_(3-delta) and La_(0.5-2y)Sr_(0.5+2y)Fe_(1-y)Nb_yO_(3-delta) perovskites

摘要

The effects of doping the mixed-conducting (La,Sr)FeO_(3-delta) system with Ce and Nb have been examined for the solid-solution series, La_(0.5-2x)Ce_xSr_(0.5+x)FeO_(3-delta) (x = 0-0.20) and La_(0.5-2y)Sr_(0.5+2y)Fe_(1-y)Nb_yO_(3-delta) (y = 0.05-0.10). Mossbauer spectroscopy at 4.1 and 297 K showed that Ce~(4+) and Nb~(5+) incorporation suppresses delocalization of p-type electronic charge carriers, whilst oxygen nonstoichiometry of the Ce-containing materials increases. Similar behavior was observed for La_(0.3)Sr_(0.7)Fe_(0.90)Nb_(0.10)O_(3-delta) at 923-1223 K by coulometric titration and thermogravimetry. High-temperature transport properties were studied with Faradaic efficiency (FE), oxygen-permeation, thermopower and total-conductivity measurements in the oxygen partial pressure range 10~(-5)-0.5 atm. The hole conductivity is lower for the Ce- and Nb-containing perovskites, primarily as a result of the lower Fe~(4+) concentration. Both dopants decrease oxide-ion conductivity but the effect of Nb-doping on ionic transport is moderate and ion-transference numbers are higher with respect to the Nb-free parent phase, 2.2 X 10~(-3) for La_(0.3)Sr_(0.7)Fe_(0.9)Nb_(0.1)O_(3-delta) cf. 1.3 X 10~(-3) for La_(0.5)Sr_(0.5)FeO_(3-delta) at 1223 K and atmospheric oxygen pressure. The average thermal expansion coefficients calculated from dilatometric data decrease on doping, varying in the range (19.0-21.2) X 10~(-6)K~(-1) at 780-1080K.