Some factors that determine proton conductivity in nonstoichiometric complex perovskites

Complex perovskites, of the types A(2)(B'B ")O-6 and A(3)(B'B-2")O-9, (in w hich A ions are always 2 +, B' ions are 3 + and 2 +, respectively, and B " ions are 5 +), can become good high-temperature protonic conductors when th ey are made nonstoichiometric by increasing the concentration of the B' ion s at the expense of the B " ions. After heating in water vapor, uptake of H 2O occurs, and protons enter the structure. We consider the effects of seve ral variables on the bulk ionic conductivity and activation energy for prot onic conduction, including degree of nonstoichiometry, the effect of orderi ng on the B-sites, the effect of varying the radius of the B' ion and the e ffect of changing the A ion from Sr2+ to Ba2+. The results show that disord er favors a higher conductivity, that the degree of order can change with n onstoichiometry, and that Ba compounds always have higher conductivity and lower activation energy than the corresponding Sr compounds. (C) 1999 Elsev ier Science B.V. All rights reserved.