Defects and conductivity in Sr-doped LaNb3O9

Abstract

The work is based on possible protonic transport in LaNb3O9. This ABO3–type perovskite is known to have large voids in the lattice due to many inherent A-site metal vacancies. Acceptor-doping of this material may facilitate the transport of protons through the formation of oxygen vacancies. Strontium as an acceptor is tested in an attempt to substitute the A-site lanthanum. The product is nominally La1-xSrxNb3O9-á. Theoretical models of the resulting defect chemistry of doped metal oxides are presented and used in the interpretation of the results. 2.5 % and 5 % Sr-doped LaNb3O9 were synthesized by solid state reaction from the basic oxides. The electrical conductivity was measured as a function of temperature, pO2 (10-25 – 1 atm) and pH2O, using the 4-point van der Pauw method. pH2O dependencies of the conductivity indicated no protonic association or transport. pO2 dependencies of the conductivity showed that the material was independent of oxygen partial pressure at very low and high pO2. In the intermediate pO2 range (here: 10-12 – 10-4 atm) the slope of the dependency was found to be -1/4. This behavior is consistent with the model of a donor-doped material. Several isotherms were tested (650 – 1100 °C), and the difference in conductivity was proportional with temperature. The conductivity arises from electronic defects, mainly p-type at high pO2 and n-type at low pO2. The measurements give reason to question the effectivity of strontium as acceptor in LaNb3O9.