Crystal Structure, Electrical Conductivity and Hydration of the Novel Oxygen-Deficient Perovskite La2ScZnO5.5, Doped with MgO and CaO

Abstract

This paper demonstrates the possibility of creating oxygen deficiency in perovskites A+3B+3O3 by introducing two types of cations with different charges into the B-sublattice. For this, it is proposed to introduce a two-charged cation, for example, Zn2+, as an alternative to alkaline earth metals. Previously, this possibility was demonstrated for aluminate LaAlO3 and indate LaInO3. In this article, we have focused on the modification of the scandium-containing perovskite LaScO3. The novel oxygen-deficient perovskite La2ScZnO5.5 and doped phases La1.9Ca0.1ScZnO5.45, La2Sc0.9Ca0.1ZnO5.45, and La2Sc0.9Mg0.1ZnO5.45 were obtained via a solid-state reaction process. Their phase composition and hydration were investigated by XRD and TGA + MS techniques. The conductivities of these materials were measured by the electrochemical impedance technique under atmospheres of various water vapor partial pressures. All phases crystallized in orthorhombic symmetry with the Pnma space group. The phases were capable of reversible water uptake; the proton concentration increased in the order of La2ScZnO5.5 < La2Sc0.9Mg0.1ZnO5.45 < La2Sc0.9Ca0.1ZnO5.45 ≈ La1.9Ca0.1ScZnO5.45 and reached ~90% hydration limit for Ca2+-doped phases. The total conductivities increased with the increase in the free lattice volume in the sequence of σLa2ScZnO5.5 < σLa2Sc0.9Mg0.1ZnO5.45 < σLa1.9Ca0.1ScZnO5.45 < σLa2Sc0.9Ca0.1ZnO5.45, the activation energy decreased in the same sequence. The sample La2Sc0.9Ca0.1ZnO5.45 showed the highest conductivity of about 10−3 S∙cm−1 at 650 °C (dry air pH2O = 3.5·10−5 atm). Water incorporation was accompanied by an increase in conductivity in wet air (pH2O = 2·10−2 atm) due to the appearance of proton conductivity. The sample La2Sc0.9Ca0.1ZnO5.45 showed a conductivity of about 10−5 S∙cm−1 at 350 °C (pH2O = 2·10−2 atm). A comparison of conductivities of obtained phase La2ScZnO5.5 with the conductivities of La2AlZnO5.5 and La2InZnO5.5 was made; the nature of the B-cation did not significantly affect the total conductivity.