The design of new oxide compounds that can be used as oxygen- or proton-conducting electrolytes for solid oxide fuel cells is actively in progress. Despite the intensive research activities regarding electrolytes with perovskite/fluorite structures, the search for other structural alternatives is of paramount importance. In this study we focus on a novel material with significantly improved properties for the electrochemical purposes. The two-layered BaNd₂In₂O₇ perovskite with a Ruddlesden–Popper structure was investigated as a protonic conductor for the first time. In detail, its local structure, water uptake, and the ionic (O²⁻, H⁺) conductivity were comprehensively studied. The nature of rare-earth elements (M = La, Nd) in the structure of BaM₂In₂O₇ on the structural and transport properties was revealed. The presented analysis showed that the composition of BaNd₂In₂O₇ is nearly pure proton conductor below 350 °C. This work opens up a new way in the design of protonic conductors with double-layered perovskite structure.

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 La₂ScZnO_5.5 and doped phases La_1.9Ca_0.1ScZnO_5.45, La₂Sc_0.9Ca_0.1ZnO_5.45, and La₂Sc_0.9Mg_0.1ZnO_5.45 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 La₂ScZnO_5.5 < La₂Sc_0.9Mg_0.1ZnO_5.45 < La₂Sc_0.9Ca_0.1ZnO_5.45 ≈ La_1.9Ca_0.1ScZnO_5.45 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 σLa₂ScZnO_5.5 < σLa₂Sc_0.9Mg_0.1ZnO_5.45 < σLa_1.9Ca_0.1ScZnO_5.45 < σLa₂Sc_0.9Ca_0.1ZnO_5.45, the activation energy decreased in the same sequence. The sample La₂Sc_0.9Ca_0.1ZnO_5.45 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 La₂Sc_0.9Ca_0.1ZnO_5.45 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 La₂ScZnO_5.5 with the conductivities of La₂AlZnO_5.5 and La₂InZnO_5.5 was made; the nature of the B-cation did not significantly affect the total conductivity.