BaTbO ₃ -Based Perovskites with Different Configurational Entropy: Structure, Hygroscopicity, and Proton Conductivity

Acceptor-substituted BaTbO 3 -based perovskites differing in configurational entropy have been prepared using mechanical activation in a high-energy eccentric vibrating mill, followed by firing in the range 1100–1500 °C for 4 h. In this work, the low-entropy perovskites (LEP) with Δ S config = 0.822R, the medium- and high-entropy perovskites with Δ S config = 1.317R (MEP) and Δ S config = 1.552R (HEP) were synthesized, respectively. The structure of the solid solutions has been studied by X-ray diffraction, their microstructure has been examined by SEM, and their proton conductivity has been determined using impedance spectroscopy in dry and wet atmospheres (air, oxygen, and argon). In addition, the materials have been characterized by differential scanning calorimetry, thermogravimetry, X-ray photoelectron spectroscopy, and temperature-programmed reduction. A relation between the structure and proton conductivity of acceptor-substituted barium terbate-based solid solutions has been found for the first time. Stabilization of a tetragonal (sp. gr. I 4/ mcm ) or cubic (sp. gr. Pm -3 m ) perovskite phase has been shown to depend on the doping level. The LEP perovskites with a low degree of substitution x = 0.2 have been shown to have a tetragonal structure (sp. gr. I 4/ mcm ). The MEP and HEP perovskites with a degree of substitution x = 0.4 have a cubic structure (sp. gr. Pm –3 m ). The impact of the entropy factor has been shown for medium- and high-entropy perovskites with the same degree of substitution x = 0.4: BaTb 0.6 (5Ln) 0.4 O 3−δ and BaTb 0.6 (9Ln) 0.4 O 3−δ . The high Δ S config led to the synthesis of a nanopowder with a crystallite of ∼18–23 nm, with high hygroscopicity, and no sintering is observed up to 1500 °C. However, the MEP exhibits higher sinterability than LEP solid solutions. LEP and MEP barium terbates ceramics with a total conductivity of 5.5 × 10 –4 and 7.5 × 10 –4 S cm –1 have a broad electrolytic domain, extending to an oxygen partial pressure as high as 1 atm at ∼410 °C. The proton conductivity of the cubic BaTbO 3 -based MEP solid solutions has been shown for the first time to exceed that of the LEP tetragonal perovskites by a factor of 1.5–2.