The road to high-entropy: Combinatorial sequential doping study of Ba2In2O5

Abstract

Ba₂In₂O₅ is a rare-earth free material extensively explored for its remarkable oxygen-ion conductivity properties, highly sought after in the field of solid-state ionic and related devices. These properties can be activated at lower temperatures, and augmented with mixed ionic-electronic conductivity, by stabilization of its cubic perovskite phase through a variety of doping on the indium site. Following this reported flexibility, this study attempts a systematic doping of Ba₂In₂O₅ with a variety of metals (Co, Cr, Fe, Mn, Ni, Sn, Ti, V, Zr), with incremental steps approached in a combinatorial fashion; starting from one dopant, we provide various combinations of 2, 3, and 4 dopant options, with the latter representing the first high-entropy perovskite doping attempt on this system. Out of the 48 attempted composition, 8 yielded novel stable Pm-3m perovskites, of which one high-entropy perovskite and two medium-entropy perovskites; additionally, three more composition in medium entropy range and one more composition in high entropy range approached stability with minimal splitting. Through thermogravimetric analysis, all cubic compounds showcased reversible evolution of oxygen upon heating. The phase stabilization of a single phase multi-doped system appears to be the result of a complex relationship between element sizes, charges and orbital configurations. While increases in entropy of the indium sublattice were always accompanied by reduced oxygen mobility onset temperatures, we critically assessed the pairwise interaction between different metals and their impact on mobile oxygen quantities in different temperature ranges (500 to 900 °C). Is high-entropy always achievable in a system with appropriate flexibility? If so, it is actually beneficial in its impact on the materials properties? The results of this study highlight interesting avenues for compositionally complex oxides.