Structure and electrical conductivity of compositionally complex double perovskite cobaltites

Abstract

In this study, compositionally complex cobaltites with the general formula BaLnCo2O6−δ with three to eight different lanthanides at the Ln-site were synthesized using the solid-state reaction method and studied. Analysis of entropy metrics and configurational entropy calculations indicated that these compounds are medium entropy oxides. All of these crystallize as tetragonal double perovskites from the space group P4/mmm. The unit cell parameters are controlled by the average ionic radius, not the configurational entropy. On the other hand, the oxygen non-stoichiometry is consistently higher than in the case of low entropy double perovskite cobaltites. The total electrical conductivity of all materials in studied conditions is well above 50 S/cm, peaking at 1487 S/cm for BaLa1/3Nd1/3Gd1/3Co2O6−δ at 300°C. The electrical conductivity decreases with the number of substituents.

1. Introduction

High entropy materials have attracted considerable attention in recent years. It began with high entropy alloys, in which at least five different chemical elements were incorporated into a single lattice site to create a material with high configurational entropy [1]. The predicted benefits of this approach were the potential to develop new types of materials with intriguing properties stemming from the entropy stabilisation of new phases, strain induced by size mismatch due to the presence of multiple elements in one lattice, or other types of synergistic effects arising from compositional complexity. Subsequently, the high entropy approach was extended to inorganic compounds, such as oxides [2]. This led to the synthesis of novel types of ionic conductors [3–5], dielectrics [6], catalysts [7–8], and magnetic materials [9–10].

The materials studied in this work are oxides from the group of BaLnCo2O6−δ layered perovskites. This is because these materials exhibit interesting electrical properties that yield them promising for electrochemical applications. Generally, BaLnCo2O6−δ oxides are mixed ionic–electronic conductors [11–18]. The defects with the highest concentration in these compounds are primarily electron holes and oxygen vacancies [13,19]. The formation of protonic defects in wet conditions has also been reported for selected compositions within this group [11–12,20–21], thereby expanding the potential range of ionic charge carriers for partial protonic conductivity. The partial protonic conductivity of 10−5 S/cm was confirmed for at least one material from this group [21]. Another interesting aspect of these cobaltites is that they can exsolve nanoparticles in oxidizing conditions, which opens a possibility for enhancing catalytic activity [22–26]. Performance studies demonstrate that these materials can be utilized in well-performing high-temperature fuel cells or electrolyzers [13,27]. The cobaltites have been reported to be stable in pressurized steam [28], and their fabrication can be easily adjusted to optimize microstructure [29–30], which further signifies potential for eventual commercial use. All in all, the BaLnCo2O6−δ layered perovskites constitute an intriguing group of materials, and the introduction of compositional complexity may result in the formation of new materials with unforeseen properties.

It is important to note that the effect of introducing multiple elements into the structure of metals differs from the same approach applied to oxides. Introducing five different elements in equimolar proportions into a single lattice site in a metal is sufficient to yield configurational entropy above 1.5R (R—the universal gas constant), which is regarded as a threshold that must be met to classify a material as “high entropy” [1]. Every oxide possesses at least two different crystallographic sites; therefore, one must consider the changes in compositional complexity across each of the sublattices and then evaluate its effect on the entropy of the entire crystal structure. Dippo and Vecchio [31] studied this issue in detail and introduced a formula including the configurational entropy effects of each sublattice. They introduced a concept of entropy metrics (EM) that includes the effect coming from multiple sublattices by adding the total number of sublattices. The EM should be greater than 1.5 for high-entropy materials or above 1.0 for materials with middle entropy. The formulas used to calculate each factor are provided as follows: S config = −R∑ S ∑ iaS XS i ln(XS i ) ∑ SaS (1) EM = S config R · L (2) S config XS i where —configurational entropy; R—universal gas constant; aS—number of sites on the S sublattice; —site fraction of element species i; L—the total number of sublattices. It turns out that introducing even several elements into one sublattice for oxides is not sufficient.