Key Takeaways & Executive Findings
- •• A novel high-entropy oxide (La0.2Gd0.2Tm0.2Lu0.2Y0.2)3.12Ti0.88O6.44 was synthesized to isolate the ionic conductivity component in a predominantly hole-conducting Tb-based titanate. • The high-entropy analogue exhibited proton conductivity of ~7 × 10−6 S/cm at 600°C, confirmed by an isotope effect with D2O showing lower conductivity than H2O. • The parent Tb3.12Ti0.88O6.44 showed maximum hole conductivity of ~22 S/cm at 600°C, highlighting the challenge of separating ionic contributions in mixed conductors. • This work provides a strategy for detecting and quantifying ionic conductivity in electronic conductors, relevant for SOFC/SOEC materials development.
Abstract
A series of solid solutions with high content of Tb2O3–(TbxTi1−x)4O8−2x (x = 0.667–0.830) are synthesized in the Tb2O3–TiO2 system via co-precipitation and/or mechanical activation. This is followed by high-temperature annealing for 4–22 h. The X-ray diffraction method showed that the fluorite structure was realized for (TbxTi1−x)4O8−2x (x = 0.75–0.817). The solid solution Tb3.12Ti0.88O6.44 (64mol% Tb2O3 (x = 0.78)) with a fluorite structure exhibited a maximum hole conductivity of ~22 S/cm at 600°C. To separate the ionic component of the conductivity in the electronic conductor Tb3.12Ti0.88O6.44, its high entropy analogue, (La0.2Gd0.2Tm0.2Lu0.2Y0.2)3.12Ti0.88O6.44, was synthesized in which all rare-earth elements (REE) cations exhibited valency of +3. Consequently, the contribution of ionic (proton) conductivity (~7 × 10−6 S/cm at 600°C) was revealed with respect to the background of dominant hole conductivity. The proton conductivity of high-entropy oxide (HEО) (La0.2Gd0.2Tm0.2Lu0.2Y0.2)3.12Ti0.88O6.44 was confirmed by the detection of the isotope effect, where the mobility of the heavier O–D ions was lower than that of the O–H hydroxyls, resulting in lower conductivity in D2O vapors when compared to H2O.
1. Introduction
Currently, the search for new materials with high oxygen ion or proton conductivity for solid oxide fuel cells (SOFC) with oxygen- and proton-conducting electrolytes is ongoing. The most reliable oxygen-ion electrolyte for SOFCs is yttrium-stabilized zirconia (YSZ) owing to its availability and high ionic conductivity. High-temperature solid oxide electrolysis cells (SOECs) based on YSZ have been proposed for the large-scale production of H2 [1–3]. The most commonly used anode material for SOFCs and electrolysis cells is Ni-YSZ cermet. As in the past, active searches are ongoing for new cathode materials as an alternative to La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) [4–7].
Investigations of oxygen ion and proton conductivity have been carried out for a long time, mainly in R2O3–ZrO2 systems (R = La–Lu, Y, Sc). First, ZrO2-based ceramics doped with rare-earth elements (REE) oxides with a low degree of substitution up to 12mol% R2O3 were investigated. Among these oxides, ceramics with high oxygen-ion conductivity, including 8mol% Y2O3 stabilized ZrO2 (8YSZ) [8–9] and scandia-stabilized zirconia (ScSZ), are of practical interest [10–12]. Yttria-stabilized zirconia (6mol%–8mol% Y2O3 or 6YSZ–8YSZ) is currently used as a material for thermal barrier coatings (TBCs) to protect the high-temperature parts of aeroengines and gas turbines [13]. Solid solutions of HfO2 with a low degree of substitution by REE oxides exhibit low oxygen ion conductivities [14–15].
In R2O3–ZrO2 systems (R = La–Lu, Y, Sc) with a higher R2O3 content, compounds with a pyrochlore or fluorite structure with the nominal formula R2Zr2O7 are formed. Although ceramics based on rare-earth zirconates, termed as oxygen-ion and proton conductors [16–21], have attracted significant research attention, high oxygen-ion conductivity comparable to those of zirconates have been found in pure and acceptor-doped rare-earth titanates and hafnates [22–23]. For a long time, proton conductivity was associated only with pyrochlores zirconates acceptor-doped into the lanthanide sublattice [16,24–25]. Based on pyrochlores titanates (Ln2Ti2O7), obtaining solid solutions is possible, but the degree of substitution when doped with lanthanides in the Ti position does not usually exceed 0.1. Furthermore, at high degrees of substitution up to composition Ln2TiO5, nano- and micro-impurities have been detected via spectroscopy [17,26–27], which are either fluorite phase or hexagonal phase, depending on the ionic radius of REE cation. A recent study of REE titanates and hafnates with fluorite structure and high REE content up to 62mol% showed that titanates with fluorite structure (LnxTi1−x)4O8−2x (Ln = Yb, Er, Ho, 0.667 ≤ x ≤ 0.765) as well as hafnates Gd2HfO5 and Gd1.9La0.1HfO5 exhibit proton conductivity [17,28]. The proton contribution in titanates (LnxTi1−x)4O8−2x (Ln = Yb, Er, Ho, 0.667 ≤ x ≤ 0.765) was observed to increase with increasing ionic radius of the lanthanide, and the highest value was obtained for holmium-containing solid solutions [17]. The synthesis of (La0.2Gd0.2Ho0.2Lu0.2Y0.2)2ZrO5, a high-entropy analog of Gd2ZrO5, was successful [29]. Gd2ZrO5 is not a single-phase material when it is synthesized at temperatures up to 1600°C, unlike Gd2HfO5 [29], and it always contains an admixture of bixbyite [30–33]. The use of ...
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A.V. Shlyakhtina, E.D. Baldin, N.V. Gorshkov, D.N. Stolbov, N.V. Lyskov (2025). High-entropy oxide ceramics for detecting the ionic conductivity component in electron conductors. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-025-3206-7
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Frequently Asked Questions
What is the main objective of this study?
The study aims to detect and quantify the ionic (proton) conductivity component in a predominantly hole-conducting Tb-based titanate by synthesizing a high-entropy oxide analogue with all +3 rare-earth cations.
How was the proton conductivity confirmed?
Proton conductivity was confirmed by observing an isotope effect: the conductivity in D2O vapor was lower than in H2O, indicating that heavier O–D ions have lower mobility than O–H hydroxyls.
What is the significance of the high-entropy oxide approach?
The high-entropy oxide approach allows the separation of ionic and electronic conductivity contributions by replacing the mixed-valent Tb cations with a combination of stable +3 rare-earth cations, thereby eliminating hole conductivity and revealing the intrinsic ionic conductivity.
What are the potential applications of this research?
This research is relevant for developing materials for solid oxide fuel cells (SOFCs) and electrolysis cells, particularly for understanding and optimizing mixed ionic-electronic conductors used in electrodes and electrolytes.
What was the maximum hole conductivity observed in the parent material?
The parent material Tb3.12Ti0.88O6.44 exhibited a maximum hole conductivity of approximately 22 S/cm at 600°C.
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