Synthesis and characterization of dense, rare-earth based high entropy fluorite thin films

High entropy oxides (HEOs) with 5 or more cations in equimolar proportions that result in a phase-pure material, are a new class of materials attracting a lot of attention in recent years. HEOs exhibit interesting optical, electrochemical, magnetic and catalytic properties. To get a comprehensive understanding of the physics behind the complex interactions taking place in these materials, it is important to evaluate the material in (near-fully) dense forms, such as pellets or thin films. The fluorite structured high entropy oxide, (CeLaSmPrY)O2−x has been investigated only in the powder form and there are no studies on the dense form of fluorite (CeLaSmPrY)O2−x. One of the main reasons is that (CeLaSmPrY)O2−x undergoes a structural transition from fluorite to bixbyite (at 1000 °C) and typically temperatures above the transition (>1200 °C) are required for achieving high densities via conventional sintering. In this study, we synthesize dense films of fluorite structured (CeLaSmPrY)O2−x by sol-gel as well as pulsed laser deposition processes. The films synthesized via sol-gel process exhibit equiaxed grains and polycrystalline morphology, whereas columnar and epitaxial films are obtained using pulsed laser deposition. Thus, microstructural tuning of dense fluorite (CeLaSmPrY)O2−x films has been demonstrated while maintaining the basic characteristics of the HEO as observed in the powder form, therefore, paving the way towards more comprehensive studies for possible applications

Phase–Property Diagrams for Multicomponent Oxide Systems toward Materials Libraries

Exploring the vast compositional space offered by multicomponent systems or high entropy materials using the traditional route of materials discovery, one experiment at a time, is prohibitive in terms of cost and required time. Consequently, the development of high-throughput experimental methods, aided by machine learning and theoretical predictions will facilitate the search for multicomponent materials in their compositional variety. In this study, high entropy oxides are fabricated and characterized using automated high-throughput techniques. For intuitive visualization, a graphical phase–property diagram correlating the crystal structure, the chemical composition, and the band gap are introduced. Interpretable machine learning models are trained for automated data analysis and to speed up data comprehension. The establishment of materials libraries of multicomponent systems correlated with their properties (as in the present work), together with machine learning-based data analysis and theoretical approaches are opening pathways toward virtual development of novel materials for both functional and structural applications

Elucidation of the Transport Properties of Calcium‐Doped High Entropy Rare Earth Aluminates for Solid Oxide Fuel Cell Applications

Solid oxide fuel cells (SOFCs) are paving the way to clean energy conversion,relying on efficient oxygen-ion conductors with high ionic conductivitycoupled with a negligible electronic contribution. Doped rare earth aluminatesare promising candidates for SOFC electrolytes due to their high ionicconductivity. However, they often suffer from p-type electronic conductivity atoperating temperatures above 500°C under oxidizing conditions caused bythe incorporation of oxygen into the lattice. High entropy materials are a newclass of materials conceptualized to be stable at higher temperatures due totheir high configurational entropy. Introducing this concept to rare earthaluminates can be a promising approach to stabilize the lattice by shifting thestoichiometric point of the oxides to higher oxygen activities, and thereby,reducing the p-type electronic conductivity in the relevant oxygen partialpressure range. In this study, the high entropy oxide (Gd,La,Nd,Pr,Sm)AlO3issynthesized and doped with Ca. The Ca-doped (Gd,La,Nd,Pr,Sm)AlO3compounds exhibit a higher ionic conductivity than most of thecorresponding Ca-doped rare earth aluminates accompanied by a reduction ofthe p-type electronic conductivity contribution typically observed underoxidizing conditions. In light of these findings, this study introduces highentropy aluminates as a promising candidate for SOFC electrolytes

High Entropy Approach to Engineer Strongly Correlated Functionalities in Manganites

Technologically relevant strongly correlated phenomena such as colossal magnetoresistance (CMR) and metal-insulator transitions (MIT) exhibited by perovskite manganites are driven and enhanced by the coexistence of multiple competing magneto-electronic phases. Such magneto-electronic inhomogeneity is governed by the intrinsic lattice-charge-spin-orbital correlations, which, in turn, are conventionally tailored in manganites via chemical substitution, charge doping, or strain engineering. Alternately, the recently discovered high entropy oxides (HEOs), owing to the presence of multiple-principal cations on a given sub-lattice, exhibit indications of an inherent magneto-electronic phase separation encapsulated in a single crystallographic phase. Here, the high entropy (HE) concept is combined with standard property control by hole doping in a series of single-phase orthorhombic HE-manganites (HE-Mn), (Gd$_{0.25}$La$_{0.25}$Nd$_{0.25}$Sm$_{0.25}$)$_{1-x}$Sr$_x$MnO$_3$ (x = 0–0.5). High-resolution transmission microscopy reveals hitherto-unknown lattice imperfections in HEOs: twins, stacking faults, and missing planes. Magnetometry and electrical measurements infer three distinct ground states—insulating antiferromagnetic, unpercolated metallic ferromagnetic, and long-range metallic ferromagnetic—coexisting or/and competing as a result of hole doping and multi-cation complexity. Consequently, CMR ≈1550% stemming from an MIT is observed in polycrystalline pellets, matching the best-known values for bulk conventional manganites. Hence, this initial case study highlights the potential for a synergetic development of strongly correlated oxides offered by the high entropy design approach

High‐Entropy Sulfides as Highly Effective Catalysts for the Oxygen Evolution Reaction

With respect to efficient use of diminishing or harder to reach energy resources, the catalysis of processes that will otherwise require high overpotentials is a very important application in today\u27s world. As a newly developed class of materials, high-entropy sulfides (HESs) are promising electrocatalysts for a variety of different reactions. In this report, HESs containing five or six transition metals are synthesized in a one-step mechanochemical process. Seven HESs of Pnma (M:S≈1:1) and three Pa-3 (M:S = 1:2) structures are investigated as electrocatalysts for the oxygen evolution reaction (OER). The performances and properties of the HESs with different compositions and structures are compared with each other and with commercial IrO2 as reference material, in terms of OER overpotential, Tafel slope, electrochemically active surface area, ionic conductivity, and durability. The structural and chemical properties of these HESs are determined by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and energy-dispersive X-ray spectroscopy. Most of the HESs show excellent and promising performance as OER electrocatalysts under alkaline conditions, and outperform the reference OER catalyst IrO2

High‐Entropy Sulfides as Highly Effective Catalysts for the Oxygen Evolution Reaction

With respect to efficient use of diminishing or harder to reach energy resources, the catalysis of processes that will otherwise require high overpotentials is a very important application in today's world. As a newly developed class of materials, high‐entropy sulfides (HESs) are promising electrocatalysts for a variety of different reactions. In this report, HESs containing five or six transition metals are synthesized in a one‐step mechanochemical process. Seven HESs of Pnma (M:S≈1:1) and three Pa‐3 (M:S = 1:2) structures are investigated as electrocatalysts for the oxygen evolution reaction (OER). The performances and properties of the HESs with different compositions and structures are compared with each other and with commercial IrO₂ as reference material, in terms of OER overpotential, Tafel slope, electrochemically active surface area, ionic conductivity, and durability. The structural and chemical properties of these HESs are determined by X‐ray diffraction, transmission electron microscopy, scanning electron microscopy, X‐ray photoelectron spectroscopy, and energy‐dispersive X‐ray spectroscopy. Most of the HESs show excellent and promising performance as OER electrocatalysts under alkaline conditions, and outperform the reference OER catalyst IrO₂

Influence of Zr-doping on the structure and transport properties of rare earth high-entropy oxides

Fluorite-type ceria-based ceramics are well established as oxygen ion conductors due to their high conductivity, superseding state-of-the-art electrolytes such as yttria-stabilized zirconia. However, at a specific temperature and oxygen partial pressure they occasionally exhibit electronic conduction attributed to polaron hopping via multivalent cations (e.g. Pr and Ce). (Ce, La, Pr, Sm, Y)O _2− _δ is a high-entropy oxide with a fluorite-type structure, featuring low concentrations of multivalent cations that could potentially mitigate polaron hopping. However, (Ce, La, Pr, Sm, Y)O _2− _δ undergoes a structural transition to the bixbyite-type structure above 1000 °C. In this study, we introduce Zr doping into (Ce, La, Pr, Sm, Y)O _2− _δ to hinder the structural transition at elevated temperatures. Indeed, the fluorite structure at elevated temperatures is stabilized at approximately 10 at.% Zr doping. The total conductivity initially increases with doping, peaking at 5 at.% Zr doping, and subsequently decreases with further doping. Interestingly, electronic conductivity in (Ce, La, Pr, Sm, Y) _1− _x Zr _x O _2− _δ under oxidizing atmospheres is not significant and is lowest at 8 at.% Zr. These results suggest that ceria-based high-entropy oxides can serve as oxygen ion conductors with a significantly reduced electronic contribution. This work paves the way for new compositionally complex electrolytes as well as protective coatings for solid oxide fuel cells

High‐entropy sulfides as highly effective catalysts for the oxygen evolution reaction

With respect to efficient use of diminishing or harder to reach energy resources, the catalysis of processes that will otherwise require high overpotentials is a very important application in today's world. As a newly developed class of materials, high-entropy sulfides (HESs) are promising electrocatalysts for a variety of different reactions. In this report, HESs containing five or six transition metals are synthesized in a one-step mechanochemical process. Seven HESs of Pnma (M:S≈1:1) and three Pa-3 (M:S = 1:2) structures are investigated as electrocatalysts for the oxygen evolution reaction (OER). The performances and properties of the HESs with different compositions and structures are compared with each other and with commercial IrO2 as reference material, in terms of OER overpotential, Tafel slope, electrochemically active surface area, ionic conductivity, and durability. The structural and chemical properties of these HESs are determined by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and energy-dispersive X-ray spectroscopy. Most of the HESs show excellent and promising performance as OER electrocatalysts under alkaline conditions, and outperform the reference OER catalyst IrO2

High‐Entropy Sulfides as Highly Effective Catalysts for the Oxygen Evolution Reaction

With respect to efficient use of diminishing or harder to reach energy resources, the catalysis of processes that will otherwise require high overpotentials is a very important application in today's world. As a newly developed class of materials, high‐entropy sulfides (HESs) are promising electrocatalysts for a variety of different reactions. In this report, HESs containing five or six transition metals are synthesized in a one‐step mechanochemical process. Seven HESs of Pnma (M:S≈1:1) and three Pa‐3 (M:S = 1:2) structures are investigated as electrocatalysts for the oxygen evolution reaction (OER). The performances and properties of the HESs with different compositions and structures are compared with each other and with commercial IrO2 as reference material, in terms of OER overpotential, Tafel slope, electrochemically active surface area, ionic conductivity, and durability. The structural and chemical properties of these HESs are determined by X‐ray diffraction, transmission electron microscopy, scanning electron microscopy, X‐ray photoelectron spectroscopy, and energy‐dispersive X‐ray spectroscopy. Most of the HESs show excellent and promising performance as OER electrocatalysts under alkaline conditions, and outperform the reference OER catalyst IrO2