High entropy oxide

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Structure of high-entropy oxide (MgNiCoCuZn)0.2O with site occupancies shown. Oxygen atoms are shown in red. Rock Salt HEO (MgNiCoCuZn)O With Site Occupancies.png
Structure of high-entropy oxide (MgNiCoCuZn)0.2O with site occupancies shown. Oxygen atoms are shown in red.

High-entropy oxides (HEOs) are complex oxides that contain five or more principal metal cations and have a single-phase crystal structure. The first HEO, (MgNiCuCoZn)0.2O in a rock salt structure, was reported in 2015 by Rost et al. [1] HEOs have been successfully synthesized in many structures, including fluorites, [2] perovskites, and spinels. [3] HEOs are currently being investigated for applications as functional materials. [3] [4] [5]

Contents

History

In the realm of high-entropy materials, HEOs are preceded by high-entropy alloys (HEAs), which were first reported by Yeh et al. in 2004. [6] HEAs are alloys of five or more principal metallic elements. Some HEAs have been shown to possess desirable mechanical properties, such as retaining strength/hardness at high temperatures. [7] HEA research substantially accelerated in the 2010s. [8]

The first HEO, (MgNiCuCoZn)0.2O in a rock salt structure, was reported in 2015 by Rost et al. [1] Similar to HEAs, (MgNiCuCoZn)0.2O is a multicomponent single-phase material. The cation site in (MgNiCuCoZn)0.2O material is compositionally disordered, similar to HEAs. However, unlike HEAs, (MgNiCuCoZn)0.2O contains an ordered anion sublattice. Following the discovery of HEOs in 2015, the field rapidly expanded. [3] [4]

Since the discovery of HEOs, the field of high-entropy materials has expanded to include high-entropy metal diborides, high-entropy carbides, high-entropy sulfides, and high-entropy alumino-silicides. [4]

Predicting HEO Formation

Principle of Entropy Stabilization

The formation of HEOs is based on the principle of entropy stabilization. Thermodynamics predicts that the structure which minimizes Gibbs free energy for a given temperature and pressure will form. The formula for Gibbs free energy is given by:

where G is Gibbs free energy, H is enthalpy, T is absolute temperature, and S is entropy. It can clearly be seen from this formula that a large entropy reduces Gibbs free energy and thus favors phase stability. It can also be seen that entropy becomes increasingly important in determining phase stability at higher temperatures. In a multi-component system, one component of entropy is the entropy of mixing (). For an ideal mixture, takes the form:

where R is the ideal gas constant, n is the number of components, and ci is the atomic fraction of component i. The value of increases as the number of components increases. For a given number of components, is maximized when the atomic fractions of the components approach equimolar amounts.

Evidence for entropy stabilization is given by the original rock salt HEO (MgNiCuCoZn)0.2O. Single-phase (MgNiCuCoZn)0.2O may be prepared by solid-state reaction of CuO, CoO, NiO, MgO, and ZnO. [1] Rost et al. reported that under solid state reaction conditions that produce single-phase (MgNiCuCoZn)0.2O, the absence of any one of the five oxide precursors will result in a multi-phase sample, [1] suggesting that configurational entropy stabilizes the material.

Other Considerations

It can clearly be seen from the formula for Gibbs free energy that enthalpy reduction is another important indicator of phase stability. For an HEO to form, the enthalpy of formation must be sufficiently small to be overcome by configurational entropy. Furthermore, the discussion above assumes that the reaction kinetics allow for the thermodynamically favored phase to form.

Synthesis Methods

Solid-State Reaction

Bulk samples of HEOs may be prepared by the solid-state reaction method. In this technique, oxide precursors are ball milled and pressed into a green body, which is sintered at a high temperature. The thermal energy provided accelerates diffusion within the green body, allowing new phases to form within the sample. Solid-state reactions are often carried out in the presence of air to allow oxygen-rich and oxygen-deficient mixtures to release and absorb oxygen from the atmosphere, respectively. Oxide precursors are not required to have the same crystal structure as the desired HEO for the solid-state reaction method to be effective. For example, CuO and ZnO may be used as precursors to synthesize (MgNiCuCoZn)0.2O. At room temperature, CuO has the tenorite structure and ZnO has the wurtzite structure.

Polymeric Steric Entrapment

Polymeric steric entrapment is a wet chemistry technique for synthesizing oxides. It is based on similar principles as the sol–gel process, which has also been used to synthesize HEOs. [9] [10] Polymeric steric entrapment requires water-soluble compounds containing the desired metal cation (e.g., metal acetates, metal chlorides) to be placed in a solution with water and a water-soluble polymer (e.g., PVA, PEG). In solution, the cations are thoroughly mixed and held close together by the polymer chains. [11] The water is driven off to produce a foam whose organic components are burned off with a calcining step, producing a fine and pure mixed oxide powder, [12] which may be pressed into a green body and sintered. This method was first reported by Nguyen et al. in 2011. [12] In 2017, Kriven and Tseng reported the first polymeric steric entrapment HEO synthesis. [13]

Polymeric steric entrapment can be used to synthesize bulk HEO samples that are difficult to successfully synthesize the solid-state method. For example, Musico et al. synthesized the high entropy cuprate (LaNdGdTbDy)0.4CuO4 using solid-state reaction and polymeric steric entrapment. [11] X-ray diffraction of the sample prepared with solid-state reaction showed small inclusions of a second phase, and energy-dispersive X-ray spectroscopy showed inhomogeneous distributions of some cations. Neither impurity peaks nor evidence of inhomogeneous cation distribution was found in the sample of this material prepared with polymeric steric entrapment.

Other Techniques

Other techniques that have been used to synthesize HEOs include:

HEO Materials

The first HEOs synthesized had the rock-salt structure. Since then, the family of HEOs has expanded to include perovskite, spinel, fluorite, and other structures. [22] [11] [23] [24] [25] [26] Some of these structures, such as the perovskite structure, are notable in that they have two cation sites, each of which may independently possess compositional disorder. For example, high entropy perovskites (GdLaNdSmY)0.2MnO3 (A-site configurational entropy), Gd(CoCrFeMnNi)0.2O3 (B-site configurational entropy), and (GdLaNdSmY)0.2(CoCrFeMnNi)0.2O3 (A-site and B-site configurational entropy) have been synthesized. [27] [28]

Examples of HEO materials and their crystal structures
StructureExampleReference
Rock Salt(MgNiCuCoZn)0.2ORost et al [1]
Fluorite(GdLaCeHfZr)0.2O2 and (GdLaYHfZr)0.202;

(CeZrHfSnTi)0.2O2

Anandkumar et al [29] ;

Chen et al [26]

Spinel(CoCrFeMnNi)0.6O4Dabrowa et al [23]
PerovskiteSr(ZrSnTiHfMn)0.2O3Jiang et al [24]
Pyrochlore(GdEuSmNdLa)0.4Zr2O7Teng et al [25]
Cuprate Perovskite(LaNdGdTbDy)0.4CuO4Musico et al [11]

Properties and Applications

In contrast to HEAs, which are typically investigated for their mechanical properties, HEOs are often studied as functional materials. The original HEO, (MgNiCuCoZn)0.2O, has been investigated as a promising material for applications in energy production and storage, e.g. as anode material in Li-ion batteries, [30] or as large k dielectric material, [31] or in catalysis. [32] [33]

Low Thermal Conductivity

It has been shown that increasing the configurational entropy of a material reduces its lattice thermal conductivity. [34] Correspondingly, HEOs typically have lower thermal conductivities than materials with the same crystal structure and only one cation per lattice site. [35] [36] The thermal conductivity of HEOs is usually greater than or comparable to the thermal conductivity of amorphous materials containing the same components. [3] However, crystalline materials typically have higher elastic moduli than amorphous materials of the same components. The combination of these factors leads to HEOs occupying a unique region of the property space by having the highest elastic modulus to thermal conductivity ratios of all materials. [35]

Property Tunability Through Cation Selection

HEOs enhance functional property tunability through cation selection. Magnetic, [37] [38] catalytic, [39] and thermophysical [40] properties may be tuned by modifying the cation composition of a given HEO. Many material applications demand a highly specific set of properties. For example, thermal barrier coatings require thermal expansion coefficient matching with a metal surface, high-temperature phase stability, low thermal conductivity, and chemical inertness, among other properties. [41] Due to their innate tunability, HEOs have been proposed as candidates for advanced material applications such as thermal barrier coatings. [40]

Terminology

The definition of high-entropy oxide is debated. In oxide literature, the term is commonly used to refer to any oxide with at least five principal cations. [42] However, it has been suggested that this is a misnomer, as most reports neglect to calculate configurational entropy. [42] Additionally, a survey of 10 HEOs found that only 3 were entropy-stabilized. [43] It has been suggested that the term HEO be replaced with three terms: compositionally complex oxide, high-entropy oxide, and entropy-stabilized oxide. [42] In this scheme, compositionally complex refers to materials with multiple elements occupying the same sublattice, high-entropy refers to materials where configurational entropy plays a role in stabilization, and entropy-stabilized refers to materials where entropy dominates the enthalpy term and is necessary for the formation of a crystalline phase.

See also

Related Research Articles

<span class="mw-page-title-main">Perovskite (structure)</span> Type of crystal structure

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<span class="mw-page-title-main">Yttrium barium copper oxide</span> Chemical compound

Yttrium barium copper oxide (YBCO) is a family of crystalline chemical compounds that display high-temperature superconductivity; it includes the first material ever discovered to become superconducting above the boiling point of liquid nitrogen [77 K ] at about 93 K.

<span class="mw-page-title-main">Zinc oxide</span> White powder insoluble in water

Zinc oxide is an inorganic compound with the formula ZnO. It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cement, lubricants, paints, sunscreens, ointments, adhesives, sealants, pigments, foods, batteries, ferrites, fire retardants, semi conductors, and first-aid tapes. Although it occurs naturally as the mineral zincite, most zinc oxide is produced synthetically.

<span class="mw-page-title-main">Solid oxide fuel cell</span> Fuel cell that produces electricity by oxidization

A solid oxide fuel cell is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Fuel cells are characterized by their electrolyte material; the SOFC has a solid oxide or ceramic electrolyte.

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<span class="mw-page-title-main">Ferrite (magnet)</span> Ferrimagnetic ceramic material composed of rust and a metallic element

A ferrite is a ceramic material made by mixing and firing iron(III) oxide with one or more additional metallic elements, such as strontium, barium, manganese, nickel, and zinc. They are ferrimagnetic, meaning they are attracted by magnetic fields and can be magnetized to become permanent magnets. Unlike other ferromagnetic materials, most ferrites are not electrically conductive, making them useful in applications like magnetic cores for transformers to suppress eddy currents. Ferrites can be divided into two families based on their resistance to being demagnetized.

<span class="mw-page-title-main">Nanorod</span>

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<span class="mw-page-title-main">Gallium(III) oxide</span> Chemical compound

Gallium(III) oxide is an inorganic compound and ultra-wide-bandgap semiconductor with the formula Ga2O3. It is actively studied for applications in power electronics, phosphors, and gas sensing. The compound has several polymorphs, of which the monoclinic β-phase is the most stable. The β-phase’s bandgap of 4.7–4.9 eV and large-area, native substrates make it a promising competitor to GaN and SiC-based power electronics applications and solar-blind UV photodetectors. The orthorhombic ĸ-Ga2O3 is the second most stable polymorph. The ĸ-phase has shown instability of subsurface doping density under thermal exposure. Ga2O3 exhibits reduced thermal conductivity and electron mobility by an order of magnitude compared to GaN and SiC, but is predicted to be significantly more cost-effective due to being the only wide-bandgap material capable of being grown from melt. β-Ga2O3 is thought to be radiation-hard, which makes it promising for military and space applications.

<span class="mw-page-title-main">Geikielite</span> Magnesium titanium oxide mineral

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The spinels are any of a class of minerals of general formulation AB
2X
4 which crystallise in the cubic (isometric) crystal system, with the X anions arranged in a cubic close-packed lattice and the cations A and B occupying some or all of the octahedral and tetrahedral sites in the lattice. Although the charges of A and B in the prototypical spinel structure are +2 and +3, respectively, other combinations incorporating divalent, trivalent, or tetravalent cations, including magnesium, zinc, iron, manganese, aluminium, chromium, titanium, and silicon, are also possible. The anion is normally oxygen; when other chalcogenides constitute the anion sublattice the structure is referred to as a thiospinel.

<span class="mw-page-title-main">Perovskite solar cell</span> Alternative to silicon-based photovoltaics

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<span class="mw-page-title-main">High-entropy alloy</span> Alloys with high proportions of several metals

High-entropy alloys (HEAs) are alloys that are formed by mixing equal or relatively large proportions of (usually) five or more elements. Prior to the synthesis of these substances, typical metal alloys comprised one or two major components with smaller amounts of other elements. For example, additional elements can be added to iron to improve its properties, thereby creating an iron-based alloy, but typically in fairly low proportions, such as the proportions of carbon, manganese, and others in various steels. Hence, high-entropy alloys are a novel class of materials. The term "high-entropy alloys" was coined by Taiwanese scientist Jien-Wei Yeh because the entropy increase of mixing is substantially higher when there is a larger number of elements in the mix, and their proportions are more nearly equal. Some alternative names, such as multi-component alloys, compositionally complex alloys and multi-principal-element alloys are also suggested by other researchers.

<span class="mw-page-title-main">Ceria-zirconia</span>

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<span class="mw-page-title-main">HKUST-1</span>

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Thermal shock synthesis (TSS) is a method in which materials are synthesized via rapid, high-temperature heating. In the TSS process, temperatures as high as 3000 K are applied for a duration of just seconds or milliseconds, followed by rapid cooling (a TSS image shown in Fig. 1). In this regard, TSS is distinct from conventional high-temperature syntheses that feature slow and near-equilibrium heating at limited temperature ranges (e.g., 1500 K for furnace heating) for extended periods of time (typically hours) and generally slow heating and cooling (~10 K/min).

<span class="mw-page-title-main">High-entropy-alloy nanoparticles</span>

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Nickel niobate is a complex oxide which as a solid material has found potential applications in catalysis and lithium batteries.

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