# High-Entropy Hexagonal Manganites for Fast Oxygen Absorption and Release

Frida Hemstad Danmo, Aamund Westermoen, Kenneth Marshall, Dragos Stoian, Tor Grande, Julia Glaum, Sverre M. Selbach

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Frida Hemstad Danmo, Aamund Westermoen, Kenneth Marshall, Dragos Stoian, Tor Grande, et al. High-Entropy Hexagonal Manganites for Fast Oxygen Absorption and Release. Chemistry of Materials, 2024, 36 (6), pp.2711–2720. 10.1021/acs.chemmater.3c02702. hal-05097668

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# High-Entropy Hexagonal Manganites for Fast Oxygen Absorption and Release

Frida Hemstad Danmo, Aamund Westermoen, Kenneth Marshall, Dragos Stoian, Tor Grande, Julia Glaum, and Sverre M. Selbach*

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Cite This: *Chem. Mater.* 2024, 36, 2711–2720

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## ACCESS

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Supporting Information

**ABSTRACT:** Hexagonal manganites (RMnO₃) display a large capacity for oxygen storage and release at temperatures below 400 °C. A challenging trade-off is that larger R³⁺ cations improve both the absorption capacity and the exchange kinetics but also destabilize the layered hexagonal structure with respect to the competing orthorhombic perovskite phase. Here, high-entropy RMnO₃ materials with 5 or 6 rare earth elements are prepared as both bulk and nanocrystalline materials, and the hexagonal phase is stabilized for compositions RMn₁-xTiₓO₃ where R = Y, Gd, and Er and x = 0, 0.15. These high-entropy hexagonal manganites show similar oxygen storage capacity as the best-known single or double R³⁺ compositions and greatly enhanced oxidation kinetics. Even bulk samples show significant oxygen absorption at rapid cooling rates of up to 20 K/min in O₂. Importantly, both the oxygen capacity and the oxidation kinetics are insensitive to the exact R³⁺ composition. This improves the commercial application potential of hexagonal manganites for oxygen storage and separation from air. Finally, sustainability implications are discussed.

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## INTRODUCTION

High-entropy chemical compositions are a rapidly emerging approach to design functional oxides. Following early studies of mechanical properties of metal alloys with 5 or more elements in single-phase solid solution,¹,² high entropy has later been explored for oxides and ceramics,³⁻⁹ e.g., rock salts,¹⁰ perovskites,¹¹,¹² oxyfluorides,¹³ and carbides.¹⁴ High-entropy compounds are usually defined by having a configurational entropy, S_config larger than 1.5R, where R is the universal gas constant, which is achieved by 5 or more equimolar elements on the same sublattice.¹⁵ High configurational entropy can stabilize single-phase materials even if the compositional end members are unstable.²,¹⁰ High ionic conductivity is a prominent example of enhanced properties in high-entropy oxides.⁴,⁵,¹⁶

Hexagonal manganites, RMnO₃+δ (R = Sc, Y, Ho–Lu), can reversibly absorb large amounts of oxygen as interstitial ions below 400 °C¹⁷⁻²¹ and are thus promising oxygen storage materials (OSMs) for oxygen separation, e.g., through chemical looping air separation (CLAS). Such applications require materials with a high reversible oxygen storage capacity (OSC), fast bulk ion transport, and oxygen surface exchange rates.²²⁻²⁴ However, the oxidation kinetics of these materials are generally slow, with time to full oxidation on the order of several hours.²⁵ This implies a smaller available OSC when using short dwell times or fast heating and cooling rates,²⁰,²⁵⁻³⁰ which are both necessary for efficient O₂ gas production by cyclic processes like CLAS.²³ Both the OSC and

oxidation kinetics of RMnO₃ are known to improve with larger R³⁺ cations, either as the sole R-site cation or as a substituted element in YMnO₃¹⁹,²⁵,²⁶,²⁸⁻³³ or through aliovalent doping, e.g., with Zr⁴⁺ on the R-site³⁴ or Ti⁴⁺ on the Mn³⁺ site.³⁰,³³,³⁵⁻³⁷

Unfortunately, larger rare earth cations destabilize the desired hexagonal layered phase with respect to the orthorhombic perovskite structure, which cannot accommodate interstitial oxygen.³⁸ The Goldschmidt tolerance factor, t = (r<sub>R</sub> + r<sub>O</sub>)/2¹/²(r<sub>Mn</sub> + r<sub>O</sub>), must be ≤0.855 (r<sub>R</sub>³⁺ ≤ r<sub>Ho</sub>³⁺) to favor the hexagonal P6₃<sub>cm</sub> structure, while for larger rare earths like Dy³⁺, the P6₃<sub>cm</sub> hexagonal structure becomes metastable with respect to the orthorhombic Pnma perovskite structure. Several strategies have been demonstrated to extend the stability range of the hexagonal structure, like epitaxial strain,³⁹ synthesis at high temperatures and low pO₂,¹⁷ or prereduction of an amorphous gel in a H₂ atmosphere prior to crystallization. The two latter examples give a smaller tolerance factor due to reduction of a fraction of Mn³⁺ to larger radius Mn²⁺, and the material is easily reoxidized at lower temperatures after high-temperature synthesis.

Received: October 20, 2023

Revised: February 16, 2024

Accepted: February 20, 2024

Published: March 1, 2024

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<table><caption>Table 1. Composition, Naming, and Tolerance Factor of Samples Used in This Study<sup>a</sup></caption><thead><tr><th>composition</th><th>nano-crystalline</th><th>bulk</th><th>Goldschmidt tolerance factor</th></tr></thead><tbody><tr><td>Y<sub>0.167</sub>Gd<sub>0.167</sub>Tb<sub>0.167</sub>Dy<sub>0.167</sub>Ho<sub>0.167</sub>Er<sub>0.167</sub>MnO<sub>3</sub></td><td>HEO1-n</td><td>HEO1-b</td><td>0.865</td></tr><tr><td>Y<sub>0.167</sub>Gd<sub>0.167</sub>Tb<sub>0.167</sub>Dy<sub>0.167</sub>Ho<sub>0.167</sub>Er<sub>0.167</sub>Mn<sub>0.85</sub>Ti<sub>0.15</sub>O<sub>3</sub></td><td>HEO2-n</td><td>HEO2-b</td><td>0.870</td></tr><tr><td>Y<sub>0.167</sub>Gd<sub>0.233</sub>Tb<sub>0.167</sub>Dy<sub>0.167</sub>Ho<sub>0.167</sub>Er<sub>0.1</sub>Mn<sub>0.85</sub>Ti<sub>0.15</sub>O<sub>3</sub></td><td>HEO3-n</td><td></td><td>0.871</td></tr><tr><td>Y<sub>0.145</sub>Gd<sub>0.210</sub>Tb<sub>0.190</sub>Dy<sub>0.165</sub>Ho<sub>0.155</sub>Er<sub>0.135</sub>Mn<sub>0.85</sub>Ti<sub>0.15</sub>O<sub>3</sub></td><td>HEO4-n</td><td></td><td>0.870</td></tr><tr><td>Y<sub>0.2</sub>Gd<sub>0.2</sub>Dy<sub>0.2</sub>Ho<sub>0.2</sub>Er<sub>0.2</sub>Mn<sub>0.85</sub>Ti<sub>0.15</sub>O<sub>3</sub></td><td>HEO5-n</td><td>HEO5-b</td><td>0.869</td></tr></tbody></table>

<sup>a</sup>Tolerance factors were calculated with C.N. = 8 on the R-site and C.N. = 5 on the Mn-site.

Here, we demonstrate single-phase hexagonal manganite materials with high-entropy compositions of RMnO<sub>3</sub>, where R = Y, Gd, and Er, with excellent oxygen storage capacity and outstanding oxidation kinetics. High configurational entropy favors single-phase hexagonal materials with oxygen storage capacities comparable to the best-reported single-R compounds. Importantly, the oxidation kinetics is significantly improved, especially for bulk materials, and both the capacity and kinetics are insensitive to the exact rare earth cation composition. The improved kinetics are discussed with respect to structural disorder resulting from high configurational entropy and the size of the R<sup>3+</sup> cations causing an expansion of the ab-plane. Finally, sustainability aspects are addressed.

## EXPERIMENTAL DETAILS

**Synthesis.** Bulk samples were prepared through solid-state synthesis by mixing R<sub>2</sub>O<sub>3</sub> (R = Y, Gd, Dy, Ho, Er, Mn), Tb<sub>4</sub>O<sub>7</sub>, and TiO<sub>2</sub> in stoichiometric amounts before pressing into pellets that were fired at 1450 °C for 1 h in air. Nanocrystalline samples were prepared using a modified citric acid synthesis method previously reported by Bergum et al.<sup>40</sup> and Danmo et al.,<sup>30,33</sup> with the composition and naming of all samples shown in Table 1. For the precursor solutions, metal acetates R(CH<sub>3</sub>CO<sub>2</sub>)<sub>3</sub>·xH<sub>2</sub>O (R = Y, Ho, Er, Dy, Tb, Mn) were each dissolved in a mixture of citric acid and deionized water and stirred on a hot plate set to 150 °C for 3 h. Titanium(IV) isopropoxide as the Ti<sup>4+</sup> precursor was added to a mixture of deionized water and citric acid. The precursor solutions were added in stoichiometric amounts together with ethylene glycol. The resulting solution was kept on a hot plate at 150 °C for 24–48 h until a viscous gel was formed. The gel was then dried at 120 °C for 3 days before calcination in air at 600 °C for 6 h, resulting in an amorphous powder. These powders were prereduced in 5% H<sub>2</sub> in N<sub>2</sub> at 250 °C for 10 h to promote the formation of the desired hexagonal structure,<sup>30</sup> and then crystallized at 900 °C in a N<sub>2</sub> atmosphere for 1 h. More details about the synthesis are given in the Supporting Information.

**Characterization.** X-ray diffraction (XRD) was done with a Bruker D8 Focus with Cu Kα radiation. Oxygen stoichiometry was studied by thermogravimetric analysis (TGA) with a Netzsch STA 449C Jupiter with 30 mL min<sup>-1</sup> O<sub>2</sub> gas flow. Mass changes were measured during heating to 800 °C and subsequent cooling to room temperature, using heating and cooling rates of 5, 10, or 20 °C min<sup>-1</sup>. δ values were calculated relative to a reference point where all Mn is assumed to be found as Mn<sup>3+</sup>, chosen as the point of lowest mass at T > 700 °C, corresponding to δ = 0 for undoped and δ = 0.075 for Ti-doped samples. High-temperature XRD of the samples HEO1-n and HEO2-n was done with a Bruker D8 ADVANCE with Cu Kα radiation. Data were first collected in O<sub>2</sub> at 25 °C intervals during heating to 725 °C, then upon subsequent cooling to 50 °C, and finally during reheating to 725 °C. This was repeated in a N<sub>2</sub> atmosphere. Pawley

refinements of lattice parameters and crystallite size were performed with the Bruker AXS TOPAS 5 software.<sup>41</sup>

Synchrotron HT-XRD and X-ray absorption near edge structure (XANES) measurements during in situ switching of the atmosphere from N<sub>2</sub> to O<sub>2</sub> was performed using BM31 at the Swiss-Norwegian Beamlines (SNBL) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Samples were mounted in quartz capillaries, and a 10 mL min<sup>-1</sup> gas flow was used for all experiments. The wavelength was 0.38591 Å for diffraction, while XANES fluorescence signal was measured at the Mn K-edge and the Tb L<sub>3</sub>-edge. Lattice parameters were determined by Pawley refinement,<sup>41</sup> and Larch<sup>42</sup> was used for normalization of XANES data. More details about the characterization are given in the Supporting Information.

## RESULTS

All nanocrystalline samples are phase pure after synthesis, and their diffractograms can be indexed with the hexagonal P6<sub>3</sub>cm space group, Figure S1, with trace amounts of the orthorhombic perovskite Pnma phase in some of the samples. The bulk HEO2-b and HEOS-b samples are also phase pure, while the HEO1-b sample contains both the P6<sub>3</sub>cm and Pnma phases. Refined lattice parameters (Table 2) for all nano-

<table><caption>Table 2. Sample Name, Refined Lattice Parameters, and Crystallite Sizes for As-Prepared Materials<sup>a</sup></caption><thead><tr><th>sample name</th><th>a (Å)</th><th>c (Å)</th><th>crystallite size (nm)</th></tr></thead><tbody><tr><td>HEO1-n</td><td>6.181(8)</td><td>11.412(0)</td><td>28 ± 1</td></tr><tr><td>HEO2-n</td><td>6.207(7)</td><td>11.382(2)</td><td>23 ± 1</td></tr><tr><td>HEO3-n</td><td>6.225(8)</td><td>11.374(0)</td><td>31 ± 1</td></tr><tr><td>HEO4-n</td><td>6.213(2)</td><td>11.411(6)</td><td>23 ± 1</td></tr><tr><td>HEOS-n</td><td>6.189(9)</td><td>11.406(5)</td><td>30 ± 1</td></tr><tr><td>HEO1-b</td><td>6.166(3)</td><td>11.382(0)</td><td></td></tr><tr><td>HEO2-b</td><td>6.246(3)</td><td>11.235(2)</td><td></td></tr><tr><td>HEOS-b</td><td>6.232(8)</td><td>11.250(0)</td><td></td></tr></tbody></table>

<sup>a</sup>X-ray diffraction data were refined within the P6<sub>3</sub>cm space group in all cases. Compositions are given in Table 1. Digits in parentheses are the first uncertain digits for each refined lattice parameter.

crystalline materials reveal expansion of the unit cell compared to undoped and single R<sup>3+</sup> cation RMnO<sub>3</sub>,<sup>30,33,43</sup> as expected from the larger average ionic radii of the cations incorporated in the present compositions.<sup>44</sup> The increasing size of the ab-plane is correlated to the increasing tolerance factors. The nanocrystalline samples have crystallite sizes ranging from 23 to 31 nm, as expected from the citric acid synthesis method,<sup>30,40</sup> while SEM images of the bulk powders (Figure S2) show particle sizes from tens to hundreds of microns in size. Different lattice parameters for nanocrystalline and bulk samples of the same composition are expected due to the size-dependence of lattice parameters<sup>40</sup> and the necessarily
<figure><img src="image_10.png" /><figcaption>**Figure 1.** Nanocrystalline samples. Oxygen stoichiometry during cooling as a function of temperature was measured with different cooling rates in O₂ atmosphere. The dashed horizontal line indicates the oxygen stoichiometry where all Mn and Tb is assumed to be found as Mn³⁺ and Tb³⁺, which corresponds to δ = 0 and δ = 0.075 for the undoped and Ti-doped samples, respectively. Corresponding data³³ for DyMnO₃ (DMO) (gray line) and HoMnO₃ (HMO) (dashed gray line) are plotted in (panel a), and for DyMn₀.₈₅Ti₀.₁₅O₃ (DMTO) (gray line) and HoMn₀.₈₅Ti₀.₁₅O₃ (HMTO) (dashed gray line) in (panel d).</figcaption></figure>

different thermal histories of as-prepared samples, which in turn give different oxygen stoichiometries, which also affect the lattice parameters.

We first investigate oxygen absorption by thermogravimetry (TGA) upon cooling in an O₂ atmosphere. All nanocrystalline materials absorb large amounts of oxygen at temperatures below 400 °C, resulting in δ ≈ 0.30 for HEO1-n, δ ≈ 0.35 for HEO2-n and HEO3-n, δ ≈ 0.37 for HEO4-n, and δ ≈ 0.32 for HEO5-n (Figure 1). Corresponding wt % values are 1.89, 2.22, 2.23, 2.34 and 2.05, and in µmol/g, the values are 1186, 1390, 1393, 1463 and 1280, respectively. Interestingly, all nanocrystalline materials are highly insensitive to the cooling rates investigated, which are limited to up to 20 °C/min by the instrument. This implies very fast oxidation kinetics compared to literature reports.²⁵,²⁸,³³ The samples containing both Ti and Tb (Figure 1b–d) show the highest oxygen storage capacities, as expected from previous studies of Ti and Tb in hexagonal manganites.²⁸,³⁰,³³,³⁵ The oxygen content of these samples is also above the δ = 0.33 limit for oxidation of 2/3 of Mn only, giving 2 oxygen interstitials per 30-atom unit cell.³⁸ Oxidation of Tb³⁺ to Tb⁴⁺ can charge compensate more oxygen interstitials than possible in samples with only “inert” R cations. Ti-doped samples (Figure 1b–e) show significant oxygen content at T > 350 °C, as Ti increases the thermal stability of interstitial oxygen,³⁰,³³ and a stable plateau is not reached at T > 700 °C, where all Mn is presumed to be Mn³⁺, indicating a small underestimation in the assumption of δ for these samples. Compared to our recent work on single-R

nanocrystalline materials,³⁰,³³ the effect of high entropy is much stronger for materials without Ti donor doping (Figure 1a) than for Ti-doped (Figure 1d). The HEO2-4 compositions, with much smaller amounts of expensive Dy, perform even better than the previously investigated DyMn₀.₈₅Ti₀.₁₅O₃.³⁰,³³

Next, we investigate the effect of the crystallite size by measuring oxygen absorption in bulk materials. The HEO2-b and HEO5-b samples measured with a cooling rate of at 5 °C min⁻¹ show a small decrease in oxygen storage capacity (Figure 2) compared to the isostoichiometric nanocrystalline materials in Figure 1, from δ = 0.35 to δ = 0.27 for HEO2 and from δ = 0.32 to δ = 0.22 for HEO5. There is also a small offset in the temperature ranges of oxidation and reduction for these samples, indicating that the materials are not fully oxidized due to kinetic limitations. HEO2-b has a subtly higher maximum oxygen content during cooling than HEO5-b, as this composition contains Tb, which likely increases the total oxygen storage capacity of the material.²⁸ The maximum oxygen content of both bulk samples decreases only subtly when increasing the heating and cooling rates, even if the particles are much larger (Figure S2) with much longer diffusion distances. This is highly surprising, considering the difference in kinetics of oxidation of bulk vs nanocrystalline materials reported for perovskite La₁-xSrxMnO₃.⁴⁵ Compared to literature reports²⁰,²⁵,²⁷⁻²⁹,³¹,⁴⁶ on single- or double-R hexagonal manganites, the insensitivity of the presented high-entropy bulk materials to cooling (and heating) rate is
<figure><img src="image_11.png" /><figcaption>**Figure 2.** Bulk samples. Oxygen stoichiometry as a function of temperature in O₂ atmosphere during heating (dashed lines) and cooling (solid lines) for samples (a) HEO2-b and (b) HEO5-b.</figcaption></figure>

unprecedented and implies very fast oxidation (and reduction) kinetics. For context, most studies of oxygen absorption in bulk hexagonal manganites have been performed with 0.1–1.0 °C/min cooling rates, with typically a strong reduction in the OSC observed upon increasing the cooling rate to 5 °C/min. The oxidation kinetics of bulk high-entropy materials can compete with those of nanocrystalline materials.

To investigate the effect of Ti-doping, the HEO1-n and HEO2-n samples were studied further, and their oxygen stoichiometry under different heating and cooling rates is shown in Figure 3. HEO1-n, without Ti⁴⁺, shows a small oxygen absorption at 500 °C during heating, highlighted in the inset of Figure 3a, which may stem from the oxidation of Tb³⁺ to Tb⁴⁺. During heating, HEO1-n shows less oxygen absorption with increasing heating rates and shows a visible hysteresis in oxygen content during heating and cooling, with a width of ~20 °C. The width of the hysteresis does not change with heating rate, as observed for single-R cation hexagonal manganites.³³ The temperature of maximum oxygen content during heating is subtly shifted upward for both samples with increasing heating rates. In contrast, the Ti-doped HEO2-n sample shows no thermal hysteresis in oxygen content, in accordance with previous findings for Ti⁴⁺-doped RMnO₃.³⁰,³³,³⁵ The oxygen absorption of HEO2-n is less sensitive to faster heating rates than that for HEO1-n.

When interstitial oxygen enters the crystal lattice upon oxidation, hexagonal manganites typically display an expansion of the ab-plane and a contraction in the c direction, as illustrated in Figure S3 for HEO1-n and HEO2-n. This implies that XRD can be used to indirectly probe the degree of oxidation through chemical expansion.³⁰ The relationship between lattice parameters and oxidation is further supported by XANES spectra revealing oxidation of Mn³⁺ to Mn⁴⁺ and of

<figure><img src="image_12.png" /><figcaption>**Figure 3.** Effect of Ti-doping. Oxygen stoichiometry in O₂ atmosphere measured during heating (dashed lines) and cooling (solid lines) using different heating and cooling rates for samples (a) HEO1-n and (b) HEO2-n. The horizontal dashed lines indicate the oxygen stoichiometry, where all Mn and Tb are assumed to be found as Mn³⁺ and Tb³⁺, respectively. The inset in (a) shows the small oxidation occurring at 500 °C during heating, which is attributed to the oxidation of Tb³⁺ to Tb⁴⁺. Cooling data³³ for DyMnO₃ (DMO) (panel a) and DyMn₀.₈₅Ti₀.₁₅O₃ (DMTO) (panel b) at 20 °C/min heating and cooling rates are included as gray dotted lines for comparison.</figcaption></figure>

Tb³⁺ to Tb⁴⁺ (Figure S4). Lattice parameters *a* and *c* of HEO1-n and HEO2-n from high-temperature XRD measurements are presented in Figure 4. The diffractograms for the HEO1-n sample were refined by using the R3c space group, while P6₃cm was used for HEO2-n. Both samples show a large contraction along the *c* axis below 400 °C in O₂, which is a signature of oxygen absorption. For HEO1-n, the largest changes in the *a* and *c* parameters occur between 325 and 350 °C, in accordance with the narrow temperature interval for oxygen absorption and release found from TGA (Figure 1a). A small hysteresis in the *a* parameter is visible at ~150 °C, which may indicate some oxidation below the bulk oxidation temperature ~300 °C. This could be due to the oxidation of a fraction of Tb³⁺ to Tb⁴⁺ at ~150 °C. Only thermal expansion is observed upon heating and cooling HEO1-n in a N₂ atmosphere.

The Ti-doped HEO2-n sample shows a significant chemical expansion of the *c* parameter up to 600 °C during heating and cooling in O₂, as expected from the TGA results (Figure 1b). Above 600 °C, the increase in the *c* parameter becomes less distinct, but the continuous increase still indicates that the material has not reached a plateau at its assumed minimum oxygen content of δ = 0.075, as stoichiometric hexagonal manganites show a negative thermal expansion coefficient along the *c*-axis.⁴⁷,⁴⁸ The large gap between the *c* parameter values in O₂ and N₂ at higher temperatures also indicates that δ
<figure><img src="image_13.png" /><figcaption>**Figure 4.** Chemical expansion. Refined lattice parameters *a* and *c* from HT-XRD as a function of temperature for (a) HEO1-*n* and (b) HEO2-*n* upon heating and cooling in O₂ and N₂ atmosphere.</figcaption></figure>

> 0 at these temperatures. No large expansion of the *ab*-plane is visible during the oxidation of this sample; instead, a small contraction in the *a* parameter at *T* < 300 °C can be observed, similar to what has been reported for Ti-doped DyMnO₃.³⁰ A small contraction of the *c* parameter can be seen in a N₂ atmosphere, indicating that some incorporation of interstitial oxygen also occurs at low pO₂ (nominally 5.0 purity, ~10⁻⁴ to 10⁻⁶ atm). No other changes in lattice parameters occur during heating and cooling in a N₂ atmosphere, except what can be attributed to thermal expansion.

We now investigate the kinetics of oxidation by synchrotron XRD upon in situ switching from a N₂ to an O₂ atmosphere. In situ XANES upon atmosphere switch shows progressive oxidation of Mn³⁺ to Mn⁴⁺ with time (Figure S5). Diffractograms for HEO1-*n* and HEO2-*n* collected at 200 and 300 °C are shown in Figure 5, and more temperatures are presented in Figures S6–S9 in the Supporting Information. Both samples show shifts in peak positions upon O₂ exposure, a signature of interstitial oxygen entering the lattice. The oxidation time decreases with increasing temperature for both samples, and the Ti-doped sample oxidizes faster than the undoped sample, as expected.³³ The visible R3c reflections at ~6.8° 2θ and ~6.9° 2θ in the N₂ atmosphere for HEO1-*n* indicate that the material contains significant amounts of interstitial oxygen even before being re-exposed to oxygen, indicating that both the initial rate of oxidation and the total time for oxidation are underestimated.

Since oxidation is correlated with lattice parameters through chemical expansion, the time it takes for the lattice parameters to stabilize reflects the oxidation kinetics. Refined lattice parameters as a function of time after in situ switching of the atmosphere from N₂ to O₂ at different temperatures are shown in Figure 6. Both samples show a net contraction in the *a* and *c* axes during oxidation, and the time to reach equilibrium decreases as expected with increasing temperatures. The contraction along *c* is lower in magnitude for HEO1-*n* than for HEO2-*n*, which is related to the former being slightly oxidized (indicated by the appearance of the R3c reflections in Figure 5) prior to exposure to O₂. The net contraction in *c* also decreases with temperature for HEO1-*n*, as TGA measure-

ments (Figure 3) show that the sample has a smaller OSC at *T* > 350 °C. When comparing the lattice parameters measured before oxidation in Figure 6 with the lattice parameters found for the as-prepared HEO1-*n* in Table 2 [*a* = 6.181(8) Å, *c* = 11.412(0) Å], we find a net expansion, not contraction, of the *ab*-plane during oxidation. During initial oxidation, the *ab*-plane expands, while contraction is observed as the oxidation progresses. This is also observed for the Ti-doped sample, which displays a subtle expansion of the *a* parameter before contraction during oxidation.

## ■ DISCUSSION

**Kinetics of Oxidation.** All investigated nanocrystalline samples absorb and release oxygen quickly (Figure 1), with no significant loss of OSC when increasing the cooling rate up to 20 °C min⁻¹. In contrast, literature reports on undoped Y-, Ho-, or Dy-based materials all show a significant reduction in OSC during cooling, even at a 5 °C min⁻¹ cooling rate.²⁵,²⁸,³³ YMnO₃ substituted with larger R such as Tb, Gd, and Sm show no significant change in OSC when increasing the cooling rate from 1 to 5 °C min⁻¹,²⁸ indicating that the larger R improves the kinetics of oxidation. However, these R-substituted samples also show a significant increase in thermal hysteresis with increasing cooling rates, implying significantly slower oxidation kinetics than those for the high-entropy materials studied here. Thermal hysteresis upon heating and cooling shows that the oxygen exchange is not fast enough to maintain equilibrium oxygen content, and the lack of such hysteresis for the Ti-doped HEO2-*n* samples (Figure 3) implies that Ti⁴⁺ further improves the oxidation kinetics and that oxygen release is also fast. Substituted large R³⁺ cations such as Gd³⁺ or Sm³⁺ have also shown indications of slower oxygen release due to stabilization of the oxidized phase.²⁸

Both bulk samples show fast oxygen absorption at a 5 °C min⁻¹ cooling rate (Figure 2) and only a subtle decrease in oxygen content with faster heating and cooling rates of 10 and 20 °C min⁻¹. This shows that even bulk powders with micrometer-sized particles display remarkably fast oxygen exchange. Compared to literature reports for 5 °C min⁻¹ cooling rate, oxygen contents of δ = 0.28 for HEO2-*b* and δ = 0.22 for HEOS-*b* are comparable or better than many nanocrystalline single-R hexagonal manganites²⁵,²⁸ and significantly better than other reported bulk materials.²⁰,²⁷ This suggests the use of bulk high-entropy hexagonal manganites, prepared by cheap and easily scalable solid state methods, for industrial oxygen separation applications.

The HEO1-*n* and HEO2-*n* samples oxidize fast (Figure 6) and the lattice parameters stabilize within 20 min or less at all temperatures, with shorter times at higher temperatures. Ti-doped HEO2-*n* is subtly faster than the undoped HEO1-*n*, as expected from previous studies of Ti⁴⁺-doping.³³ As some R3c is present before switching gas from N₂ to O₂ (Figure 5), the time necessary for full oxidation from a completely unoxidized state is believed to be longer but only subtly, as the rate of oxidation is expected to be the fastest for the lowest degree of oxidation. The oxidation time for HEO1-*n* is substantially shorter than previous findings for RMnO₃ without aliovalent dopants, with, e.g., Y₀.₉₅Pr₀.₀₅MnO₃ reported to need several hours to oxidize in air at multiple temperatures²⁵ and HoMnO₃ being far from fully oxidized after 20 min at 250 °C in an O₂ atmosphere.³³ This shows that high-entropy compositions greatly improve the oxidation rate of hexagonal manganites. This may be attributed to lattice site disorder
<figure><img src="image_14.png" /><figcaption>**Figure 5.** Synchrotron X-ray diffractograms and 2D contour plots of samples (a–d) HEO1-*n* and (e–h) HEO2-*n* measured at 200 and 300 °C as a function of time after in situ switching of atmosphere from N₂ (blue) to O₂ (black). The baseline intensity at 6.0° 2θ indicates the time in minutes after gas switch. Vertical dashed blue lines indicate the initial position of the (004)₁₈₅/(0012)₁₆₁ reflection measured in N₂. The subscripts 185 and 161 refer to space groups *P*6₃*cm* and *R*3*c*, respectively.</figcaption></figure>

causing overlap in site energies, which promotes ionic transport.⁴⁹ Site disorder has, e.g., been shown to enhance Li⁺ transport in solid electrolytes,⁴⁹,⁵⁰ and similar microscopic mechanisms may also be relevant for other 2D and 3D ionic conductors with high configurational entropy. A broader distribution of site energies, as described for bulk ion mobility in Li- and Na-conductors,⁴⁹ may also enhance surface exchange of oxygen as the lattice disorder may induce sites with very high catalytic activity.

Chemical expansion of the *c* axis (Figure 6) can be directly correlated with the degree of oxidation, and the surface exchange rates of oxygen for the two samples can be calculated using a model developed by Moreno et al.⁵¹ Calculated surface exchange rates are shown in Figure 7 and the inferred activation energies of HEO1-*n* and HEO2-*n* are 0.59 and 0.58 eV, respectively. In comparison, the activation energies of single-R nanocrystalline HoMn₀.₈₅Ti₀.₁₅O₃ and DyMn₀.₈₅Ti₀.₁₅O₃ are 0.54 and 0.68 eV, respectively.³³ The
<figure><img src="image_15.png" /><figcaption>**Figure 6.** Refined lattice parameters as a function of time after in situ switching from a N₂ to an O₂ atmosphere for (a) HEO1-*n* and (b) HEO2-*n*. Both samples were refined within the *R*3c space group. The colored horizontal lines indicate the value of the lattice parameters in the final oxidized state at different temperatures.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Figure 7.** Surface exchange rates for HEO1-*n* and HEO2-*n*. Surface exchange rates for HoMn<sub>0.85</sub>Ti<sub>0.15</sub>O<sub>3</sub> and DyMn<sub>0.85</sub>Ti<sub>0.15</sub>O<sub>3</sub> are included for comparison.<sup>33</sup></figcaption></figure>

surface exchange rates are higher for high-entropy compositions than both Ti-doped and undoped single-R counterparts and comparable to values reported for perovskites and other oxides at several hundred degrees higher temperatures.<sup>51-59</sup> The surface exchange rates for HEO1-*n* are in addition likely an underestimation, as they are calculated from only the last and slower part of the oxidation process.

**Oxygen Storage Capacity.** All samples show a large OSC, with the highest found for HEO4-*n* with δ = 0.37 (Figure 1), corresponding to 2.34 wt % or 1463 µmol/g. Such high OSCs are commonly found for hexagonal manganites with large R<sup>3+</sup>

cations ($r_R^{3+} \ge r_{Dy}^{25,26,28,29}$) and can thus not be attributed to high configurational entropy. Oxidation of Tb<sup>3+</sup> to Tb<sup>4+</sup> may also contribute to high OSC, supported by the XANES spectra in Figure S4c,d and the increasing OSC with increasing Tb-content (Figure 1). It is not known where in the crystal structure Oᵢ charge-compensating Tb<sup>4+</sup> are found, e.g., if Oᵢ resides in the Mn–O layer<sup>60</sup> or in the R–O layer. As Tb is found as both Tb<sup>3+</sup> and Tb<sup>4+</sup> at T > 900 °C in high pO₂,<sup>61</sup> the dashed minima lines, corresponding to all Mn being present as Mn<sup>3+</sup>, in Figures 1–3 may underestimate the oxygen content at higher temperatures.

**Phase Stability.** As configurational entropy favors single-phase solid solutions, the presence of many elements may stabilize phases even if the compositional end members are strongly metastable, like, e.g., TbMnO<sub>3</sub><sup>39</sup> and GdMnO<sub>3</sub>. The high-entropy compositions used for this study enable incorporation of a large R<sup>3+</sup> in the hexagonal manganite structure. As all nanocrystalline samples and the two bulk HEO2-*b* and HEO5-*b* samples show high phase purity, with only trace amounts of the orthorhombic phase found for HEO2-*n*, HEO3-*n* and HEO5-*n*, the differences in composition within the present study (Table 1) do not significantly affect the phase stability. This indicates that the high configurational entropy is imperative for stabilizing the hexagonal structure even for tolerance factors as high as t = 0.871, which is higher than the “limit” of t = 0.855–0.862 usually reported for these materials.<sup>62</sup>

The HEO1-*b* sample was not phase pure, even though it has the lowest tolerance factor of all of the samples studied (Table 1). This tolerance factor is determined by the size of the cations in the material and was calculated by assuming the oxidation states of R and Mn to be R<sup>3+</sup> and Mn<sup>3+</sup>. As the
oxidation states of Mn and Tb depend on both the temperature and atmosphere, this assumption is imprecise. In reducing conditions or at higher temperatures, Ti⁴⁺ can cause reduction of Mn to Mn²⁺ instead of being charge compensated by excess Oᵢ, which lowers the tolerance factor. It is therefore likely that the “effective” tolerance factor of Ti-doped samples is lower at the synthesis temperature (1450 °C for bulk samples) than that of undoped samples, which promotes formation of the hexagonal phase. For Tb, the opposite effect occurs; at higher temperatures, less Tb⁴⁺ will be present,⁶¹ and the larger size of Tb³⁺ (1.04 Å, C.N. = 8) compared to that of Tb⁴⁺ (0.88 Å, C.N. = 8)⁴⁴ will then increase the tolerance factor, promoting formation of the orthorhombic perovskite phase. As the HEO1-b sample needs to be synthesized at higher temperatures through the solid-state synthesis method and contains Tb but no Ti, the “effective” tolerance factor is likely too high to achieve phase-pure hexagonal manganite.

**Applications and Sustainability.** The thermal hysteresis for oxygen content for HEO1-*n* is only ~20 °C, which is small compared to previous reports for other RMnO₃.²⁰,²¹,²⁸ The HEO1-*n* material is thus well equipped for temperature swing absorption processes, as only a small temperature change is needed for absorption and release. Both larger R and Ti-doping stabilize higher oxygen contents at higher temperatures, where the exchange kinetics are naturally faster. However, Ti-doping “smears out” the temperature regime of oxidation compared to undoped samples. The broader temperature regime and faster oxygen exchange kinetics make Ti-doped samples like the HEO2-*n* materials more suitable for pressure swing absorption than temperature swing.

All high-entropy compositions studied here show excellent OSC and kinetics, with small variations between different compositions. This suggests the use of rare earth sources as raw materials for hexagonal manganite synthesis without the need for expensive process steps to separate the different rare earths. Many of the rare earth elements are scarce and expensive and have been categorized as critical raw materials by the EU. Particularly, large rare earths, such as Nd, Pr, and Dy, are important for magnets in wind turbine generators and electric vehicles.⁶³⁻⁶⁶ Furthermore, the chemical similarity between the different rare earths also makes their isolation and purification expensive, as they are usually found together in the same ore. The tolerance factor of hexagonal manganites will benefit from using small and relatively cheap rare earth cations, such as Er and Y, together with larger cations, such as Gd, Ce, and La, which are also cheaper than, e.g., Dy and Tb. This could allow comparatively affordable materials with excellent OSC and oxidation kinetics.⁶⁷,⁶⁸ High-entropy alloys also have a high tolerance for impurities, enabling the use of scrap metals or recycled materials for synthesis,¹ and this likely applies to high-entropy oxides as well.

With respect to sustainability, it is generally preferable to keep the number of elements low to reduce the elemental footprint⁶⁹ from mining and refinement as well as to simplify recycling. However, high-entropy hexagonal manganites with smaller rare earths than Nd and Pr, and properties insensitive to the exact composition, present the attractive possibility of using the remaining less critical rare earths for oxygen storage and separation applications after the more critical elements have been extracted.

## CONCLUSIONS

The oxygen absorption properties of high-entropy hexagonal manganites have been studied using thermogravimetric analysis (TGA), high-temperature X-ray diffraction (HT-XRD) and X-ray absorption near-edge structure (XANES). High configurational entropy compositions were found to stabilize the layered hexagonal structure for higher tolerance factors and to improve the kinetics of oxidation compared with previously reported hexagonal manganite materials. The oxygen storage capacity was high (δ > 0.3 in RMnO₃+δ) for all nanocrystalline samples, with increased oxygen absorption by adding Ti and Tb. Bulk high-entropy materials also displayed strongly improved oxygen absorption capacity and kinetics compared with previous reports on bulk materials. The oxygen absorption properties were found to be insensitive to variations in chemical composition, implying that structural disorder itself contributes to the improved oxidation kinetics. Improved properties and insensitivity to exact rare earth composition enhance the application potential of hexagonal manganites for oxygen separation and release.

## ASSOCIATED CONTENT

### Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.chemmater.3c02702.

X-ray diffractograms of as-prepared samples; scanning electron micrographs of bulk powders, X-ray absorption near-edge structure spectra, high-temperature X-ray diffractograms, synthesis details, and characterization details (PDF)

## AUTHOR INFORMATION

### Corresponding Author

Sverre M. Selbach - Department of Materials Science and Engineering, NTNU Norwegian University of Science and Technology, Trondheim NO-7491, Norway; ● orcid.org/0000-0001-5838-8632; Email: selbach@ntnu.no

### Authors

Frida Hemstad Danmo - Department of Materials Science and Engineering, NTNU Norwegian University of Science and Technology, Trondheim NO-7491, Norway

Aamund Westermoen - Department of Materials Science and Engineering, NTNU Norwegian University of Science and Technology, Trondheim NO-7491, Norway

Kenneth Marshall - The Swiss-Norwegian Beamlines (SNBL), European Synchrotron Radiation Facility, Grenoble 38043, France

Dragos Stoian - The Swiss-Norwegian Beamlines (SNBL), European Synchrotron Radiation Facility, Grenoble 38043, France; ● orcid.org/0000-0002-2436-6483

Tor Grande - Department of Materials Science and Engineering, NTNU Norwegian University of Science and Technology, Trondheim NO-7491, Norway; ● orcid.org/0000-0002-2709-1219

Julia Glaum - Department of Materials Science and Engineering, NTNU Norwegian University of Science and Technology, Trondheim NO-7491, Norway; ● orcid.org/0000-0002-4871-4438

Complete contact information is available at: https://pubs.acs.org/10.1021/acs.chemmater.3c02702
**Notes**

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

Dr. Elvia Anabela Chavez Panduro is acknowledged for help with the HT-XRD measurements. We would also like to thank the Swiss-Norwegian Beamlines (SNBL) and BM31 at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, for providing the beamtime and the staff for their support. Financial support from NTNU Norwegian University of Science and Technology is acknowledged.

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