PNNL

The Science

High-entropy oxides (HEOs) possess exceptional compositional flexibility and structural stability, making them highly promising for energy and catalytic applications. Understanding the electrocatalytic activity and stability of HEOs requires precise control of composition and cation oxidation states. Researchers demonstrated that oxygen partial pressure and strontium (Sr) doping significantly tunes cation-specific oxidation states and cation segregation in epitaxial La_1-xSr_x(5B)O₃ (where 5B = Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2) thin films. They observed that both higher oxygen, partial pressure, and Sr doping preferentially promote chromium (Cr) oxidation from Cr³⁺ to Cr⁶⁺ and partially oxidizes cobalt and nickel (Ni) while leaving the manganese (Mn⁴⁺) and iron (Fe³⁺) unchanged. Sr doping induces pronounced Cr segregation, marked by depletion at the film/substrate interface and enrichment at the surface. The films undergo partial amorphization in heavily Sr-doped samples. 

The Impact

This research advances scientific understanding of how doping and local composition fluctuations affect HEOs. Cr segregation is driven by selective oxidation and strain relief during growth, influences catalytic activity, and raises concerns about the formation of toxic Cr⁶⁺-containing phases. These findings emphasize the critical role of elemental doping, surface, and interface in controlling the structure and properties of HEO as thin films. The results also highlight the importance of designing materials to mitigate segregation and prevent harmful phase formation. 

Summary

HEOs offer exceptional flexibility in tuning electronic structure and catalytic performance through multi-cation composition. However, the role of A-site doping in controlling charge distribution and stability remains poorly understood. Researchers investigated how Sr substitution affects oxidation states, charge transfer, and elemental segregation in epitaxial La_1-xSr_x(5B)O₃ (where 5B = Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2) thin films. Using a combination of experimental techniques, they found that Sr doping selectively oxidizes Cr (to Cr⁶⁺) and increases cobalt and Ni valence, while the Mn and Fe oxidations remain stable. 

Microscopy and depth profiling revealed pronounced Cr segregation with Sr enrichment at the film surface, forming amorphous Sr and Cr-rich regions and Cr depletion near the substrate interface. This segregation is driven by oxidation-induced lattice strain and amplified by Sr doping. The resulting Cr⁶⁺ phases are potentially unstable and toxic, posing challenges for electrochemical applications. These findings establish that Sr doping profoundly alters charge distribution and microstructural stability of HEOs through coupled redox, strain, and segregation processes. The work highlights potential problems associated with Sr–Cr combinations in HEOs, demonstrates the need to optimize growth conditions to suppress elemental segregation, and establishes principles for HEO design where the competition between entropy and epitaxy needs to be strategically balanced. 

Contact

Le Wang, Pacific Northwest National Laboratory, le.wang@pnnl.gov 

Yingge Du, Pacific Northwest National Laboratory, yingge.du@pnnl.gov 

Funding

This work was supported by the Department of Energy (DOE), Office of Science, Basic Energy Sciences, Division of Materials Sciences and Engineering, Synthesis and Processing Science Program, FWP 10122. Soft XAS measurements were performed at the Soft X-ray beamline (SR14ID01) at the Australian Synchrotron. Hard XAS measurements used the Submicron Resolution X-ray Spectroscopy (SRX) beamline at 5-ID of the National Synchrotron Light Source II, a DOE Office of Science user facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. Part of the STEM and focused ion beam SEM work was carried out using microscopes that are funded by a grant from the Washington State Department of Commerce’s Clean Energy Fund. A portion of work was performed via a project award (Award DOI: 10.46936/cpcy.proj.2021.60271/60008423) from the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility at Pacific Northwest National Laboratory sponsored by the Biological and Environmental Research program under Contract No. DE-AC05-76RL01830.