Speaker
Description
High-entropy oxides (HEOs) are materials in which five or more cations share a common crystallographic lattice that is stabilized by high configurational disorder [1]. The presence of multiple principal elements allows their deliberate selection, thus tuning or fundamentally altering the properties of the compound. Despite often crystallizing in simple average structures, HEOs exhibit significant local distortions due to differences in the ionic radii, electronic configurations, and bonding preferences of the constituent cations. Understanding these distortions is essential for establishing structure-property relationships in HEOs.
In this work, the local atomic structure of the prototypical rock-salt HEO $(MgCoNiCuZn)O$ [2] was investigated and compared with two related compounds, $(MgCoNiZn)O$ and $(MgCo_{0.3}NiCu_{0.1}Zn)O$, to determine the effect of Jahn-Teller distortions at Cu sites on the overall lattice. To probe the local environment of individual cations, extended X-ray absorption fine structure (EXAFS) spectroscopy measurements were carried out at the Co, Ni, Cu, and Zn K-edges at the P65 [3] and P64 [4] beamlines of the PETRA III synchrotron facility. The experimental spectra were analysed using reverse Monte Carlo simulations [5], allowing the simultaneous fitting of multiple absorption edges while accounting for contributions from distant coordination shells.
Significant distortions of the first coordination shell around both Co and Cu ions were observed in $(MgCoNiCuZn)O$. However, no distortions around Co were found in samples with lower Cu content, namely $(MgCoNiZn)O$ and $(MgCo_{0.3}NiCu_{0.1}Zn)O$. The distortion of the local environment around Cu is consistent with the first-order Jahn-Teller effect, as previously reported by Rost et al. [2]. The distortion of the local environment around Co may be induced by the Cu Jahn-Teller distortions; however, the exact mechanism underlying this effect requires further investigation.
Acknowledgments
This study was supported by the Latvian Council of Science project No. LZP-2023/1-0476. Beamtimes were allocated under proposals I-20240967 EC and I-20250904 EC.
References
[1] J. Ren et al., “A review of high-entropy materials with their unique applications,” Adv Compos Hybrid Mater, vol. 8, p. 195, 2025. doi: 10.1007/s42114-025-01275-4.
[2] C. M. Rost et al., “Entropy-stabilized oxides,” Nat Commun, vol. 6, p. 8485, 2015. doi: 10.1038/ncomms9485.
[3] E. Welter et al., “A beamline for bulk sample x-ray absorption spectroscopy at the high brilliance storage ring PETRA III,” AIP Conf. Proc., vol. 2054, p. 040002, 2019. doi: 10.1063/1.5084603.
[4] W. A. Caliebe et al., “High-flux XAFS-beamline P64 at PETRA III,” AIP Conf. Proc., vol. 2054., p. 060031, 2019. doi: 10.1063/1.5084662.
[5] J. Timoshenko et al., “EXAFS study of hydrogen intercalation into ReO3 using the evolutionary algorithm,” J. Phys.: Condens. Matter, vol. 26, p. 055401, 2014. doi: 10.1088/0953-8984/26/5/055401.
