X-raying Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O: disentangling elemental contributions in a prototypical high-entropy oxide

TL;DR

This work addresses how individual chemical species in high-entropy oxides influence structure, electronics, and magnetism. It employs a combination of element-specific X-ray techniques (XRD, PDF, XANES/EXAFS, and RIXS) across a panel of compositions to disentangle site-resolved contributions, revealing a sizable Jahn-Teller distortion at Cu and mapping magnetic exchange interactions among Cu, Ni, and Co. The findings show that Ni and Mg mitigate local distortions while Cu promotes them, and they quantify key magnetic couplings, offering a quantitative framework for understanding and designing ESOs with tailored structural and magnetic properties. The approach demonstrates the power of combining spectroscopy with composition-tuned samples to deconvolve complex, disordered systems and provides insights valuable for predicting stability and functionality in high-entropy oxide materials.

Abstract

We employ several X-ray based techniques, including X-ray diffraction, absorption and resonant inelastic scattering, to disentangle the contributions of individual chemical species to the structural, electronic and magnetic properties of high-entropy oxides. In the benchmark compound Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O and related systems, we unambiguously resolve a sizable Jahn-Teller distortion at the Cu sites, more pronounced in the absence of Ni2+ and Mg2+, suggesting that these ions promote positional order, whereas Cu2+ ions act to destabilize it. Moreover, we detect magnetic excitations and estimate the strength of the interactions between pairs of different magnetic elements. Our results provide valuable insights into the role of the various chemical species in shaping the physical properties of high-entropy oxides.

Figures (10)

• Figure 1: Rietveld refinements of XRD patterns collected at room temperature.
• Figure 2: Sketch of the rocksalt unit cell, with grey and red spheres representing the metal and the oxygen ions, respectively, and the PDF curves of 5HEO, noCu and noMg. The principal interatomic distances used to interpret the PDF data, as well as a MO_6 octahedron are shown.
• Figure 3: Cu (a), Ni (b) and Co (c) K edge XANES spectra for noCu, 5HEO and noMg samples. The corresponding experimental (open circles) and simulated (thin line) EXAFS oscillations are reported in panels (b), (e) and (h), and (c), (f) and (i), respectively.
• Figure 4: Average Co-O, Ni-O and Cu-O bond lengths for noCu, 5HEO and noMg as extracted from Cu, Ni and Co K edge EXAFS, respectively.
• Figure 5: (a) Cu L$_3$ edge RIXS spectra taken at 30 K with an energy of the incident photons corresponding to the main peak in the XAS profile (vertical red bar in the inset of panel (c)). The scheme of the experimental RIXS geometry is shown in the inset. (b) Experimental and fitting curves of the $d$-$d$ excitations in 5HEO at 30 K as explained in the text. The inset shows the crystal field splitting of $e_g$ and $t_{2g}$ levels in a Jahn-Teller distorted octahedral system. (c) Temperature dependence of the RIXS spectra of 5HEO.