Configurational Entropy-Driven Phase Stability and Thermal Transport in Rock-Salt High-Entropy Oxides

TL;DR

The paper investigates how configurational entropy drives phase stability and thermal transport in rock-salt high-entropy oxides (HEOs) by systematically varying cation diversity from 2 to 5 elements, including Li/Na/K dopants. It demonstrates that single-phase rock-salt phases can be stabilized at intermediate entropy levels ($ΔS_{conf} \approx 0.95R$), challenging the conventional $1.61R$ benchmark, and reveals a strong coupling between $ΔS_{conf}$ and thermal conductivity $κ$, with pronounced phonon-scattering leading to low $κ$ in more disordered variants. Alkali and Li doping further suppress $κ$ and, in Li-doped samples, enhance thermoelectric performance to a maximum $zT$ of about 0.15 at 1173 K, due to reduced $κ$ and favorable carrier transport. Overall, the work shows configurational entropy as a tunable parameter for stabilizing disordered solid solutions and tailoring phonon transport in oxide thermoelectrics, while noting potential limitations from ionic conduction and thermal stability at elevated temperatures.

Abstract

High-entropy oxides (HEOs) offer a unique platform for exploring the thermodynamic interaction between configurational entropy and enthalpy in stabilizing complex solid solutions. In this study, a series of rock-salt structured oxides with varying configurational entropy, ranging from binary to multi-cation systems, to elucidate the competing roles of enthalpy and entropy in phase stabilization is investigated. Compositions including (Ni$_{0.8}$Cu$_{0.2}$)O to(NiCuZnCoMg)$_{0.9}$A$_{0.1}$O (A = Li, Na, K) were synthesized and their stuctural, microstructural and thermal properties have been discussed. X-ray diffraction combined with thermal cycling confirms that even a medium configurational entropy ($\sim$ 0.95R) can induce single-phase behavior stabilized by configurational entropy ($ΔS_{conf}$), challenging the traditional threshold of $1.5\,R$. High-resolution TEM and EDS mapping reveal nanocrytalline features and homogeneous elemental distribution respectively, while XPS analysis confirms divalent oxidation states. A strong coupling between high configurational entropy with thermal conductivity ($κ$) has been observed. First, a sharp decrease in $κ$ with increasing $ΔS_{conf}$ is seen and then decomposed samples (while cooling) show high $κ$, demonstrating the role of $ΔS_{conf}$ on $κ$. Furthermore, Li-doped compositions exhibit improved thermoelectric performance, with a maximum figure of merit ($zT$) of $\sim$0.15 at 1173K\, driven by low thermal conductivity and favorable carrier transport. The results highlight that configurational entropy, even at intermediate values, plays a significant role in stabilizing disordered single-phase oxides and tailoring phonon transport.

Figures (15)

• Figure 1: (a) X-ray diffraction pattern for rock-salt structure oxide, (b) Rietveld refinement profile for Li-doped (NiCuZnCoMg)0.9Li0.1O (MO17), confirming phase purity and rock-salt structure, (c) Lattice parameters as a function of composition, showing expansion with increasing cation diversity and alkali doping, (d) X-ray diffraction pattern of the rock-salt samples decomposed at 600$^{\circ}$C, and (e) the Tauc plot obtained from the UV-visible spectroscopy measurement showing direct optical band gaps between 1.25–1.35 eV across all compositions.
• Figure 2: High-resolution transmission electron microscopy (HRTEM) images and selected-area electron diffraction (SAED) patterns for MO11–MO14. Top row: low-magnification images reveal nanocrystalline domains (50–150 nm). Middle row: HRTEM fringes confirm crystalline rock-salt planes with local lattice distortions. Bottom row: SAED patterns display cubic diffraction rings with diffuse scattering, consistent with entropy-driven disorder.
• Figure 3: TEM–EDS elemental mapping of rock-salt oxides MO11–MO14. All systems show homogeneous nanoscale distribution of constituent elements (Ni, Cu, Zn, Co, Mg, O), confirming solid-solution formation without clustering or secondary phases. Increasing cationic complexity maintains compositional uniformity, highlighting entropy-driven mixing.
• Figure 4: Survey X-ray photoelectron spectroscopy (XPS) spectra of representative rock-salt oxides (MO11, MO12, MO13, MO14, MO17, MO20, MO21). All expected elements (Ni, Cu, Zn, Co, Mg, O) are detected, with no extraneous peaks, confirming chemical incorporation of designed cations.
• Figure 5: High-resolution XPS spectra of Cu 2p, Ni 2p, Zn 2p, Mg 2p, and O 1s core levels in rock-salt oxides. Spectra confirm divalent oxidation states of cations: Cu$^{2+}$ (shake-up satellites), Ni2+ (multiplet splitting), Zn$^{2+}$ (sharp 2p doublet), Mg$^{2+}$ (symmetric 2p peak), and lattice O$^{2-}$ with minor vacancy-related components.