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Entropy-stabilized oxides are single-phase, multicomponent oxides that are stabilized by a large entropy of mixing, ΔS, overcoming a positive enthalpy. Due to the −TΔS term in the Gibbs' free energy, G, it can be hypothesized that entropy-stabilized oxides demonstrate a robust thermal stability. Here, we investigate the high temperature stability (1300–1700 °C) of the prototypical entropy-stabilized rocksalt oxide (MgCoNiCuZn)0.2O in air. We find that at temperatures >1300 °C, the material gradually loses Cu and Zn with increasing temperature. Cu is lost through a selective melting as a Cu-rich liquid phase is formed. Zn is sublimed from the rocksalt phase at approximately similar temperatures to those corresponding to the Cu loss, significantly below both the melting temperature of ZnO and its solubility limit in a rocksalt phase. The elemental loss progressively reduces the entropy of mixing and results in a multiphase solid upon quenching to room temperature. We posit that the high-temperature solubility of Cu and Zn is correlated providing further evidence for entropic stabilization over general solubility arguments. 

Abstract Ferroelectricity in hafnia films has triggered significant research interest over the past decade due to its immense promise for next‐generation memory devices. However, the origin of ferroic behavior at the nanoscale and the means to control it remain an open question, with the consensus being that it deviates from conventional ferroelectrics. In this work, a novel approach is presented to tune ferroelectric properties of hafnia through environmental control using piezoresponse force microscopy (PFM). A reversible transition from non‐ferroelectric to ferroelectric behavior by modulating the surrounding atmosphere is demonstrated. Notably, the domain relaxation dynamics exhibit striking sensitivity to environmental factors, including ambient conditions, specific gas compositions (N₂, CO₂, O₂), and humidity levels. The critical role of surface water removal, gas molecule adsorption, and their interactions with near‐surface oxygen vacancies is identified and the injected charge in determining ferroelectricity in uncapped hafnia films. These insights reveal a significant strategy for stabilizing ferroic responses by carefully regulating the chemical environment, offering new possibilities for precise control in hafnia‐based films. 

The study of high-entropy materials has attracted enormous interest since they could show new functional properties that are not observed in their related parent phases. Here, we report single crystal growth, structure, thermal transport, and magnetic property studies on a novel high-entropy oxide with the spinel structure (MgMnFeCoNi)Al2O4. We have successfully grown high-quality single crystals of this high-entropy oxide using the optical floating zone growth technique for the first time. The sample was confirmed to be a phase pure high-entropy oxide using x-ray diffraction and energy-dispersive spectroscopy. Through magnetization measurements, we found (MgMnFeCoNi)Al2O4 exhibits a cluster spin glass state, though the parent phases show either antiferromagnetic ordering or spin glass states. Furthermore, we also found that (MgMnFeCoNi)Al2O4 has much greater thermal expansion than its CoAl2O4 parent compound using high resolution neutron Larmor diffraction. We further investigated the structure of this high-entropy material via Raman spectroscopy and extended x-ray absorption fine structure spectroscopy (EXAFS) measurements. From Raman spectroscopy measurements, we observed (MgMnFeCoNi)Al2O4 to display a combination of the active Raman modes in its parent compounds with the modes shifted and significantly broadened. This result, together with the varying bond lengths probed by EXAFS, reveals severe local lattice distortions in this high-entropy phase. Additionally, we found a substantial decrease in thermal conductivity and suppression of the low temperature thermal conductivity peak in (MgMnFeCoNi)Al2O4, consistent with the increased lattice defects and strain. These findings advance the understanding of the dependence of thermal expansion and transport on the lattice distortions in high-entropy materials. 

As various property studies continue to emerge on high entropy and entropy-stabilized ceramics, we seek a further understanding of the property changes across the phase boundary between “high-entropy” and “entropy-stabilized” phases. The thermal and mechanical properties of bulk ceramic entropy stabilized oxide composition Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O are investigated across this critical transition temperature via the transient plane-source method, temperature-dependent x-ray diffraction, and nano-indentation. The thermal conductivity remains constant within uncertainty across the multi-to-single phase transition at a value of ≈2.5 W/mK, while the linear coefficient of thermal expansion increases nearly 24% from 10.8 to 14.1 × 10−6 K−1. Mechanical softening is also observed across the transition. 

Abstract Interest in high‐entropy inorganic compounds originates from their ability to stabilize cations and anions in local environments that rarely occur at standard temperature and pressure. This leads to new crystalline phases in many‐cation formulations with structures and properties that depart from conventional trends. The highest‐entropy homogeneous and random solid solution is a parent structure from which a continuum of lower‐entropy offspring can originate by adopting chemical and/or structural order. This report demonstrates how synthesis conditions, thermal history, and elastic and chemical boundary conditions conspire to regulate this process in Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O, during which coherent CuO nanotweeds and spinel nanocuboids evolve. We do so by combining structured synthesis routes, atomic‐resolution microscopy and spectroscopy, density functional theory, and a phase field modeling framework that accurately predicts the emergent structure and local chemistry. This establishes a framework to appreciate, understand, and predict the macrostate spectrum available to a high‐entropy system that is critical to rationalizing property engineering opportunities. 

Abstract High‐entropy materials defy historical materials design paradigms by leveraging chemical disorder to kinetically stabilize novel crystalline solid solutions comprised of many end‐members. Formulational diversity results in local crystal structures that are seldom found in conventional materials and can strongly influence macroscopic physical properties. Thermodynamically prescribed chemical flexibility provides a means to tune such properties. Additionally, kinetic metastability results in many possible atomic arrangements, including both solid‐solution configurations and heterogeneous phase assemblies, depending on synthesis conditions. Local disorder induced by metastability, and extensive cation solubilities allowed by thermodynamics combine to give many high‐entropy oxide systems utility as electrochemical, magnetic, thermal, dielectric, and optical materials. Though high‐entropy materials research is maturing rapidly, much remains to be understood and many compositions still await discovery, exploration, and implementation. 

Abstract Manipulating a crystalline material's configurational entropy through the introduction of unique atomic species can produce novel materials with desirable mechanical and electrical properties. From a thermal transport perspective, large differences between elemental properties such as mass and interatomic force can reduce the rate at which phonons carry heat and thus reduce the thermal conductivity. Recent advances in materials synthesis are enabling the fabrication of entropy‐stabilized ceramics, opening the door for understanding the implications of extreme disorder on thermal transport. Measuring the structural, mechanical, and thermal properties of single‐crystal entropy‐stabilized oxides, it is shown that local ionic charge disorder can effectively reduce thermal conductivity without compromising mechanical stiffness. These materials demonstrate similar thermal conductivities to their amorphous counterparts, in agreement with the theoretical minimum limit, resulting in this class of material possessing the highest ratio of elastic modulus to thermal conductivity of any isotropic crystal.