Katharine Page, "Complexity in High Entropy Oxides- A Modern Frontier"
Next generation materials of every kind rely on specific atom, charge, or spin motifs for creating and harnessing their properties. High entropy oxides (HEOs), materials exhibiting a single-phase crystal structure containing five or more different cations on single crystallographic lattice sites, have attracted great interest in diverse fields because of their inherent opportunities to tailor and combine material functionalities. Controlling the nature and extent of chemical and structural order in the class represents a grand challenge towards realizing their vast potential. Notably, combinations of neutron, X-ray, and electron scattering probes, deployed across length scales, will be required to untangle their complexity. As examples, we present investigation of the specific cation site preferences, chemical short-range order, and electrocatalytic properties achieved in spinel and fluorite compositionally complex oxides through compositional tuning and variation in synthesis/processing conditions. We also explore the complex configurational diversity present in an entropy stabilized rock salt oxide, discussing approaches to quantifying the extent of chemical disorder and local lattice distortion. Experimentally derived models are supported by Density Functional Theory calculations and Metropolis Monte Carlo simulations. This work hints at the exquisite level of detail that may be required in experimental and computational approaches to guide structure-property breakthroughs in emerging HEO materials. Current challenges and future opportunities within this modern frontier will be discussed.
Raymond E. Schaak, "Understanding how high entropy materials form and transform at low-to-moderate temperatures"
Bulk high entropy materials, including alloys and oxides, have been studied for many years, in part because of how their complex compositions can lead to synergistic and emergent properties. More recently, nanocrystalline high entropy materials have been of interest in the heterogeneous catalysis community because of their beneficial combination of synergistic effects, which impart unique catalytic functions, and high surface areas, which maximize active site density. Many high entropy materials are synthesized at high temperatures, which maximizes the contribution of configurational entropy. We have been exploring the formation of high entropy materials at lower temperature, either by annealing solution-mixed precursors or by precipitating solution-mixed precursors directly in a solvent. This talk will highlight the synthesis and characterization of a bulk high entropy oxide spinel, (Fe0.2Co0.2Ni0.2Cu0.2Zn0.2)Al2O4, that exhibits a narrower band gap than any of its end members due to electronic effects that are a direct result of incorporating a large number of transition metals. This narrowed band gap is advantageous for electrocatalysis, making (Fe0.2Co0.2Ni0.2Cu0.2Zn0.2)Al2O4 an active catalyst for the oxygen evolution reaction. This talk will also highlight recent efforts in the solution-phase synthesis of colloidal nanoparticles of high entropy materials. For example, nanoparticle libraries of composition-tunable high entropy rare earth (RE) oxychlorides, REOCl, that can homogeneously incorporate up to thirteen distinct rare earth metals can be synthesized directly in solution. The 13-metal compound (LaCePrNdSmEuGdDyHoErYbScY)OCl also exhibits a narrower band gap than any of its end members. Colloidal nanoparticle libraries of high entropy alloys, intermetallic compounds, and metal sulfides, made using several distinct synthetic strategies, will also be discussed. Our emphasis in these colloidal syntheses, which all occur at temperatures below 350 °C, is to identify and understand the pathways by which high entropy phases form. We find, in many cases, that colloidal high entropy nanoparticle formation is enabled by chemical reactivity. Chemical reactions tend to proceed through a multi-step pathway that often involves segregated (heterogeneous) intermediates that transform, through diffusion, into homogeneously mixed products. Knowledge of these pathways is important for designing syntheses of targeted high entropy nanoparticles, as well as for future efforts to control their sizes and shapes.
Alannah Hallas, "Understanding the Role of Entropy in High Entropy Oxides"
The field of high entropy oxides (HEOs) flips traditional materials science paradigms on their head by seeking to understand what properties arise in the presence of profound configurational disorder. This disorder, which emerges as the result of multiple elements sharing a single crystalline lattice, can take on a kaleidoscopic character due to the vast numbers of possible elemental combinations and appears to imbue some HEOs with functional properties that far surpass their conventional analogs. However, the actual degree of configurational disorder, its role in stabilizing the HEO phase, and its effect on other physical properties such as magnetism all remain open questions. In my talk, I will discuss my group's efforts towards addressing these questions using x-ray and neutron methods.
Zachary Mansley, "Unraveling structural complexity in high entropy layered oxides using electron microscopy"
High entropy layered oxides (HELOs) are an exciting class of future battery cathode materials, incorporating the chemical and structural tunability of high entropy oxides with the well-established phase space of layered cathode materials such as LiCoO2. HELOs can form in different phases and contain many defects complicating bulk characterization. In this talk I will discuss the study of a HELO Li1.5MO3-δ (M = Mn, Al, Fe, Co, Ni), wherein XRD, XAS, STEM imaging, and electron diffraction patterns reveal the significance of defects within the material; with imaging and diffraction simulation utilized to compare experimental results to atomic modeling. Without rigorous, atomic-scale characterization of HELO materials, phase identification is unreliable and important features such as defect ordering can be easily overlooked – a problem when attempting to rationalize electrochemical properties.
