$\eta$-Carbide-type compounds have recently emerged as a diverse class of materials in the study of superconductivity. These phases contribute to a growing family of metal-rich quantum materials that exhibit unusual superconducting properties emerging from complex metallic bonding. Several members of the $\eta$-carbide-type phases have been found to be bulk superconductors, such as ${\mathrm{Nb}}_4{\mathrm{Rh}}_2{\mathrm{C}}_{1-\delta }$, ${\mathrm{Ta}}_4{\mathrm{Rh}}_2{\mathrm{C}}_{1-\delta }$, ${\mathrm{Ti}}_4{\mathrm{Ir}}_2{\mathrm{O}}_{1-\delta }$, and ${\mathrm{Ti}}_4{\mathrm{Co}}_2{\mathrm{O}}_{1-\delta }$–with transition temperatures up to $T_{\mathrm{c}}\approx$ 10 K and upper critical fields as high as ${\mu }_0{H}_{\mathrm{c}2}\left(0\right)\approx$ 30 T. Whereas the transition temperatures may fall within the range typical for intermetallic superconductors, the pronounced violation of the weak-coupling Pauli limit in many of these crystallographically high-symmetry materials is noteworthy. Here, we review recent progress on superconducting $\eta$-carbide-type phases, emphasizing how crystal symmetry, synthetic challenges, transition-metal composition, and electronic structure govern their superconducting properties. Furthermore, we outline open questions and future directions, including the possible discovery of additional $\eta$-carbide-type materials.

A thermal gradient not only generates a heat current but could also result in a usable voltage difference if applied to a suitable material. Conversely, it is possible to use voltage differences to drive heat currents. The effects related to the mutual conversion of temperature and voltage differences are known as the thermoelectric (TE) effects. TE materials have been used for energy harvesting and solid-state cooling applications for decades. Nevertheless, their performances need significant enhancements to overcome the growing demands for applications. The challenge is that a good TE material needs to have a high electrical conductivity like in a metal, a high Seebeck coefficient like in an insulator, and a low thermal conductivity like in a glass. A number of strategies have been proposed to meet these diverse demands. Dimension reduction is a promising route to enhance the efficiencies of conventional TE materials, and it was proposed long before the innovation of 2D materials. The emergence of 2D materials has stimulated intensive research into their TE properties, with numerous approaches proposed to enhance their performance. Here, we review the recent literature on the TE properties of 2D materials by focusing on the employed enhancement strategies. In addition to the effects of quantum confinement, strategies such as nanostructuring, strain engineering, edge/surface functionalization, creating defects/vacancies, and doping are discussed thoroughly. Approaches to incorporate the intrinsic electronic properties, to utilize the materials with intrinsically low lattice thermal conductivity, and proposed techniques to further lower thermal conductivity are also reviewed. Lastly, the properties of emerging 2D materials with potentially high TE efficiencies are summarized.

While redox activity in electrode materials is well studied in the context of energy storage, its connection to magnetic and electronic phenomena is perhaps not as widely appreciated. Electrochemistry can be leveraged to provide dynamic control over emergent electronic and magnetic behavior in solid materials. In this perspective we review how alkali-ion (de)insertion has been utilized to alter spin states, induce charge ordering, and drive transitions between insulating, metallic, and magnetically frustrated phases for range of battery materials, including layered $\mathrm{LiCo}{\mathrm{O}}_2$ and $\mathrm{LiNi}{\mathrm{O}}_2$, olivine-$\mathrm{LiFeP}{\mathrm{O}}_4$, and spinel $\mathrm{Li}{\mathrm{Mn}}_2{\mathrm{O}}_4$. We also describe less conventional systems like Wadsley–Roth shear phases and breathing-kagome networks to showcase the broader potential for quantum control via redox chemistry. The evolution of emergent properties like spin frustration, Jahn–Teller distortions, and electron localization/delocalization is related to ion content and crystallographic structure. We conclude that electrochemistry can provide a powerful platform for exploring and manipulating physical phenomena in correlated and quantum materials and discuss some experimental challenges.

Electrochemical energy storage plays a key role in supporting the continued electrification of the energy grid, transportation, and other technological sectors. Current ion batteries have been successfully implemented in mobile devices and light-duty transport vehicles, but they have insufficient energy densities for higher-power applications such as aviation, heavy-duty vehicles, or the support of intermittent energy sources. The specific capacity of the cathode limits the energy density of an ion battery, which motivates a transition to new cathode chemistries. Conversion materials, in which the redox-active ion is fully reduced in a multielectron reaction, are promising candidates with double to quadruple the specific capacities of commercial intercalation materials. However, the complexity of conversion reactions and associated phase transitions correlate with poor electrochemical reversibility. Here, we provide several examples of candidate conversion materials and highlight unifying features that describe their phase evolution. We argue that materials with structural simplicity, topotactic relationships between reactants and intermediates, and access to metastable structural polymorphs offer opportunities to design a reversible conversion material.

Transition-metal oxides have been a central subject of condensed matter physics for decades. In addition to novel electronic states driven by the influence of strong correlation, relativistic spin-orbit coupling effects have recently attracted much attention for their potential to explore topological phenomena. In this article, we review various experimental and theoretical studies on transition-metal oxides with focus on thin films and heterostructures where their physics is much influenced by correlation effects and spin-orbit coupling. The combination of the heterostructure geometry together with correlation and topology leads to a variety of novel states here reviewed. We also discuss perspectives for future research in this broad promising area.

Understanding the origin of distinct charge density wave (CDW) instabilities in layered $R{\mathrm{Te}}_n$ ($n=2$, 3) compounds ($R$, rare earth element) has been an important issue. In this research update, we have investigated the electronic structures of ${\mathrm{PrTe}}_n$ ($n=2$, 3) and ${\mathrm{ErTe}}_3$ layered CDW compounds employing angle-resolved photoemission spectroscopy (ARPES) and soft x-ray absorption spectroscopy (XAS). The trivalent valency of $R^{3+}$ ions is confirmed for ${\mathrm{PrTe}}_n$ ($n=2$, 3) and ${\mathrm{ErTe}}_3$, supporting that $R$-Te slabs serve as charge reservoirs and that the CDW instability occurs in the partially filled Te sheets. Both $R4d\to 4f$ resonant photoemission spectroscopy and photon-energy map measurements provide evidence that $R4f$ electrons do not contribute directly to the CDW formation but that the indirect contribution from Pr $4f$ electrons through the Pr $4f-\mathrm{Te} 5p$ hybridization is feasible in ${\mathrm{PrTe}}_n$ ($n=2$, 3). Circular and linear dichroism ARPES measurements indicate that the chirality of the Te $5p$ orbitals certainly plays a role in the CDW formation of $R{\mathrm{Te}}_3$ ($R=\mathrm{Pr}$, Er) while it is relatively weak in ${\mathrm{PrTe}}_2$, and that the $E_{\mathrm{F}}$-crossing orbitals, responsible for the CDW formation, are ordered in plane (in the $ac$ plane) in all of them. Different CDW-induced Fermi surface reconstructions between $R{\mathrm{Te}}_3$ and $R{\mathrm{Te}}_2$ are due to (i) the existence of two Te sheets and one Te sheet per unit cell in $R{\mathrm{Te}}_3$ and $R{\mathrm{Te}}_2$, respectively, so as to produce different numbers of hole carriers, and (ii) the different lattice parameters of Te sheets in $R{\mathrm{Te}}_n$, leading to the different densities of states at $E_{\mathrm{F}}$.

Well-ordered nanonetwork materials with triply periodic minimal surface texture are appealing and promising for innovative properties such as optical and mechanical metamaterials as inspired by nature (e.g., the photonic property from the wing structure of a butterfly and the high-impact property from the dactyl club of a mantis shrimp). Network structures possess self-supporting frameworks, open-cell character, high porosity, and large specific surface area, giving specific functions and complexity for practical applications. Here, a facile approach with simple routes for acquiring of metastable network phases beyond conventional phase diagrams is proposed and examined. By taking advantage of controlled self-assembly for high-interaction-parameter (χ) block copolymers (BCPs), it is feasible to acquire network phases from the use of a selective solvent for self-assembly under controlled evaporation of the solvent. In contrast with the thermodynamically stable equilibrium phases from intrinsic BCPs and their blends with conventional network phases, a variety of kinetically trapped network phases with a high degree of ordering can be obtained from a single-composition lamellar phase, giving an easy method to acquire metastable network phases even for a primitive phase with large packing frustration (i.e., entropic penalty). Furthermore, the windows for network phases can even be expanded through controlled self-assembly of star-BCPs as compared with the linear-conformation diblocks. As a result, the topological architecture of BCPs could be another controlling factor to vary the phase behaviors, which may provide easy access to a variety of metastable network phases from thermodynamically stable phases through a kinetically controlled process for self-assembly.

Designing conformationally dynamic molecules that self-assemble into predictable nanostructures remains an important unmet challenge. This paper describes how atomic-scale cryogenic transmission electron microscopy (cryo-TEM) can be used to explore the relationship between molecular structure and self-assembly of block copolymers. We examined sheetlike micelles formed in water using a series of diblock copolypeptoids with the same hydrophilic block and three distinct crystalline hydrophobic blocks. Our cryo-TEM images revealed all the structures share nansoscale features, but differ in their intermolecular packing geometries. Different molecular arrangements, parallel and antiparallel V-shaped crystal motifs, were revealed by two-dimensional atomic-scale through-plane images. However, images from tilted samples revealed an unexpected feature when the hydrophobic polypeptoid block comprised phenyl rings with substituted bromine atoms at the para position. The nanosheets contained atomic-scale corrugations that were absent in the other systems which comprised unsubstituted aliphatic and aromatic side chains. We hypothesize that these corrugations are due to the dipolar characteristics of the brominated phenyl group and interactions between this group and water molecules.

The exploration of nanoscale morphologies with three-dimensional spatial arrangements in block copolymers involves the manipulation of compositional fluctuations at interfaces, induction of conformational asymmetry, and design of complex architectures. Despite the abundance of triply periodic minimal surfaces identified to date, the experimental demonstration of thermodynamically stable complex nanostructures with high-packing frustration remains limited. This research update focuses on the importance of molecular interactions for stabilizing complex nanostructures and proposes the use of end-group chemistry as a versatile method for realizing thermodynamically stable network structures with high-packing frustration in simple linear diblock copolymers. The proposed approach substantially alters phase diagrams and reveals unprecedented network structures in block copolymers. Ongoing research has the potential to generate even more diverse and stable nanostructures, thereby advancing nanotechnology and expanding our understanding of polymers and materials science.

Soft materials with nanostructures, such as Frank-Kasper (FK) and related spherical phases, have garnered significant interest in recent years. However, there is still a lack of rational approaches to design and fabricate these specified spherical phases. This is primarily due to the hierarchical self-assembly that commonly occurs during phase formations, which obscures the correlations between discrete molecules and the resulting superlattices. To address this challenge, giant molecules (GMs), which are macromolecules with precisely controlled chemistry and architecture, present a promising platform to explore the underlying principles of self-assembly. In this contribution, our focus is primarily on the features of the intermediate stage of self-assembly (i.e., mesoatoms) and a review of experimental guidelines in molecular systems to control the characteristics of self-assembled phases. We will first introduce the experimentally accessible spherical phases up to the current stage. The phases are categorized based on their structural features, including the simple spherical packing phases, FK phases, quasicrystalline phases, and quasi-FK phases. Subsequently, we will delve into the mesoatomic parameters that can be experimentally controlled, including the individual size ($V$), size difference ($\nu$), stoichiometry (ϕ) and size distribution (${D–}_{\mathrm{size}}$) of mesoatoms. By envisioning the perspective from mesoatoms of GMs, we aim to facilitate the construction of spherical phases through rational molecular designs.

Block copolymer (BCP) thin films readily self-assemble into intricate nanoscale morphologies, with structure dictated by chain architecture. As the field searches for more control over structure formation, it is natural to investigate more sophisticated control mechanisms, such as synthesizing more complex chain architectures or applying directing fields. We review the use of a simple but powerful method for controlling self-assembly: blending of BCPs with homopolymers or other BCPs. Blending enables tailoring ordering kinetics, the spatial distribution of chains throughout the film thickness, and the nanoscale morphology itself. Blending is thus emerging as a remarkably simple and powerful means of tailoring self-assembled morphologies.

Block copolymers have served as versatile model compounds to understand the self-assembly of inhomogeneous materials. However, the close-packed structures and relevant polytypic crystal systems in block copolymer materials still require a better understanding. In this research update, we review early and recent advancements in close-packed structures in block copolymer materials and attempts to present a framework to understand the origin of polytypism in polymeric and relevant systems. We propose a critical role of interstitial space as a structure director in polytypism and introduce the interstitial space distribution factor as a semiquantitative parameter to address the difference in the configurations of interstitial space distribution in polytypes of the same class. We also note that the random stacking of two-dimensional close-packed structures is a class of aperiodic crystals.

Over the past decade, materials that combine broken inversion symmetry with metallic conductivity have gone from a thought experiment to one of the fastest growing research topics. In 2013, the observation of the first uncontested polar transition in a metal, ${\mathrm{LiOsO}}_3$, inspired a surge of theoretical and experimental work on the subject, uncovering a host of materials which combine properties previously thought to be contraindicated [Shi et al., Nat. Mater. 12, 1024 (2013)]. As is often the case in a nascent field, the sudden rise in interest has been accompanied by diverse (and sometimes conflicting) terminology. Although “ferroelectriclike” metals are well defined in theory, i.e., materials that undergo a symmetry-lowering transition to a polar phase while exhibiting metallic electron transport, real materials find a myriad of ways to push the boundaries of this definition. Here, we review and explore the burgeoning polar metal frontier from the perspectives of theory, simulation, and experiment while introducing a unified taxonomy. The framework allows one to describe, identify, and classify polar metals; we also use it to discuss some of the fundamental tensions between theory and models of reality inherent in the terms “ferroelectric” and “metals.” In addition, we highlight shortcomings of electrostatic doping simulations in modeling different subclasses of polar metals, noting how the assumptions of this approach depart from experiment. We include a survey of known materials that combine polar symmetry with metallic conductivity, classified according to the mechanisms used to harmonize those two orders and their resulting properties. We conclude by describing opportunities for the discovery of novel polar metals by utilizing our taxonomy.

As the majority of the input energy in power generation or energy consumption processes goes to waste as heat, thermophotovoltaic (TPV) devices enable energy recovery from (waste) heat. In TPV devices, the power output and conversion efficiency are impacted by thermal emitters. Since TPV devices operate at higher temperatures, emitters that can withstand hot environments without significant degradation of their emission performance are required. Refractory metals are commonly used as the emitter material due to their higher melting point and optical properties. This paper reviews physical and chemical properties of 15 refractory metals that may affect the emitter's performance at high temperatures: melting point, crystal structure, lattice constant, standard reduction potential, diffusion coefficient, Young's modulus, thermal expansion coefficient, and refractive index. Biological hazards and prices of the metals are also explored. Then, selective TPV emitters fabricated with the refractory metals are compared regarding their thermal stability. Finally, material properties are discussed toward achieving thermally robust TPV emitters.

As a sustainable technology, radiative cooling has received considerable attention due to its potential in energy sustainability. Unlike conventional cooling techniques, radiative cooling does not consume electricity during its operation and is therefore particularly attractive in reducing the energy demand for cooling and addressing global warming by reducing carbon emissions. The general principle requires a radiative cooler to be thermally emissive to dissipate heat via thermal radiation. During the daytime, the cooler needs to minimize the solar heating effect to ensure subambient temperatures. Guided by these criteria, researchers have developed various materials with engineered optical, thermal, and mechanical features. In this review, we will first explore the fundamentals of heat transfer in radiative cooling processes. Subsequently, we will summarize the state-of-the-art progress on material synthesis and system designs. Building upon those recently developed features, we will review how this technology has been implemented in practical applications, ranging from thermal management of buildings, semiconductor cooling, personal comfort design, and atmospheric water harvesting. Finally, we will conclude this review by identifying and discussing some of the remaining challenges requiring future research and development.

The recent development of multicomponent, high-entropy oxides (HEOs) has sparked research into ceramics with significant compositional and structural diversity. The wide range of compositions and structures achievable with these HEOs presents a promising research opportunity, as these materials may be custom-designed to serve in several important applications, from chemical catalysis to lithium-ion batteries. Many of the unique properties which make these materials viable for a variety of applications are attributed to the characteristic single-phase (SP) structure of HEOs. As such, some researchers of HEOs have focused on understanding the driving force which results in this transition from a multiphase to a SP structure. The continued development of these materials relies on establishing a sound fundamental understanding of these critical phase transformations. To that end, in this review, we will go over the recent research advancements investigating phase transitions in HEOs. In this review, we will leverage mathematical and empirical evidence to compare the effects of enthalpic and entropic contributions to the free energy of mixing and, in turn, their impact on the transition to SP, HEOs. Emphasis will be placed on the role of entropy stabilization in these phase transitions as well as other property effects arising from high-entropic lattice disorder.

Benefiting from the high specific capacity and output voltage, Ni-rich layered oxides are one of the most promising commercial cathodes for next-generation high-energy-density Li-ion batteries (LIBs). However, typical Ni-rich cathode materials generally inherit polycrystal (PC) morphology from their precursors, which induces severe crack generations and side reactions, resulting in the rapid decay of structural and electrochemical stability upon cycling. In contrast, the Ni-rich cathodes with single-crystal (SC) morphology display remarkable structural stability and long cycle life by means of superior mechanical strength and limited side reactions, which is expected to solve the instinctive problems of PC counterparts. Herein, the synthesis strategies and the growth mechanisms of SC Ni-rich oxides are introduced and summarized in detail. Moreover, the significant differences in electrochemical behaviors between PC and SC cathodes are comprehensively compared in various voltage windows. Furthermore, the corresponding structural evolutions and morphology changes are also systematically investigated and analyzed. Additionally, the state-of-the-art characterization techniques for SC materials are elaborated. Altogether, in this review, we not only unravel the fundamental understandings of SC Ni-rich cathode materials but also provide an effective guide for realizing high-energy-density LIBs with long cycle life.

Alkali metal ion batteries, and in particular Li-ion batteries, have become a key technology for current and future energy storage, already nowadays powering many devices in our daily lives. Due to the inherent complexity of batteries and their components, the use of computational approaches on all length and timescales has been largely evolving in recent years. Gaining insight in complex processes or predicting new materials for specific applications are two of the main perspectives computational studies can offer, making them an indispensable tool of modern material science and hence battery research. After a short introduction to battery technology, this review will first focus on the theoretical concepts that underlie the functioning of Li- and post-Li-ion batteries. This will be followed by a discussion of the most prominent computational methods and their applications, currently available for the investigation of battery materials on an atomistic scale.