In compositionally complex materials, there is controversy on the effect of enthalpy versus entropy on the structure and short-range ordering in so-called high-entropy materials. To help address this controversy, we synthesized and probed 40 M4AlC3 layered carbide phases containing two to nine metals and found that short-range ordering from enthalpy was present until the entropy increased enough to achieve complete disordering of the transition metals in their atomic planes. We transformed all of these layered carbide phases into two-dimensional (2D) sheets and showed the effects of the order versus disorder on their surface properties and electronic behavior. This study suggests the key effect that the competition between enthalpy and entropy has on short-range order in multicompositional materials.

The recent progress on the liquid crystalline (LC) dispersion of two-dimensional (2D) transition metal carbides (MXenes) has propelled this unique nanomaterial into a realm of high-performance architectures, such as films and fibers. Additionally, compared to architectures made from typical non-LC dispersions, those derived from LC MXene possess tunable ion transport routes and enhanced conductivity and physical properties, demonstrating great potential for a wide range of applications, such as electronic displays, smart glasses, and thermal camouflage devices. This review provides an overview of the progress achieved in the production and processing of LC MXenes, including critical discussions on satisfying the required conditions for LC formation. It also highlights how acquiring LC MXenes has broadened the current solution-based manufacturing paradigm of MXene-based architectures, resulting in unprecedented performances in their conventional applications (e.g., energy storage and strain sensing) and in their emerging uses (e.g., tribology). Opportunities for innovation and foreseen challenges are also discussed, offering future research directions on how to further benefit from the exciting potential of LC MXenes with the aim of promoting their widespread use in designing and manufacturing advanced materials and applications.

Transition metal carbides have been adopted in energy storage, conversion, and extreme environment applications. Advancements in their 2D counterparts, known as MXenes, enable the design of unique structures at the ~1 nm thickness scale. Alkali cations have been essential in MXenes manufacturing processing, storage, and applications, however, exact interactions of these cations with MXenes are not fully understood. In this study, using Ti3C2Tx, Mo2TiC2Tx, and Mo2Ti2C3Tx MXenes, we present how transition metal vacancy sites are occupied by alkali cations, and their effect on MXene structure stabilization to control MXene’s phase transition. We examine this behavior using in situ high-temperature x-ray diffraction and scanning transmission electron microscopy, ex situ techniques such as atomic-layer resolution secondary ion mass spectrometry, and density functional theory simulations. In MXenes, this represents an advance in fundamentals of cation interactions on their 2D basal planes for MXenes stabilization and applications. Broadly, this study demonstrates a potential new tool for ideal phase-property relationships of ceramics at the atomic scale. The effect of defects on MXene structural transitions in high-temperature environments is shown. Further, defect occupied alkali cations are shown to improve MXenes’ phase stability and control surface diffusion related phase changes.

To advance the MXene field, it is crucial to optimize each step of the synthesis process and create a detailed, systematic guide for synthesizing high‐quality MXene that can be consistently reproduced. In this study, a detailed guide is provided for an optimized synthesis of titanium carbide (Ti 3 C 2 T x ) MXene using a mixture of hydrofluoric and hydrochloric acids for the selective etching of the stoichimetric‐Ti 3 AlC 2 MAX phase and delamination of the etched multilayered Ti 3 C 2 T x MXene using lithium chloride at 65 °C for 1 h with argon bubbling. The effect of different synthesis variables is investigated, including the stoichiometry of the mixed powders to synthesize Ti 3 AlC 2 , pre‐etch impurity removal conditions, selective etching, storage, and drying of MXene multilayer powder, and the subsequent delamination conditions. The synthesis yield and the MXene film electrical conductivity are used as the two parameters to evaluate the MXene quality. Also the MXenes are characterized with scanning electron microscopy, x‐ray diffraction, atomic force microscopy, and ellipsometry. The Ti 3 C 2 T x film made via the optimized method shows electrical conductivity as high as ≈21,000 S/cm with a synthesis yield of up to 38 %. A detailed protocol is also provided for the Ti 3 C 2 T x MXene synthesis as the supporting information for this study.

Inside Back Cover In article number 2300030, Anasori and co-workers present a detailed guide for the optimized synthesis of titanium carbide (Ti3C2Tx) MXene and investigate the effects of selective etching and delamination conditions. The resulting Ti3C2Tx MXene film exhibits high electrical conductivity (≈21,000 S/cm) and a synthesis yield of 38 %. This study also provides a detailed step-by-step recipe for high-quality Ti3C2Tx MXene synthesis.