Order to disorder transition due to entropy in layered and 2D carbides

There is controversy surrounding the moniker “high-entropy” materials due to the unclear effect of entropy and enthalpy. The unique nanolayered structure of MAX phases, with its structural covalent-metallic-covalent carbide interfaces, allowed us to address this controversy systematically. Here, we synthesized nearly 40 known and novel MAX phases containing 2 to 9 metals and found that their enthalpic preference for short-range order remains until entropy increases enough to achieve all configurations of the transition metals in their atomic planes. In addition, we transformed all these MAX phases into two-dimensional (2D) MXenes and showed the effects of the order vs. disorder on their surface properties and electronic behavior. This study indicates that short-range ordering in high-entropy materials determines the impact of entropy vs. enthalpy on their structures and properties.