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

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. Editor's summary: As the number of elements in a material increases, the possibility of greater entropic contributions to ordering or disordering also increases. Wyatt et al. synthesized 40 MAX materials (where M is a transition metal, A is a group 13 or 14 element, and X is either C or N) using 2 to 9 M elements. They were able to quantify the impact of entropy versus enthalpy on the resulting structure and properties. Enthalpy-driven short-range order persisted until configurational entropy became sufficiently large to randomize metal configurations. Compositional disorder in the precursor phases directly influenced the surface terminations (–O, –OH, and –F) and electronic properties when converted to MXenes. —Marc S. Lavine [ABSTRACT FROM AUTHOR]