Ab initio Monte Carlo prediction of chemical order and disorder in multicomponent MXenes

Understanding the chemical order and disorder in multicomponent MXenes is crucial for their precise design and synthesis. Using an integrated and improved ab initio Monte Carlo (MC) framework, we systematically investigate the minimum energy configurations (MECs) of (TiMo)-based double transition metal (DTM) carbide MXenes and demonstrate that surface termination and coordination environments play critical roles in governing chemical order and disorder in these materials. Specifically, the formation of out-of-plane MXene (o-MXenes) with Mo segregation to outermost metallic layers (M') is only driven by the oxygen (O) termination at prismatic sites. In contrast, O termination at octahedral sites and fluorine (F) termination at both prismatic and octahedral sites always promote the formation of o-MXenes with Ti-segregated to M' layers. Furthermore, varying the F/O ratio at prismatic termination sites or alternating the atomic coordination within the MXene lattices changes the energetic preference between ordered and disordered states in (TiMo)-based MXenes. The reshaping of chemical order and disorder induced by surface termination and coordination environment is observed in other DTM and high-entropy MXenes, suggesting the generality of these phenomena.