Ab initio Monte Carlo prediction of order-to-disorder transitions in multicomponent MXenes

TL;DR

This paper introduces an advanced first-principles Monte Carlo method to predict order-to-disorder phase transitions in multicomponent MXenes, highlighting the influence of surface termination and atomic coordination.

Contribution

It develops an improved ab initio Monte Carlo framework with structural relaxation and selective atom swapping for accurate MXene phase predictions.

Findings

Surface termination critically influences MXene ordering.

Oxygen termination drives Mo segregation to outer layers.

F/O ratio and atomic coordination can induce order-disorder transitions.

Abstract

This letter predicts unprecedented order-to-disorder transition behaviors in multicomponent MXenes using an integrated and improved first-principles Monte Carlo (MC) framework. The improvements include (i) structural relaxation and (ii) selective atom swapping during MC iterations for more accurate and efficient predictions. Using (TiMo)-based double transition metal (DTM) carbide MXenes as a model system, ab initio MC simulations reveal that surface termination and coordination environments play critical roles in governing chemical ordering in MXenes. 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, changing the F/O ratio at prismatic termination sites or alternating the atomic coordination within the MXene lattices can induce an order-to-disorder transition in DTM MXenes.