Phase Stability of MXenes: Understanding the Role of Coordination Symmetries, Transition Metals, and Surface Terminations

Due to diverse chemical compositions, MXenes have emerged as a promising class of two-dimensional materials for various applications such as energy storage, electromagnetic shielding, and thermoelectrics. These layered materials can have two distinct crystal coordinations for transition-metal ions, trigonal-prismatic and octahedral, resulting in different phases. Understanding the relative stability of the phases and factors governing the phase stability is crucial for the phase engineering of MXenes with desirable physical properties for specific applications. We have systematically investigated the stability of different MXene phases arising due to various combinations of coordination symmetries and identified factors governing the phase stability. Specifically, we focused on carbide MXenes with a series of transition metals and surface terminations (T = O, F, and OH). Our density functional theory simulation demonstrated that the majority of MXene compositions are most stable with octahedral coordination for transition metals, but compositions such as Mo2CO2, W2CO2, Nb2CF2, Nb2C(OH)2, Ta2CF2, and Ta2C(OH)2 prefer a phase where transition-metal ions have trigonal-prismatic coordination. The stability trends observed in the DFT simulations are explained using crystal field theory, and a model is developed to predict stable phases based on the number of electrons. Furthermore, we use the Wannierzation technique along with a tight-binding model to explain different trends in stability with atomic properties. This study provides valuable insights into the phase stability of MXenes and offers a roadmap for phase-engineering these materials with desirable physical properties for specific applications.