Two-dimensional fluids under triangle-well-like interactions: A tunable phase behavior

Abstract

We carried out a detailed molecular dynamics study to investigate the phase behavior of two-dimensional fluids where particles interact through triangular-like pair potentials. Focusing on three representative interaction models—the triangular well (TW), the Jagla-like (JL), and the inverse triangular well (ITW)—we explored how subtle changes in the potential shape influence structural organization and phase transitions. Each model exhibits distinctive behavior. The TW potential reproduces classical 2D phase features, including a well-defined vapor–liquid coexistence and the formation of compact triangular solids. The JL potential, characterized by a soft repulsive ramp, introduces structural frustration that stabilizes low-density square lattices and gives rise to rich polymorphism, including solid–solid coexistence. In contrast, the ITW potential, where attractive and repulsive features overlap, displays the most complex structural diversity, with emergent phases such as dodecagonal and honeycomb-like solids. By analyzing thermodynamic properties, radial distribution functions, and bond-orientational order parameters, we mapped out vapor–liquid, liquid–solid, and solid–solid transitions, revealing how the shape of the interaction potential plays a critical role in governing order, polymorphism, and phase stability in two-dimensional systems. Finally, we confirm the presence of the hexatic phase in systems governed by the TW potential by analyzing both bond-orientational and translational correlation functions. These findings not only support the existence of this intermediate phase but also provide a clear framework for its identification. More broadly, this work lays the groundwork for exploring the hexatic phase across a wide range of discrete pair interaction models in two-dimensional systems, offering new opportunities to understand the subtle mechanisms behind two-step melting and structural transitions.