It is well documented that the solid-state phase transition types are generally controlled by kinetic conditions (e.g., cooling/heating rate) and thermodynamic factors (e.g., temperature). Applying in situ transmission electron microscopy as well as theoretical calculations and simulations, we unexpectedly discover two competing transition types of β-phase single-crystalline tungsten nanoparticles (NPs) that are mediated by size and impurity. Under thermal heating, a nonclassical diffusion-mediated fast transition from β to α phase is identified in NPs with sizes >100 nm. Concerning the NPs with sizes <100 nm, we detect a distinct interface-controlled diffusion mechanism, driving the β to hexagonal close-packed (hcp) phase transition with much slower transition rate. By investigating the overall energy landscape of the α, β, and hcp phases as a function of both size and light element impurity (hydrogen and oxygen), we uncover the possible origin of two different transition modes. Our observations highlight the complex interplay of various parameters in mediating the solid-phase transition modes, and thus offer in-depth insights into designing the functionalized low-dimensional materials.

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• Phase transitions
• Nanoparticles
• Single crystal materials
• Ab initio molecular dynamics
• Density functional calculations
• High-resolution electron microscopy