Reversible transdimensional phase transition in a topological semimetal

The dimensionality in material science serves as a fundamental spatial metric that profoundly shapes both phases and inherent properties of materials. Phase engineering is widely employed to tailor atomic and dimensional structures, facilitating the discovery of emergent quantum phases. However, isocompositional phase transitions in two-dimensional (2D) or three-dimensional (3D) materials are typically restricted in the same dimension due to the significant differences in surface energies and out-of-plane chemical bonds. Here, we report a reversible transdimensional phase transition between a 2D ferroelectric phase and a 3D topological T’ phase in the semimetal PtBi ₂ . Reversible resistance hysteresis loops are revealed in the electrical measurements, which originate from the 2D-3D phase transition and contact resistance modulation. This transdimensional phase transition is attributed to the mechanism of intralayer splitting and interlayer reconstructions: Pt atoms migrate from one layer to the van der Waals gap and format covalent bonds with Bi atoms in the adjacent layer. Theoretical calculations further confirm that our results not only refer to transdimensional structural changes but also 2D-3D transdimensional topological transitions, establishing the transdimensional phase engineering as a platform for exploring topological and functional quantum states.