Latent Phase Transition in Two-Dimensional PdSe_{2}.

Conventional phase transitions in two-dimensional (2D) materials typically involve gradual and progressive structural evolution upon external stimuli. However, analogous to the latent heat phenomena in classical thermodynamics, where energy absorption leads to internal reconfiguration without immediate phase change, we report an unconventional "latent phase transition" in 2D palladium diselenide (PdSe_{2}), uncovered through in situ atomic scanning transmission electron microscopy. During Pd atom implantation and subsequent thermal annealing, the PdSe_{2} lattice absorbs input energy without significant structural rearrangement, maintaining its original framework over an extended temperature range. Remarkably, once a critical threshold of internal chemical pressure is reached, the structure abruptly collapses into a three-dimensional crystalline Pd_{17}Se_{15} phase. Stabilized by the robust layer structure of PdSe_{2}, this phase transition behavior closely mirrors the latent energy and sudden transformation characteristic of classical latent heat processes. Density functional theory calculations corroborate the mechanism, highlighting the role of van der Waals gap energy buffering and the weak Pd-Pd interactions. Our findings establish an unusual phase transition paradigm in 2D materials, enriching the fundamental understanding of phase engineering in low-dimensional systems.