Structural metatransition of energetically tangled crystalline phases.

We solve the longstanding puzzle of pressure induced structural evolution of SnSe using a swarm structure search method combined with first-principles phonon and kinetic barrier calculations. Our results identify a dynamic set of nearly degenerate crystalline SnSe phases that are separated by low kinetic barriers and undergo an unusual type of structural transitions characterized by a dynamically changing mix of the constituent phases. We introduce a new concept of structural metatransition to highlight the transitional nature of such phase transitions. Our theoretical prediction is corroborated by X-ray diffraction measurements, and this intriguing phenomenon offers insights into the enigmatic property variations of SnSe under pressure. This work raises prospects of considerably improving characterization and understanding of intrinsic multiphase crystals and their dynamic evolution.