Time-Temperature-Transformation Diagrams to Navigate the Nucleation and Quenchability of Metastable α-Li₃PS₄

α-Li₃PS₄ is a promising solid-state electrolyte with the highest ionic conductivity among its polymorphs. However, its formation presents a thermodynamic paradox: the α-phase is the equilibrium phase at high temperature and transforms to the stable γ-Li₃PS₄ polymorph when cooled to room temperature; however, α-Li₃PS₄ can be synthesized and quenched in a metastable state via rapid heating at relatively low temperatures. The origin of this synthesizability and anomalous stability has remained elusive. Here, we address this paradox by presenting an experimental and computational time-temperature-transformation (TTT) diagram, constructed from a temperature-size phase diagram and experimental high-time-resolution isothermal measurements. Our density functional theory calculations reveal that at the nanoscale, the α-phase is stabilized by its low surface energy, which drastically lowers its nucleation barrier across a wide temperature range. This size-dependent stabilization is directly visualized using in-situ sub-second synchrotron X-ray diffraction and electron microscopy, capturing the rapid nucleation of nano-sized α-Li₃PS₄ and its subsequent slow transformation. This work presents a generalizable framework that integrates thermodynamic and kinetic factors for understanding nucleation and phase transformation mechanisms, providing a rational strategy for the targeted synthesis of functional metastable materials.