Time-Temperature-Transformation (TTT) Diagrams to rationalize the nucleation and quenchability of metastable $\alpha$-Li$_3$PS$_4$

TL;DR

This study constructs a comprehensive TTT diagram for $ ext{Li}_3 ext{PS}_4$, revealing nanoscale stabilization mechanisms that explain its metastability and guiding targeted synthesis of metastable phases.

Contribution

It introduces a combined computational and experimental framework to understand nucleation and phase stability, specifically explaining the metastability of $ ext{Li}_3 ext{PS}_4$'s $ ext{alpha}$ phase.

Findings

Nanoscale $ ext{alpha}$-phase is stabilized by low surface energy.

Rapid nucleation of nano-sized $ ext{alpha}$-phase observed in-situ.

Size-dependent stabilization explains metastability and quenchability.

Abstract

$\alpha$-Li$_3$PS$_4$ is a promising solid-state electrolyte with the highest ionic conductivity among its polymorphs. However, its formation presents a thermodynamic paradox: the $\alpha$-phase is the equilibrium phase at high temperature and transforms to the stable $\gamma$-Li$_3$PS$_4$ polymorph when cooled to room temperature; however, $\alpha$-Li$_3$PS$_4$ 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 resolve this paradox by establishing a comprehensive time-temperature-transformation (TTT) diagram, constructed from a computational temperature-size phase diagram and experimental high-time-resolution isothermal measurements. Our density functional theory calculations reveal that at the nanoscale, the $\alpha$-phase is stabilized by its low surface energy, which drastically lowers the nucleation barrier across a wide temperature range. This size-dependent stabilization is directly visualized using in-situ synchrotron X-ray diffraction and electron microscopy, capturing the rapid nucleation of nano-sized $\alpha$-phase 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.