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

Oct 26, 2026

α-Li₃PS₄ is a promising solid-state electrolyte with the highest ionic conductivity among its polymorphs. Its formation is thermodynamically paradoxical: the α phase is the high-temperature equilibrium state, yet it transforms to stable γ-Li₃PS₄ upon cooling, while metastable α-Li₃PS₄ can be synthesized and quenched after rapid heating at relatively low temperatures. Here a comprehensive time-temperature-transformation (TTT) framework resolves the paradox using computations and high-time-resolution isothermal experiments. DFT shows nanoscale stabilization via low surface energy lowers nucleation barriers, visualized by in-situ synchrotron XRD and microscopy.

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CategoryResearch & Report

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Abstract

Thermodynamic paradox in α-Li₃PS₄ formation

Building comprehensive time-temperature-transformation (TTT) diagrams

Computational and experimental approaches

Time-Temperature-Transformation Diagrams to Navigate the Nucleation and Quenchability of Metastable α-Li₃PS4 Akira Miura,a†* Woohyeon Baek,b† Fujii,a Kiyoharu Tadanaga,a Rana Hossain,a Aichi Yamashita,c Yoshikazu Mizuguchi,c Chikako Moriyoshi,d Shintaro Kobayashi,e Shogo Kawaguchi,e Ding Jiong, f Shigeo Mori, f Atsushi Sakuda,g Akitoshi Hayashi,g,h Wenhao Sunb * a Faculty of Engineering, Hokkaido University, Kita 13, Nishi 8, Sapporo, 060-8628, Japan b Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI, 48109, USA c Department of Physics, Tokyo Metropolitan University, Hachioji, 192-0397, Japan d Graduate School of Advanced Science and Engineering, Hiroshima University, 1-3-1, Kagamiyama, Higashihiroshima 739-8526, Japan e Japan Synchrotron Radiation Research Institute, Sayo, Hyogo, 679-5198, Japan f Department of Materials Science, Graduate School of Engineering, Osaka Metropolitan University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka, 599-8531, Japan g Department of Applied Chemistry, Graduate School of Engineering, Osaka Metropolitan University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka, 599-8531, Japan h Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan † contributed equally Email: [amiura@eng.hokudai.ac.jp](amiura@eng.hokudai.ac.jp) (AM), [whsun@umich.edu](whsun@umich.edu) (WS) Abstract: 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 resolve this paradox by establishing a comprehensive time-temperature-transformation (TTT) diagram, constructed from a computational temperature-size phase diagram and experimental hightime-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 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 α-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. Metastable materials, ranging from traditional steel and glass to advanced battery materials, are crucial for a wide range of functional applications.1-3 Numerous synthesis techniques have been developed to obtain metastable materials, including topotactic intercalation, 1 ball milling,2 and rapid temperature control.3 Among these, controlling the heating profile remains a key strategy for producing metastable phases with desired properties. For instance, the classic composition-temperature phase diagram of the Fe-C system shows thermodynamically stable phases, while the temperature-time-transformation (TTT) diagram provides kinetic guidance for processing both stable and metastable phases in steel.4 Multidimensional phase diagrams, which extend beyond conventional twodimensional temperature-pressure or temperature-composition diagrams, can be powerful tools for capturing the thermodynamics and processing conditions for producing metastable materials.5 By combining first-principles theories and computations, we can achieve a predictive understanding to guide the synthesis and processing pathways of specific metastable phases.6,7 For e

FAQs

What is the thermodynamic paradox in α-Li₃PS₄ formation?

The α phase is the equilibrium high-temperature state, but it should transform to stable γ-Li₃PS₄ on cooling. Yet metastable α-Li₃PS₄ can still be synthesized and quenched by rapid heating at relatively low temperatures.

How does the proposed TTT diagram resolve the paradox?

It combines a computational temperature-size phase diagram with high-time-resolution isothermal measurements to connect thermodynamic stability with kinetic nucleation and transformation pathways.

Why is nanoscale α-phase stabilized according to the calculations?

Density functional theory indicates the α phase has low surface energy at the nanoscale, which substantially lowers the nucleation barrier across a wide temperature range.

Oct 26, 2026

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