Insights into the Rich Polymorphism of the Na⁺ Ion Conductor Na₃PS₄ from the Perspective of Variable-Temperature Diffraction and Spectroscopy

Solid electrolytes are crucial for next-generation solid-state batteries, and Na3PS4 is one of the most promising Na+ conductors for such applications, despite outstanding questions regarding its structural polymorphs. In this contribution, we present a detailed investigation of the evolution in structure and dynamics of Na3PS4 over a wide temperature range 30 < T < 600 °C through combined experimental–computational analysis. Although Bragg diffraction experiments indicate a second-order phase transition from the tetragonal ground state (α, P4̅21c) to the cubic polymorph (β, I4̅3m) above ∼250 °C, pair distribution function analysis in real space and Raman spectroscopy indicate remnants of a tetragonal character in the range 250 < T < 500 °C, which we attribute to dynamic local tetragonal distortions. The first-order phase transition to the mesophasic high-temperature polymorph (γ, Fddd) is associated with a sharp volume increase and the onset of liquid-like dynamics for sodium-cations (translational) and thiophosphate-polyanions (rotational) evident by inelastic neutron and Raman spectroscopies, as well as pair-distribution function and molecular dynamics analyses. These results shed light on the rich polymorphism of Na3PS4 and are relevant for a range host of high-performance materials deriving from the Na3PS4 structural archetype.固体电解质（Solid electrolytes）是下一代固态电池的核心关键材料，而Na₃PS₄是此类应用中最具潜力的钠离子导体（Na⁺ conductor）之一，尽管其结构多晶型仍存在诸多悬而未决的问题。在本研究中，我们通过实验与计算联用的分析手段，对Na₃PS₄在30 < T < 600 ℃的宽温度区间内的结构与动力学演化展开了详尽的研究。尽管布拉格衍射（Bragg diffraction）实验表明，在约250 ℃以上时，材料会从四方基态（α相，空间群P4̅21c）发生二级相变，转变为立方多晶型（β相，空间群I4̅3m）；但实空间对分布函数（Pair Distribution Function，PDF）分析与拉曼光谱（Raman spectroscopy）结果显示，在250 < T < 500 ℃的温度区间内，材料仍残留有四方相特征，我们将这一现象归因于动态局部四方畸变。向介相高温多晶型（γ相，空间群Fddd）的一级相变伴随显著的体积膨胀，同时钠阳离子（平移运动）与硫代磷酸根多阴离子（旋转运动）开始呈现类液动力学行为，这一结论可通过非弹性中子光谱、拉曼光谱、对分布函数分析以及分子动力学（Molecular Dynamics，MD）模拟得到验证。本研究结果阐明了Na₃PS₄丰富的多态性，对于一系列基于Na₃PS₄结构原型的高性能材料的开发具有重要参考价值。