We employ terahertz-range temperature-dependent Raman spectroscopy and first-principles lattice dynamical calculations to show that the undoped sodium ion conductors Na₃PS₄ and isostructural Na₃PSe₄ both exhibit anharmonic lattice dynamics. The anharmonic effects in the compounds involve coupled host lattice–Na⁺ ion dynamics that drive the tetragonal-to-cubic phase transition in both cases, but with a qualitative difference in the anharmonic character of the transition. Na₃PSe₄ shows an almost purely displacive character with the soft modes disappearing in the cubic phase as the change in symmetry shifts these modes to the Raman-inactive Brillouin zone boundary. Na₃PS₄ instead shows an order–disorder character in the cubic phase, with the soft modes persisting through the phase transition and remaining Raman active in the cubic phase, violating Raman selection rules for that phase. Our findings highlight the important role of coupled host lattice–mobile ion dynamics in vibrational instabilities that are coincident with the exceptional conductivity of these Na⁺ ion conductors.

中文翻译：

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钠离子导体中的非谐晶格动力学

我们采用太赫兹范围的温度相关拉曼光谱和第一性原理晶格动力学计算来表明未掺杂的钠离子导体 Na ₃ PS ₄和同构的 Na ₃ PSe ₄都表现出非谐晶格动力学。化合物中的非谐效应涉及耦合的主晶格-Na ⁺离子动力学，在这两种情况下驱动四方相到立方相变，但在转变的非谐特性方面存在质的差异。钠₃ PSe ₄随着对称性的变化将这些模式转移到拉曼非活动布里渊区边界，软模式在立方相中消失，显示出几乎纯位移的特征。相反， Na ₃ PS ₄在立方相中显示出有序-无序特征，软模式在相变中持续存在，并且在立方相中保持拉曼活性，违反了该相的拉曼选择规则。我们的研究结果强调了耦合的主晶格-移动离子动力学在振动不稳定性中的重要作用，这与这些 Na ⁺离子导体的出色导电性相一致。