BaNa₂Fe[VO₄]₂ contains a Jahn–Teller active ion (Fe^II, 3d⁶, high-spin) in an octahedral coordination. On the basis of a combination of temperature-dependent X-ray diffraction and Mössbauer and Raman spectroscopies, we demonstrate the coupling of lattice dynamics with the electronic ground state of Fe^II. We identify three lattice modes combined to an effective canted screw-type motion that drives the structural transition around room temperature from the high-temperature (P3̅) via intermediate phases to the low-temperature phase (C2/c). The dynamics of the electronic ground state of Fe(II) are evident from Mössbauer data with signatures of a motion-narrowed doublet above 320 K, a gradual evolution of the ⁵E_g electronic state below 293 K, and finally the signature of the thermodynamically preferred orbitally nondegenerate ground state (⁵A_g) of Fe(II) below 100 K. The continuous nature of the transition is associated with the temperature-dependent phonon parameters derived from Raman spectroscopy, which point out the presence of strong electron–phonon coupling in this compound. We present a microscopic mechanism and evaluate the collective component leading to the structural phase transition.

中文翻译：

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螺钉-类型运动及对协同性在BANA冲击₂的Fe [VO ₄ ] ₂

BaNa ₂ Fe [VO ₄ ] ₂包含八面体配位的Jahn–Teller活性离子（Fe ^II，3d ⁶，高自旋）。基于温度相关的X射线衍射以及Mössbauer和Raman光谱学的结合，我们证明了晶格动力学与Fe ^II的电子基态的耦合。我们确定3种格模式组合以一种有效的竖直安装螺钉-型运动驱动从高温（室温附近的结构性转变P经由中间阶段的低温相3）（c ^ 2 / ç）。Fe（II）的电子基态的动力学从Mössbauer数据中可以看出，其特征是在320 K以上运动狭窄的双峰态，在293 K以下的⁵ E _g电子态逐渐演化，最后是热力学特征。低于100 K的Fe（II）的优选轨道非简并基态（⁵ A _g）。跃迁的连续性质与拉曼光谱法得出的依赖温度的声子参数有关，后者表明存在强电子-声子耦合在这个化合物中。我们提出了一种微观机制，并评估了导致结构相变的集体成分。