[论文解读] Pressure-driven vibrational and structural peculiarities in the honeycomb layered magnetoelectrics Mn4(B)2O9 (B= Nb, Ta)

Rajesh Jana, Afsal S. Shajahan+7 more|arXiv (Cornell University)|Mar 17, 2026

Multiferroics and related materials被引用 0

一句话总结

本论文通过 Raman 光谱、同步辐射 X 射线衍射和 DFT 揭示 Mn4Nb2O9 和 Mn4Ta2O9 在高压下的多重等结构转变与长程结构变化，伴随显著的各向异性晶格压缩。

ABSTRACT

The high-pressure behavior of two Mn-based honeycomb-structured magnetoelectric materials, Mn4Nb2O9 (MNO) and Mn4Ta2O9 (MTO), was investigated using Raman spectroscopy, synchrotron x-ray diffraction, and density functional theory (DFT) calculations. In MTO, the application of a small pressure of only 0.5 GPa induces an isostructural transition driven by local symmetry breaking. With further increase in pressure, three additional isostructural transitions are observed at about 3.2, 6, and 10 GPa, followed by the onset of a long-range structural transition near 14 GPa, where the ambient P-3c1 phase begins to transform into a P2/c phase. These two phases coexist up to 27 GPa. The Nb analogue, MNO, also exhibits similar isostructural transitions at about 2, 6.6, and 10 GPa. However, the onset of the mixed P2/c and P-3c1 phases occurs at a slightly lower pressure of 12.5 GPa, with phase coexistence extending up to 26.5 GPa. These long-range transitions are supported by pressure-dependent enthalpy changes obtained from DFT calculations. Rietveld refinement reveals pronounced anisotropic lattice compression, with a 42 to 49 percent difference between the c and a axes, leading to a notable reduction in the c/a ratio. This anisotropy may strengthen interlayer coupling and promote magnetic ordering under compression, consistent with the appearance of Raman modes similar to those reported at low temperatures, together with anomalous changes in Raman mode linewidth and intensity. The marked changes in Raman self-energy parameters, anomalies in the reduced pressure-Eulerian strain profile, and the onset of local symmetry breaking at much lower pressures in MTO than in MNO highlight the important role of differences in spin-orbit coupling strength and orbital hybridization associated with Nb5+ and Ta5+ cations.

研究动机与目标

• 研究 Mn4Nb2O9（MNO）和 Mn4Ta2O9（MTO）的高压振动与结构演化。
• 比较 Nb 与 Ta 变体的压力响应以评估 B 位对其影响。
• 在压缩下识别等结构转变和长程结构变化。
• 表征各向异性晶格压缩及其对磁相互作用的影响。
• 将拉曼模式演化与潜在的磁结构耦合相关联。

提出的方法

• 对多晶样品进行高压 Raman 光谱测量，压力达 ~28 GPa。
• 对 MNO 进行高压下的同步辐射 X 射线衍射实验，压力达 26.5 GPa；对 MTO 的压力范围为 9–25 GPa。
• 密度泛函理论计算（DFT，PBE GGA，700 eV 截止，Gamma 点采样）以佐证焓驱动的转变。
• Rietveld 精细化与晶格参数分析以提取各向异性压缩（c/a 比）。
• 对 Raman 峰进行 Lorentzian 拟合以在高压下提取频率与线宽。

Figure 1: (a) Unit-cell structure of MNO/MTO crystallizing in the trigonal P-3c1 symmetry at ambient pressure. (b) Planar honeycomb layer (L1) consisting of edge-shared Mn1O 6 octahedra, viewed along the $c$ axis. (c) Buckled layer (L2) composed of two honeycomb sublayers formed by edge-shared Mn2O

实验结果

研究问题

• RQ1 在高压下 MNO 与 MTO 的等结构转变压力是多少？
• RQ2 长程结构转变何时发生，以及它们如何在 Raman 与 XRD 数据中表现？
• RQ3 在压力下各向异性晶格压缩（c 轴与 a 轴）如何演化，这对磁相互作用有何影响？
• RQ4 Nb 与 Ta 阳离子如何在自旋–声子的耦合与振动动力学方面产生差异？
• RQ5 观测到的 Raman 模型异常与潜在的磁结构耦合之间的关系是什么？

主要发现

• MTO 在约 0.5、3.2、6 与 10 GPa 附近显示等结构转变，在约 14 GPa 出现长程转变，且在 ~27 GPa 之前与基态相共存。
• MNO 在约 2、6.6 与 10 GPa 附近显示等结构转变，近 12.5 GPa 出现长程转变并且与高压相共存至 ~26.5 GPa。
• 两种化合物在压力下均经历显著的各向异性压缩，c 轴收缩更大，压力下明显降低 c/a 比。
• 在压力下出现与消失的多个 Raman 模式表明结构与振动的强烈重组，包括低压下的局部对称性破缺以及与磁性相关的声子变化。
• DFT 的焓计算支持观察到的长程转变及基态 P-3c1 与高压相之间的相竞争。
• Nb 与 Ta 变体之间的差异归因于自旋轨道耦合强度与轨道混成差异，影响声子行为及转变压力。

Figure 2: High-pressure Raman spectra of MNO up to 15 GPa in the wavenumber range 105–700 cm -1 . The emergence of new Raman modes is indicated by upward arrows, while the disappearance of modes is marked by downward arrows. The pressure evolution of individual modes is tracked by dashed black lines