多铁性四重钙钛矿 $BiMn_{7}O_{12}$ 中的两个位移型铁电相变
Two Displacive Ferroelectric Phase Transitions in Multiferroic Quadruple Perovskite $BiMn_{7}O_{12}$
AI总结：
本文通过微波、太赫兹、红外和拉曼光谱研究了 $BiMn_{7}O_{12}$ 陶瓷，揭示了其在 460 和 300 K 处的两个位移型铁电相变及低温下的多铁性本质，并证实了自旋-声子耦合。
摘要：
我们报告了对 $BiMn_{7}O_{12}$ 陶瓷进行的微波、太赫兹（THz）、红外和拉曼光谱研究，进一步阐明了两个结构相变的本质及其与该化合物铁电性的可能联系。我们观察到在冷却至 460 和 300 K 时，THz 范围内的一种极性声子发生软化，即 $BiMn_{7}O_{12}$ 在这些温度下经历从单斜 $\ extit{I2/m}$ 到极性单斜 $\ extit{Im}$ 和三斜 $\ extit{P1}$ 相的后续结构相变。软声子引起了位移型铁电相变典型的介电反常。在 5.8 GHz 下进行至 400 K 的微波测量不仅定性地证实了 300 K 处的介电反常，还揭示了在 60 K 和 28 K 磁相变附近的其他两个弱介电反常。这证明了低温相的多铁性本质，尽管在 kHz 和 Hz 频谱范围内相对较高的电导率使我们无法直接测量介电常数和铁电极化。一些拉曼模感测到发生在 60 和 25 K 附近的磁相变，表明自旋-声子耦合在该化合物及该温度范围内是相关的。Mn-O 伸缩模频率偏离非谐温度行为的现象，已通过由比热的磁性贡献计算出的自旋关联函数成功解释。
英文摘要：
We report on the microwave, terahertz (THz), infrared and Raman spectroscopic studies of $BiMn_{7}O_{12}$ ceramics, shedding more light into the nature of two structural phase transitions and their possible relation with ferroelectricity in this compound. We observed a softening of one polar phonon in the THz range on cooling towards 460 and 300 K, i.e., temperatures at which $BiMn_{7}O_{12}$ undergoes subsequent structural phase transitions from monoclinic $\textit{I2/m}$ to polar monoclinic $\textit{Im}$ and triclinic $\textit{P1}$ phases. The soft phonon causes dielectric anomalies typical for displacive ferroelectric phase transitions. Microwave measurements performed at 5.8 GHz up to 400 K qualitatively confirmed not only the dielectric anomaly at 300 K, but also revealed two other weak dielectric anomalies near the magnetic phase transitions at 60 K and 28 K. This evidences the multiferroic nature of the low-temperature phases, although the relatively high conductivity in the kHz and Hz spectral range prevented us from directly measuring the permittivity and ferroelectric polarization. Some Raman modes sense the magnetic phase transitions occurring near 60 and 25 K, showing that spin-phonon coupling is relevant in this compound and in this temperature range. The deviation of the Mn-O stretching mode frequency from the anharmonic temperature behavior was successfully explained by the spin correlation function calculated from the magnetic contribution to the specific heat.
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