[论文解读] Origin of Large Dielectric Constant with Large Remnant Polarization and Evidence of Magnetoelectric Coupling in Multiferroic La modified BiFeO3-PbTiO3 Solid Solution

Anupinder Singh, Ratnamala Chatterjee+3 more|arXiv (Cornell University)|Feb 8, 2010

Multiferroics and related materials参考文献 32被引用 42

一句话总结

本研究探讨了镧掺杂的BiFeO3-PbTiO3固溶体，揭示了晶界处空间电荷极化导致大的介电常数和剩余极化，该结论通过阻抗谱和Maxwell-Wagner建模得到证实。磁电耦合在约170 K时出现，经中子衍射和磁电测量验证，表明结构异常与多铁功能特性相关。

ABSTRACT

The presence of superlattice reflections and detailed analyses of the powder neutron and x-ray diffraction data reveal that La rich (BF$_{0.50}$-LF$_{0.50}$)$_{0.50}$-(PT)$_{0.50}$ (BF-LF-PT) has ferroelectric rhombohedral crystal structure with space group extit{$R3c$} at ambient conditions. The temperature dependence of lattice parameters, tilt angle, calculated polarization $(P_{s})$, volume, and integrated intensity of superlattice and magnetic reflections show an anomaly around 170 K. Impedance spectroscopy, dielectric and ac conductivity measurements were performed in temperature range $473K \leq T \leq 573K$ to probe the origin of large remnant polarization and frequency dependent broad transitions with large dielectric constant near $T_c^{FE}$. Results of impedance spectroscopy measurements clearly show contributions of both grain and grain boundaries throughout the frequency range ($10^{3}$ Hz$\leq f\leq 10^{7} $ Hz). It could be concluded that the grain boundaries are more resistive and capacitive as compared to the grains, resulting in inhomogeneities in the sample causing broad frequency dependent dielectric anomalies. Enhancement in dielectric constant and remnant polarization values are possibly due to space charge polarization caused by piling of charges at the interface of grains and grain boundaries. The imaginary parts of dielectric constant ($ε^{\prime\prime}$) Vs frequency data were fitted using Maxwell-Wagner model at $T_c^{FE}(\sim 523$K) and model fits very well with the data up to $10^{5}$ Hz. Magnetodielectric measurements prove that the sample starts exhibiting magnetoelectric coupling at $\sim 170$ K, which is also validated by neutron diffraction data.

研究动机与目标

• 理解La掺杂BiFeO3-PbTiO3固溶体中大介电常数和高剩余极化起源的机制。
• 研究晶界和空间电荷极化在介电异常中的作用。
• 确定磁电耦合的起始温度，并将其与结构转变相关联。
• 分析晶格参数、极化和磁序随温度的演化行为。
• 利用中子衍射和X射线衍射验证超晶格和磁性衍射峰的存在。

提出的方法

• 采用粉末中子衍射和X射线衍射测定晶体结构及超晶格衍射峰。
• 在10³–10⁷ Hz和473–573 K范围内进行阻抗谱测量，以分离晶粒与晶界贡献。
• 进行介电和交流电导率测量，分析在TcFE ~523 K附近的频率依赖性转变行为。
• 应用Maxwell-Wagner模型拟合TcFE温度下ε′′与频率的关系数据，以确认空间电荷极化机制。
• 通过磁电测量检测磁电耦合，并与中子衍射数据进行对比。
• 分析温度依赖的晶格参数、倾斜角以及超晶格和磁性衍射峰的积分强度，以识别异常行为。

实验结果

研究问题

• RQ1 La掺杂BiFeO3-PbTiO3固溶体中大介电常数和高剩余极化的原因是什么？
• RQ2 晶界如何导致频率依赖性介电异常并增强极化？
• RQ3 磁电耦合在何种温度下出现，其与结构和磁性转变之间有何关联？
• RQ4 空间电荷极化在TcFE附近观测到的介电行为中起何种作用？
• RQ5 超晶格和磁性衍射峰如何与多铁行为的起始相关联？

主要发现

• 样品在常温下具有铁电菱面体结构，空间群为R3c，经中子衍射和X射线衍射确认。
• 在约170 K时，晶格参数、倾斜角、极化强度和衍射峰强度出现异常，表明发生相变。
• 阻抗谱显示晶界电阻率和介电常数高于晶粒，导致材料非均匀性。
• 大介电常数和高剩余极化归因于晶界界面处的空间电荷极化。
• Maxwell-Wagner模型在TcFE（约523 K）下对ε′′与频率数据的拟合效果良好，支持空间电荷极化模型。
• 磁电测量与中子衍射结果共同证实磁电耦合在约170 K时开始出现。