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准二维的杂化非本征铁电体在实现强磁电耦合的单相室温多铁性方面具有很大的潜力，然而此类陶瓷样品通常有着较高的矫顽场和较低的剩余极化强度，严重阻碍了对其的研究和应用。本文成功制备了高质量且单相性较好的具有准二维的双层Ruddlesden-Popper（R-P）型氧化物Sr3Sn2-xGexO7陶瓷样品，观察到了较高的剩余极化和与Sr3Sn2O7单晶接近的矫顽场。微量Ge元素对B位Sn掺杂后极化强度显著增强，同时进一步降低了Sr3Sn2O7样品的矫顽场。结合晶格动力学研究，我们对样品的拉曼和红外光谱进行了标定，得出掺杂样品铁电性能的增强可能源于氧八面体倾侧幅度的增大和旋转幅度减小。Berry相位法和波恩有效电荷模型进一步证实了铁电性能的增强。通过紫外可见光光度计测试得到Sr3Sn2O7样品的光学带隙为3.91eV，与采用Becke-Johnson势结合局部密度近似(MBJ-LDA)所计算的结果基本一致。总之，这项研究为此类杂化非本征铁电体的制备及铁电性能的调控提供了参考，有望促进铁电陶瓷在各种电容器和非易失性存储器件中的广泛应用。Hybrid improper ferroelectricity with quasi-two-dimensional structures has attracted much attention recently due to its great potential in achieving strong magnetoelectric coupling and room-temperature multiferroicity in a single phase. However, recent studies showed that they typically exhibit high coercive field and low remnant polarization in ceramics, which severely hinder their application. In this work, high-quality Sr3Sn2O7 and Sr3Sn1.99Ge0.01O7 ceramics with a Ruddlesden-Popper (R-P) structure were successfully prepared, and their crystal structure and electrical properties were investigated in detail. It is found that Sr3Sn2O7 ceramics exhibit lower coercive field that is close to that of Sr3Sn2O7 single crystal. Moreover, via minor amount of Ge doping, significant enhancement in polarization is achieved from 0.34μC/cm2 of Sr3Sn2O7 to 0.61 μC/cm2 of Sr3Sn1.99Ge0.01O7. Combined with crystal lattice dynamic studies, we analyzed the Raman and infrared response of the samples, which shows the information of the tilting and rotation of the oxygen octahedra in the samples. The improved ferroelectricity after doping may be attributed to the increased amplitude of the tilt mode and the decreased amplitude of the rotation mode. Besides, the enhanced ferroelectric properties through Ge doping and its mechanism are further investigated by the Berry phase approach and the Born effective charge method. Furthermore, via UV-visible spectroscopy optical bandgap is determined to be 3.91 and 3.95 eV for Sr3Sn2O7 and Sr3Sn1.99Ge0.01O7 ceramics, respectively. Using the Becke-Johnson potential combined with the local density approximation correlation, the bandgap is calculated and is found to be in close agreement with the experimental results. And the electronic excitations could be assigned as O 2p to Sn 5s (Ge 4s) charge transfer excitations. The effect of Ge doping on the ability of Sr3Sn2O7 to gain and lose electrons and the bonding strength of Sn-O bonds was analyzed via two-dimensional charge density difference. In conclusion, this study provides insights into the synthesis method and modulation of ferroelectric properties of hybrid improper ferroelectrics Sr3Sn2O7, potentially facilitating their widespread application in various capacitors and non-volatile memory devices.
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Keywords:
- Sr3Sn2O7 /
- hybrid improper ferroelectricity /
- oxygen octahedron tilt and rotation /
- first-principles study
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