Physical mechanism of Ge doping enhanced Ruddlesden-Popper structure quasi-2D Sr₃Sn₂O₇ ceramic hybrid improper ferroelectricity

Abstract

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 Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> and Sr<sub>3</sub>Sn<sub>1.99</sub>Ge<sub>0.01</sub>O<sub>7</sub> 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 Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> ceramics exhibit lower coercive field that is close to that of Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> single crystal. Moreover, via minor amount of Ge doping, significant enhancement in polarization is achieved from 0.34μC/cm<sup>2</sup> of Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> to 0.61 μC/cm<sup>2</sup> of Sr<sub>3</sub>Sn<sub>1.99</sub>Ge<sub>0.01</sub>O<sub>7</sub>. 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 Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> and Sr<sub>3</sub>Sn<sub>1.99</sub>Ge<sub>0.01</sub>O<sub>7</sub> 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 Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> 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 Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub>, potentially facilitating their widespread application in various capacitors and non-volatile memory devices.