In improper ferroelectrics, the spontaneous ordering is typically driven by a structural distortion or a magnetic spin alignment. The induced electric polarization is only a secondary effect. This dependence is a rich source for unusual phenomena and ferroelectric domain configurations for proper, polarization-driven ferroelectrics. This study focuses on the polar domain structure and the hysteretic behavior at the ferroelectric phase transition in Ca₃Mn_1.9Ti_0.1O₇ as a representative of the recently discovered hybrid improper ferroelectric class of multiferroics. Combining optical second harmonic generation and Raman spectroscopy gives access to the spontaneous structural distortion and the resulting improper electric polarization. This study shows that hybrid improper ferroelectrics contrast proper and improper ferroelectrics in several ways. Most intriguingly, adjacent ferroelectric domains favor head-to-head and tail-to-tail domain walls over charge-neutral configurations. Furthermore, the phase transition occurs in an asymmetric fashion. The regime of phase coexistence of the nonpolar and polar phases shows a clear and abrupt upper temperature limit. In contrast, the coexistence toward low temperatures is best described as a fade-out process, where 100-nm-sized islands of the nonpolar phase expand deep into the polar phase.

中文翻译：

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混合不当铁电体中铁电相变和带电畴壁的不对称特征

在不合适的铁电体中，自发排序通常由结构畸变或磁自旋排列驱动。感应电极化只是次要效应。这种依赖性对于适当的极化驱动铁电体来说是异常现象和铁电畴配置的丰富来源。本研究重点研究 Ca ₃ Mn _1.9 Ti _0.1 O _{7中铁电相变时的极性畴结构和滞后行为}作为最近发现的混合不当铁电类多铁性材料的代表。结合光学二次谐波产生和拉曼光谱可以访问自发结构失真和由此产生的不正确电极化。这项研究表明，混合不当铁电体以多种方式对比正确和不当铁电体。最有趣的是，与电荷中性配置相比，相邻的铁电畴更倾向于头对头和尾对尾畴壁。此外，相变以不对称的方式发生。非极性相和极性相的相共存状态显示出明显而突然的温度上限。相比之下，向低温的共存最好描述为淡出过程，