Abstract
AbstractIn 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 Ca3Mn1.9Ti0.1O7 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.
Highlights
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The domains are separated by domain walls, which themselves may add further functionalities because they can possess structure and the hysteretic behavior at the ferroelectric phase transition in non-bulk-like properties
This study shows that hybrid improper ferroelectrics contrast proper and improper depend strongly on the crystallographic orientation of the walls
Summary
The prototype space-group symmetry of Ca3Mn1.9Ti0.1O7 is tetragonal, I4/mmm. From this, a phase transition to the orthorhombic Ccce phase takes place at, presumably, 1100 K.[11,15,23] It consists of an anti-phase rotation of the oxygen octahedra of X1 symmetry around the former fourfold axis of the tetragonal prototype phase[11] (Figure 1d,e; tilt system: a0a0b− in Glazer’s notation[24]). Making use of the linear relation between the vibrational frequency and the tilt angle (the magnitude of the order parameter is approximately proportional to the frequency of the associated mode[27,28]), and by comparison with neutron diffraction data,[18] we can approximate the tilt-related frequency shifts as 17.1 and 12.5 cm−1 deg−1 for the in-phase and the out-of-phase rotations, respectively. Www.advancedsciencenews.com phase), with every second octahedron changing the sense of rotation This contrasts with typical proper ferroelectric phase transitions, like in BaTiO3, which exhibit a softening of the order-parameter-related mode and of the associated polarization close to the phase transition.[32] the evolution of the SHG signal across the ferroelectric transition in BaTiO3 and Ca3Mn1.9Ti0.1O7 is similar at first glance, their microscopic origin is very different.[33]
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