posted on 2024-02-14, 13:04authored byLei-Yu Zhan, Yu Zhou, Na Li, Lin-Jie Zhang, Xiao-Juan Xi, Zhao-Quan Yao, Jiong-Peng Zhao, Xian-He Bu
Molecular-based multiferroic materials that possess ferroelectric
and ferroelastic orders simultaneously have attracted tremendous attention
for their potential applications in multiple-state memory devices,
molecular switches, and information storage systems. However, it is
still a great challenge to effectively construct novel molecular-based
multiferroic materials with multifunctionalities. Generally, the structure
of these materials possess high symmetry at high temperatures, while
processing an obvious order–disorder or displacement-type ferroelastic
or ferroelectric phase transition triggered by symmetry breaking during
the cooling processes. Therefore, these materials can only function
below the Curie temperature (Tc), the low of which is a severe impediment to their practical
application. Despite great efforts to elevate Tc, designing single-phase crystalline materials
that exhibit multiferroic orders above room temperature remains a
challenge. Here, an inverse temperature symmetry-breaking phenomenon
was achieved in [FPM][Fe3(μ3-O)(μ-O2CH)8] (FPM stands for 3-(3-formylamino-propyl)-3,4,5,6-tetrahydropyrimidin-1-ium,
which acts as the counterions and the rotor component in the network),
enabling a ferroelastoelectric phase at a temperature higher than Tc (365 K). Upon heating from room temperature,
two-step distinct symmetry breaking with the mm2Fm species leads to the coexistence of ferroelasticity and ferroelectricity
in the temperature interval of 365–426 K. In the first step,
the FPM cations undergo a conformational flip-induced inverse temperature
symmetry breaking; in the second step, a typical ordered–disordered
motion-induced symmetry breaking phase transition can be observed,
and the abnormal inverse temperature symmetry breaking is unprecedented.
Except for the multistep ferroelectric and ferroelastic switching,
this complex also exhibits fascinating nonlinear optical switching
properties. These discoveries not only signify an important step in
designing novel molecular-based multiferroic materials with high working
temperatures, but also inspire their multifunctional applications
such as multistep switches.