title: Symmetry breaking in molecular ferroelectrics.
abstract: Ferroelectrics are inseparable from symmetry breaking. Accompanying the paraelectric-to-ferroelectric phase transition, the paraelectric phase adopting one of the 32 crystallographic point groups is broken into subgroups belonging to one of the 10 ferroelectric point groups, i.e. C1, C2, C1h, C2v, C4, C4v, C3, C3v, C6 and C6v. The symmetry breaking is captured by the order parameter known as spontaneous polarization, whose switching under an external electric field results in a typical ferroelectric hysteresis loop. In addition, the responses of spontaneous polarization to other external excitations are related to a number of physical effects such as second-harmonic generation, piezoelectricity, pyroelectricity and dielectric properties. Based on these, this review summarizes recent developments in molecular ferroelectrics since 2011 and focuses on the relationship between symmetry breaking and ferroelectricity, offering ideas for exploring high-performance molecular ferroelectrics.
authors.affiliation: Ordered Matter Science Research Center, Southeast University, Nanjing 211189, P. R. China. xiongrg@seu.edu.cn.
authors.forename: Ping-Ping
authors.initials: PP
authors.lastname: Shi
authors.affiliation: Ordered Matter Science Research Center, Southeast University, Nanjing 211189, P. R. China. xiongrg@seu.edu.cn.
authors.forename: Yuan-Yuan
authors.initials: YY
authors.lastname: Tang
authors.affiliation: Ordered Matter Science Research Center, Southeast University, Nanjing 211189, P. R. China. xiongrg@seu.edu.cn.
authors.forename: Peng-Fei
authors.initials: PF
authors.lastname: Li
authors.affiliation: Ordered Matter Science Research Center, Southeast University, Nanjing 211189, P. R. China. xiongrg@seu.edu.cn.
authors.forename: Wei-Qiang
authors.initials: WQ
authors.lastname: Liao
authors.affiliation: Ordered Matter Science Research Center, Southeast University, Nanjing 211189, P. R. China. xiongrg@seu.edu.cn.
authors.forename: Zhong-Xia
authors.initials: ZX
authors.lastname: Wang
authors.affiliation: Ordered Matter Science Research Center, Southeast University, Nanjing 211189, P. R. China. xiongrg@seu.edu.cn.
authors.forename: Qiong
authors.initials: Q
authors.lastname: Ye
authors.affiliation: Ordered Matter Science Research Center, Southeast University, Nanjing 211189, P. R. China. xiongrg@seu.edu.cn.
authors.forename: Ren-Gen
authors.initials: RG
authors.lastname: Xiong