Direct visualization of irreducible ferrielectricity in crystals

Abstract

In solids, charge polarity can one-to-one correspond to spin polarity phenomenologically, e.g., ferroelectricity/ferromagnetism, antiferroelectricity/antiferromagnetism, and even dipole-vortex/magnetic-vortex, but ferrielectricity/ferrimagnetism kept telling a disparate story in microscopic level. Since the definition of a charge dipole involves more than one ion, there may be multiple choices for a dipole unit, which makes most ferrielectric orders equivalent to ferroelectric ones, i.e., this ferrielectricity is not necessary to be a real independent branch of polarity. In this work, by using the spherical aberration-corrected scanning transmission electron microscope, we visualize a nontrivial ferrielectric structural evolution in BaFe 2 Se 3 , in which the development of two polar sub-lattices is out-of-sync, for which we term it as irreducible ferrielectricity. Such irreducible ferrielectricity leads to a non-monotonic behavior for the temperature-dependent polarization, and even a compensation point in the ordered state. Our finding unambiguously distinguishes ferrielectrics from ferroelectrics in solids.

Ammonium Sulfate Structure-Type Hybrid Metal Halide Ferroelectric with Giant Uniaxial Spontaneous Strain

“…This is consistent with the packing manner of NH 4 + cations in the ferrielectric LT phase of (NH 4 ) 2 SO 4 (Figure S5). Therefore, the special packing of the two kinds of TMS cations in the extended lattice would give rise to residual spontaneous polarization macroscopically in the LT phase of (TMS) 2 CdI 4 . , Based on this structural hypothesis, the polarization was calculated to be 0.24 μC cm 2 through the point charge model (Figure S14). To further understand the occurrence of the ordered and disordered TMS cations and associated ferrielectricity, the molecular interactions between TMS + and CdI 4 2– groups were examined by plotting the Hirshfeld d norm surfaces and two-dimensional (2D) fingerprints of the two TMS cations (TMS1 and TMS2). , The red color in the Hirshfeld surfaces ( d norm mode) indicates the normalized contact distances with the two TMS cations.…”

Direct Evidence for an Intermediate Multiferroic Phase in LiCuFe2(VO4)3

“…The first possibility is that these two peaks could be from two independent ferroelectric components with different transition points, like recently proposed “irreducible ferrielectricity” . In LiCuFe 2 (VO 4 ) 3 , there are two different magnetic ions (Cu 2+ and Fe 3+ ), and Fe 3+ ions have two nonequivalent positions.…”

Space Group Symmetry of BaFe$_2$Se$_3$: ab initio-Experiment Phonon Study

“…At 660 K BaFe 2 Se 3 undergoes a Cmcm to P nma phase transition [26], followed by another transition at 425 K. This transition was shown, by scanning transmission electron microscopy, to be associated with an in-ladder tetramerisation, a differentiation between the two ladders of the unit cell and a room temperature polarization [16]. Putting these experimental results into perspective with our structure dynamic study, one can built a coherent picture for the BaFe 2 Se 3 phase diagram.…”

“…Prominent is that BaFe 2 Se 3 is rather ferrielectric than ferroelectric, with a strong polarization in each ladder, mostly canceling out between the ladders. Its fully ferroelectric state has an energy predicted only slightly higher than the ferrielectric one and holds a giant improper polarization predicted to be ∼ 2-3µC/cm 2 , while the ferrielectric one was predicted to be ∼ 0.2µC/cm 2 [13] and experimentally estimated around ∼ 0.6µC/cm 2 [16].…”