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We report ferrielectricity in a single-phase crystal, TSCC -- tris-sarcosine calcium chloride [(CH3NHCH2COOH)3CaCl2]. Ferrielectricity is well known in smectic liquid crystals but almost unknown in true crystalline solids. Pulvari reported it in 1960 in mixtures of ferroelectrics and antiferroelectrics, but only at high fields. TSCC exhibits a second-order displacive phase transition near Tc = 130 K that can be lowered to a Quantum Critical Point at zero Kelvin via Br- or I-substitution, and phases predicted to be antiferroelectric at high pressure and low temperatures. Unusually, the size of the primitive unit cell does not increase. We measure hysteresis loops and polarization below T = 64 K and clear Raman evidence for this transition, as well of another transition near 47-50 K. X-ray and neutron studies below Tc = 130K show there is an antiferroelectric displacement out of plane of two sarcosine groups; but these are antiparallel displacements are of different magnitude, leading to a bias voltage that grows with decreasing T. A monoclinic subgroup C2 may be possible at the lowest temperatures (T<64K or T<48K), but no direct evidence exists for a crystal class lower than orthorhombic.
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We investigate the charge and lattice states in a quasi-one-dimensional organic ferroelectric material, TTF-QCl$_{4}$, under pressures of up to 35 kbar by nuclear quadrupole resonance experiments. The results reveal a global pressure-temperature phase diagram, which spans the electronic and ionic regimes of ferroelectric transitions, which have so far been studied separately, in a single material. The revealed phase diagram clearly shows that the charge-transfer instability and the lattice symmetry breaking, which coincide in the electronic ferroelectric regime at low pressures, bifurcate at a certain pressure, leading to the conventional ferroelectric regime. The present results reveal that the crossover from electronic to ionic ferroelectricity occurs through the separation of charge and lattice instabilities.
The switching of electric polarization induced by electric fields -a fundamental functionality of ferroelectrics- is closely associated with the motions of the domain walls that separate regions with distinct polarization directions. Therefore, understanding domain-walls dynamics is of essential importance for advancing ferroelectric applications. In this Letter, we show that the topology of the multidomain structure can have an intrinsic impact on the degree of switchable polarization. Using a combination of polarization hysteresis measurements and piezoresponse force microscopy on a uniaxial organic ferroelectric, alpha-6,6-dimethyl-2,2-bipyridinium chloranilate, we found that the head-to-head (or tail-to-tail) charged domain walls are strongly pinned and thus impede the switching process; in contrast, if the charged domain walls are replaced with electrically neutral antiparallel domain walls, bulk polarization switching is achieved. Our findings suggest that manipulation of the multidomain topology can potentially control the switchable polarization.
Magnetoelectric coupling has been a trending research topic in both organic and inorganic materials and hybrids. The concept of controlling magnetism using an electric field is particularly appealing in energy efficient applications. In this spirit, ferroelectricity has been introduced to organic spin valves to manipulate the magneto transport, where the spin transport through the ferromagnet/organic spacer interfaces (spinterface) are under intensive study. The ferroelectric materials in the organic spin valves provide a knob to vary the interfacial energy alignment and the interfacial crystal structures, both are critical for the spin transport. In this review, we first go over the basic concepts of spin transport in organic spin valves. Then we introduce the recent efforts of controlling magnetoresistance of organic spin valves using ferroelectricity, where the ferroelectric material is either inserted as an interfacial layer or used as a spacer material. The realization of the ferroelectric control of magneto transport in organic spin valve, advances our understanding in the spin transport through the ferromagnet/organic interface and suggests more functionality of organic spintronic devices.
Two-dimensional polarity is intriguing but remains in the early stage. Here a structural evolution diagram is established for GeS monolayer, which leads a noncollinear ferrielectric $delta$-phase energetically as stable as the ferroelectric $alpha$-phase. Its ferrielectricity is induced by the phonon frustration, i.e., the competition between ferroelectric and antiferroelectric modes, providing more routes to tune its polarity. Besides its prominent properties like large band gap, large polarization, and high Curie temperature, more interestingly, the morphotropic phase boundary between $alpha$- and $delta$-phases is highly possible, which is crucial to obtain giant piezoelectricity for lead-free applications.
Using first-principles calculations and group-theoretical methods, we study the origin and stabilization of ferrielectricity (FiE) in CuInP$_2$Se$_6$. We find that the polar distortions of the metal atoms create most of the polarization in the FiE phase. Surprisingly, the stabilization of the FiE phase comes from an anharmonic coupling between the polar mode and a fully symmetric Raman-active mode comprising primarily of the Se atoms. This coupling is large even down to the monolayer limit, and the degree of anharmonicity is comparable to improper ferroelectrics. Our results open up possibilities for dynamical control of the single-step ferroelectric switching barrier by tuning the Raman-active mode. These findings have important implications not only for designing next-generation microelectronic devices that can overcome the voltage-time dilemma but also in explaining the unconventional responses observed in CuInP$_2$Se$_6$ and similar layered thiophosphates.
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