Polar Point Group Research Articles

Ferroelectricity with Long ion Displacements in Crystals of Non‐Polar Point Groups

AbstractIn the classical model, ferroelectricity is associated with small ion displacements from paraelectric phases with high symmetry, and ferroelectric crystals must adopt one of the ten polar point groups with low symmetry according to Neumann's principle. In this work, it is proposed that this conclusion is based on perfect bulk crystals without taking the boundaries into account. First‐principles evidence shows that ferroelectric polarizations may also be formed in some non‐polar point groups as the edges generally break the crystal symmetry. Meanwhile, such polarizations can be maintained at macroscale and are switchable when the transition between multiple equivalent states with high symmetry can be realized via long ion displacements， essentially akin to ion conductors. For example, the switching barriers can be much reduced in sliding ferroelectric bilayer systems or ionic compounds with covalent‐like directionality. Such unconventional ferroelectricity can be attributed to the boundaries and long ion displacements, and its existence in several systems like CuCrS2 is supported by experimental observations. It may explain a series of unclarified phenomena reported previously as well as significantly expand the scope of ferroelectrics, especially those with high polarizations induced by long ion displacements.

2D Molecular Ferroelectric with Large Out‐of‐plane Polarization for In‐Memory Computing

Abstract2D ferroelectric materials with out‐of‐plane polarization are crucial for future nanoscale logic devices due to the increasing demand for energy‐efficient architectures in artificial intelligence. However, only a few 2D out‐of‐plane ferroelectrics are confirmed experimentally. As an important branch of ferroelectrics, organic–inorganic hybrid perovskite ferroelectrics show flexible structures, making them eligible for constructing multifunctional materials. Here, a 2D organic–inorganic hybrid perovskite ferroelectric (6‐BHA)2CdBr4 (6‐BHA is 6‐bromohexylamine) is designed, which crystallizes in polar point group Cc. It experiences the reversal phase transition at 317.8 K and possesses multiaxial ferroelectric properties. More interestingly, it exhibits a large spontaneous polarization value of 3.26 µC cm−2 in out‐of‐plane direction of the film compared with typical 2D ferroelectrics. Moreover, an inverter based on (6‐BHA)2CdBr4 is fabricated, which serves as a proof of concept for the feasibility for logic‐in‐memory devices. This work not only enriches the family of molecular ferroelectrics but also shows the potential to create the next generation of in‐memory computing devices, nanoelectronics devices, and ultra‐high‐density memories.

Homochirality in Ferroelectrochemistry

What is the most favorite and original chemistry developed in your research group?We originally proposed the design principle for molecular ferroelectrics: ferroelectrochemistry, including quasi‐spherical theory, the introduction of homochirality, and H/F substitution. Ferroelectrochemistry changed the blind search for molecular ferroelectrics into targeted chemical design, which will develop into a new discipline.How do you get into this specific field? Could you please share some experiences with our readers?I have been devoted to the field of molecular ferroelectrics for more than 20 years. In the early stage, I worked on non‐centrosymmetric metal‐organic complexes, which are potential molecular ferroelectrics. This laid a foundation for my further study of molecular ferroelectrics. Non‐centrosymmetric crystal symmetry is only one of the necessary requirements for ferroelectrics, which must adopt one of the 10 polar crystallographic point groups and should also generally undergo symmetry‐breaking phase transitions. Due to the lack of a feasible method, the discovery of molecular ferroelectrics has long depended on blindly searching. This process is like finding a needle in a haystack. After years of exploration in this field, I fully understood the Landau phase transition phenomenological theory, Curie symmetry, and Neumann principle from a chemical perspective, and proposed the design principle for molecular ferroelectrics: ferroelectrochemistry, transforming the discovery of molecular ferroelectrics from blind search to targeted chemical design. Never give up no matter how much difficulty you have met, because maybe there is an opportunity the next second.What is the most important personality for scientific research?Curiosity, divergent thinking, perseverance, team spirit, and gratitude.How do you supervise your students?Emphasis on independent problem‐solving abilities. Encourage students to read professional books frequently while doing research.What are your hobbies? What’s your favorite book(s)?Jogging, reading, and swimming. My favorite book is The Journey to the West.Comprehensive SummaryMolecular ferroelectrics have attracted tremendous attention in the past decades due to their excellent ferroelectric performance and superiorities of easy processability, mechanical flexibility, and good biocompatibility. However, the discovery of molecular ferroelectrics is a great challenge and has long relied on blind search. This situation changed recently, with the development of ferroelectrochemistry proposed by our group. As a major design approach in ferroelectrochemistry, introducing homochirality, which facilitates the crystallization of materials in polar crystallographic point groups, greatly improves the probability of being ferroelectrics. Various new molecular ferroelectrics with splendid properties have been precisely synthesized by using this efficient and universal strategy. In this review, we summarize the advances in the chemical design of molecular ferroelectrics through the strategy of introducing homochirality.Key Scientists

Quantized ferroelectricity in multivalent ion conductors with non-polar point groups.

Ferroelectricity with switchable polarizations is generally associated with small ion-displacements and occurs only in 10 specific polar point groups, which is not a necessary requirement for ion conduction where the ions can also be electrically displaced but by much longer distances. Herein, through first-principles calculations, we predict the formation of unconventional ferroelectricity based on previous experimental reports on topotactic reaction with an aliovalent cation between trigonal layers of ion conductors. In such systems, the multivalent cations are surrounded by vacant sites that can simultaneously migrate by a much larger distance compared with conventional displacive ferroelectricity, giving rise to a quantized change in polarization even if the crystal lattices do not belong to the 10 polar groups. The deviation from classical principles can be attributed to the long ion displacements in ferroelectric ion conductors during switching that can lead to the transformation between multiple equivalent symmetrical stable states, which cannot be realized by the relatively small ion displacements in current ferroelectrics. The evenly distributed vacant sites due to Coulomb repulsion do not break the insulativity of the systems, while their inhomogeneous distribution under an electric field or in ferroelectric domain walls will give rise to high electrical conductance, which may be utilized for constructing nanoscale artificial ionic synapses that enable neuromorphic computing.

Precise Design of Molecular Ferroelectrics with High TC and Tunable Band Gap by Molecular Modification.

Molecular ferroelectric materials are widely applied in piezoelectric converters, non-volatile memorizers, and photovoltaic devices due to their advantages of adjustable structure, lightweight, easy processing, and environmental friendliness. However, designing multifunctional molecular ferroelectrics with excellent properties has always been a great challenge. Herein, a multiaxial molecular ferroelectric is successfully designed by modifying the quasi-spherical cation dabco with CuBr2 to obtain halogenated [Bretdabco]CuBr4 (Bretdabco = N-bromoethyl-N'-diazabicyclo [2.2.2]octane), which crystallizes in polar point groups (C6). Typical ferroelectric behaviors featured by the P-E hysteresis loop and switched ferroelectric domain are exhibited. Notably, the molecular ferroelectric shows a high TC of 460 K, which is rare in the field and could greatly expand the application range of this material. In addition, the band gap is adjustable through the regulation of halogen. Both the UV absorption spectra and theoretical calculations indicate that the molecular ferroelectrics belong to a direct band gap (2.14 eV) semiconductor. This tunable and narrow band gap semiconductor molecular ferroelectric material with high TC can be utilized more effectively in the study of optoelectronics and sensors, including piezoelectric energy harvesters. This research may provide a promising approach for the development of multiaxial molecular ferroelectrics with a tiny band gap and high TC.

Two new high-temperature molecular ferroelectrics [1,5-3.2.2-Hdabcn]X (X = ClO4−, ReO4−)

Molecular ferroelectrics have attracted much attention because of their excellent piezoelectricity, mechanical workability, and second harmonic effect. Here, we successfully prepared two molecular ferroelectrics [1,5–3.2.2-Hdabcn]X (X = ClO4−, 1; ReO4−, 2) by reactions of a quasi-spherical amine 1,5-diazabicycle[3.2.2]nonane (1.5–3.2.2-dabcn) with HX aqueous solution. Compounds 1 and 2 undergo high-temperature phase transitions at 381 K (1) and 396 K (2). Before and after the phase transition, they crystallize in the polar point group mm2, and the centrosymmetric point groups mmm and 4/mmm, respectively. According to Aizu rules, these two compounds experience mmmFmm2 and 4/mmmFmm2 type ferroelectric phase transitions, respectively. The ferroelectricity of both compounds is well expressed in their polycrystalline film at room temperature with low coercive voltages of 13 V for 1 and 25 V for 2. Using piezoelectric force microscopy (PFM), the 180° anti-parallel ferroelectric domains and the reversible polarization switching can be clearly observed in 1 and 2. This high-temperature molecular ferroelectric material has great application potential in flexible materials, biomechanics, intelligent wearables and other fields.

Synthesis–dependent structural and optoelectronic properties of semicrystalline LiNbO3:SiO2 hybrid silicates

Polycrystalline ceramics of lithium niobate (LNO) were prepared via standard hydrothermal and solid–state chemistry reactions, and by the sol–gel method implementing acrylamide polymerization. Congruent LiNbO3 trigonal perovskite phases crystallized in the R3c (161) space group [3 m (C3v) polar point group] were obtained in all cases. Depending on the implemented synthetic route, the average grain and crystallite sizes were estimated to 127–895 nm and 35–40 nm, respectively. As prepared LNO powders were embedded in a–SiO2 mesoporous sonogel networks at different doping rates to conform LiNbO3:SiO2 semicrystalline hybrid silicates with differentiated structural and optoelectronic (OE) properties. Intensive morphological, structural, spectroscopic, and nonlinear optical (NLO) characterizations were performed to explore these novel composites’ structural and photophysical properties for lead–free ferroelectric and optical applications. Results show that the massive porosity of the sonogel glasses and related defective bonding environment promote generalized host–guest molecular interactions influencing the oxidation states and radiative transitions of the dopant compounds via surface–assisted mechanisms and chemical charge–transfers. Additionally, due to the ferroelectric nature of LNO and average macroscopic alignment attained along the polycondensation stage of the solid solutions, SHG activity was observed from the hybrid sonogel (HSG) phases. Bulk NLO susceptibilities and molecular hyperpolarizabilities were estimated within the 9.62 × 10−11 and 10−23 esu range, respectively. Finally, depending on the synthesis methodology and in–gap hybridized levels, energy gap (Eg) narrowing was also observed for all LNO/HSG glasses, showing Eg values as low as 3.05 eV.

High-Temperature Metal-Free Molecular Ferroelectrics with Large Spontaneous Polarization.

In the past two decades, numerous molecular ferroelectrics have been reported. However, metal-free molecular ferroelectrics with high working temperatures and large spontaneous polarizations are still uncommon. Herein, we present two metal-free molecular ferroelectrics prepared from monoprotonated hexamethylenetetramine (HMTA), namely (HMTAH)Cl and (HMTAH)Br, which crystallize in a polar point group of 3m. In these crystals, the polar HMTAH+ organic cations can be reoriented 180° along the polar axis because of the quasispherical molecular geometry. As a result of the large shift of the positively charged protonated N atoms, these compounds demonstrate large spontaneous polarizations with values of 8.3 and 8.1 μC cm-2 and high working temperatures of 390 and 435 K, respectively. The ferroelectric property of these compounds is characterized with second-harmonic generation, ferroelectric hysteresis loop, and pyroelectric current measurements.

Unusual magnetic relaxation in a single-molecule magnet with toroidal magnetic moments

We report the synthesis and characterization of a single-molecule magnet composed of triangular clusters of dysprosium ions. The structural study shows that the symmetry changes from one polar point group (mm2) at room temperature to another polar point group (m) at low temperature. Magnetic studies and theory calculations illustrate that the vortex distribution of magnetic dipoles in the triangular dysprosium clusters forms a toroidal magnetic moment. Interestingly, the analysis of AC magnetic susceptibility reveals the coexistence of three distinct magnetic relaxation processes, corresponding to the Raman, Orbach, and QTM relaxation pathways, respectively. The sum of three modified Debye functions is successfully used to describe the multiple relaxation behavior.

A Polymorph of Dipeptide Halide Glycyl-L-Alanine Hydroiodide Monohydrate: Crystal Structure, Optical Second Harmonic Generation, Piezoelectricity and Pyroelectricity.

A polymorph of glycyl-L-alanine HI.H2O is synthesized from chiral cyclo-glycyl-L-alanine dipeptide. The dipeptide is known to show molecular flexibility in different environments, which leads to polymorphism. The crystal structure of the glycyl-L-alanine HI.H2O polymorph is determined at room temperature and indicates that the space group is polar (P21), with two molecules per unit cell and unit cell parameters a = 7.747 Å, b = 6.435 Å, c = 10.941 Å, α = 90°, β = 107.53(3)°, γ = 90° and V = 520.1(7) Å3. Crystallization in the polar point group 2, with one polar axis parallel to the b axis, allows pyroelectricity and optical second harmonic generation. Thermal melting of the glycyl-L-alanine HI.H2O polymorph starts at 533 K, close to the melting temperature reported for cyclo-glycyl-L-alanine (531 K) and 32 K lower than that reported for linear glycyl-L-alanine dipeptide (563 K), suggesting that although the dipeptide, when crystallized in the polymorphic form, is not anymore in its cyclic form, it keeps a memory of its initial closed chain and therefore shows a thermal memory effect. Here, we report a pyroelectric coefficient as high as 45 µC/m2K occurring at 345 K, one order of magnitude smaller than that of semi-organic ferroelectric triglycine sulphate (TGS) crystal. Moreover, the glycyl-L-alanine HI.H2O polymorph displays a nonlinear optical effective coefficient of 0.14 pm/V, around 14 times smaller than the value from a phase-matched inorganic barium borate (BBO) single crystal. The new polymorph displays an effective piezoelectric coefficient equal to deff=280 pCN-1, when embedded into electrospun polymer fibers, indicating its suitability as an active system for energy harvesting.

High-pressure single crystal growth and magnetoelectric properties of CdMn7O12

The concurrent presence of large electric polarization and strong magnetoelectric coupling is quite desirable for potential applications of multiferroics. In this paper, we report the growth of CdMn7O12 single crystals by flux method under a high pressure of 8 GPa for the first time. An antiferromagnetic (AFM) order with a polar magnetic point group is found to occur at the onset temperature of T N1 = 88 K (AFM1 phase). As a consequence, the pyroelectric current emerges at T N1 and gradually increases and reaches its maximum at T set = 63 K, at which the AFM1 phase finally settles down. Below T set, CdMn7O12 single crystal exhibits a large ferroelectric polarization up to 2640 µC m−2. Moreover, the spin-induced electric polarization can be readily tuned by applying magnetic fields, giving rise to considerable magnetoelectric coupling effects. Thus, the current CdMn7O12 single crystal acts as a rare multiferroic system where both large polarization and strong magnetoelectric coupling merge concurrently.

Single crystal growths and magnetic properties of hexagonal polar semimetals RAuGe (R = Y, Gd–Tm, and Lu)

We study structural and magnetic properties of rare-earth based semimetals RAuGe (R = Y, Gd–Tm, and Lu) using flux-grown single crystals. These compounds belong to the space group P63mc, which is associated with the noncentrosymmetric polar point group 6mm. We confirm the systematic structural evolution at room temperature as a function of ionic radius of rare earths to clarify the isopointal crossover between two polar structures: three-dimensional LiGaGe-type and quasi-two-dimensional NdPtSb-type. Magnetism shows a characteristic anisotropy in reasonable agreement with the crystal electric field (CEF) theory; the easy-plane-type anisotropy for R = Tb and Dy turns into the Ising-type anisotropy for R = Er and Tm. We evaluate the CEF parameters based on the Stevens operators to reasonably reproduce the temperature dependence of magnetic susceptibilities and specific heat for RAuGe (R = Tb–Tm). The estimated energy scale of the Ising gap (∼ 11 meV) in TmAuGe is consistent with an excitation observed in an inelastic neutron scattering experiment. These findings suggest an opportunity for interplay between conduction electrons and nontrivial spin structures in the family of magnetic polar semimetals RAuGe.

Chiroptical Activity of An Achiral Emissive Eu Metal-Organic Framework.

Chiroptical activity of achiral crystals is theoretically allowed but very unusual. There is a particularly scarcity of empirical studies on optically active achiral metal-organic frameworks (MOFs). Herein we report an achiral emissive Eu MOF and its chiroptical properties both in the ground and excited states. The framework crystallizes in an achiral space group (Pna21 ) belonging to the polar point group (mm2), where the asymmetric arrangement of racemic trinuclear Eu-oxo clusters is responsible for the optical activity. A pair of circular dichroisms (CD) and circularly polarized luminescence (CPL) peaks with opposite signs were observed for single crystals. Importantly, the luminescence dissymmetry factor can reach up to 1.1×10-3 , which is comparable in magnitude to the value of most of the chiral-linker-bridged MOFs. This work gives the first example of achiral MOFs with CPL response and should be instructive for the discovery of more CPL emitters from racemic MOF family.

Strain engineering a persistent spin helix with infinite spin lifetime

Persistent spin textures (PSTs) in solid-state materials arise from a unidirectional spin-orbit field in momentum space and offer a route to deliver long carrier spin lifetimes sought for future quantum microelectronic devices. Nonetheless, few three-dimensional materials are known to host PSTs owing to crystal symmetry and chemical requirements. There are even fewer examples demonstrated experimentally. Here we report that high-quality persistent spin textures can be obtained in the polar point groups containing an odd number of mirror operations. We use representation theory analysis and electronic structure calculations to formulate general discovery principles to identify PSTs hidden in known complex ternary layered and perovskite structures with large electric polarizations. We then show some of these materials exhibit PSTs without requiring any special crystalline symmetries. This finding removes the limitation imposed by mirror-symmetry protected PSTs that has limited compound discovery. Our general design approach enables the pursuit of persistent spin helices in materials exhibiting the $C_{3v}$ crystal class adopted by many quantum materials exhibiting large Rashba coefficients.

Robust Spin-Dependent Anisotropy of Circularly Polarized Light Detection from Achiral Layered Hybrid Perovskite Ferroelectric Crystals.

Circularly polarized light (CPL) detection has sparked overwhelming research interest for its widespread chiroptoelectronic and spintronic applications. Ferroelectric materials, especially emerging layered hybrid perovskite ferroelectrics, exhibiting striking bulk photovoltaic effect (BPVE) present significant possibilities for CPL detection by a distinctive working concept. Herein, for the first time, we demonstrate the realization of robust angular anisotropy of CPL detection in a new layered hybrid perovskite ferroelectric crystal (CPA)2FAPb2Br7 (1, CPA is chloropropylammonium, FA is formamidinium), which crystallized in an optically active achiral polar point group. Benefiting from the notable spontaneous polarization (5.1 μC/cm2) and excellent semiconducting characteristics, single crystals of 1 exhibit remarkable BPVE under light illumination, with a high current on/off switching ratio (ca. 103). More intriguingly, driven by the angular carrier drift originating from spin-dependent BPVE in optically active ferroelectrics, 1 displays highly sensitive self-powered CPL detection performance, showing a robust angular anisotropy factor up to 0.98, which is far more than those achieved by material intrinsic chirality. This work provides an unprecedented approach for realizing highly sensitive CPL detection, which sheds light on the further design of optically active ferroelectrics for chiral photonic applications.

Symmetry of antiferroelectric crystals crystallized in polar point groups.

Symmetry is an essential concept in physics, chemistry and materials science. Comprehensive, authoritative and accessible symmetry theory can provide a strong impetus for the development of related materials science. Through the sustained efforts of physicists and crystallographers, researchers have mastered the relationship between structural symmetry and ferroelectricity, which demands crystallization in the 10 polar point groups. However, the symmetry requirement for antiferroelectricity is still ambiguous, and polar crystals possessing antiferroelectricity seem contradictory. This work systematically and comprehensively studies the transformation of dipole moments under symmetry operations, using accessible geometric methods and group theory. The results indicate crystals that crystallize in polar point groups 2 (C 2), m (C 1h), mm2 (C 2v), 4 (C 4), 4mm (C 4v), 3m (C 3v), 6 (C 6) and 6mm (C 6v) also possess anti-polar structure and are capable of Kittel-type antiferroelectricity. The anti-polar direction of each point group is also highlighted, which could provide a straightforward guide for antiferroelectric property measurement. Like ferroelectric crystals, antiferroelectric crystals belonging to polar point groups have great potential to become a family of important multifunctional electroactive and optical materials. This contribution refines antiferroelectric theory, will help facilitate and stimulate the discovery and rational design of novel antiferroelectric crystals, and enrich the potential functional applications of antiferroelectric materials.

Air-Stable Dy(III)-Macrocycle Enantiomers: From Chiral to Polar Space Group

Air-Stable Dy(III)-Macrocycle Enantiomers: From Chiral to Polar Space Group

Ba4Cl] Cations Directed Perovskite-like Polar Metal Formate Frameworks {[Ba4Cl][M3(HCO2)13]}n (M = Mn, Co, and Mg): Microwave-Assisted Synthesis, Structures, and Properties.

Novel 3D metal formate frameworks {[Ba4Cl][M3(HCO2)13]}n (M = Mn for 1, Co for 2, and Mg for 3) were successfully assembled via microwave-assisted synthesis. The complexes are rare coordination polymers crystallized at space group P4cc with the polar point group C4v. In the structure, the MII ions are bridged by two types of anti-anti formate in forming a 3D pcu framework, and additional formates coordinate to the unsaturated sites of the MII ions in the framework, giving an anionic M-formate net. Ba4Cl clusters take the cavities of the net as charge balance, in which the chloride ion deviates from the center of the barium ions. The asymmetric Ba4Cl structure is transmitted throughout the crystal resulting in polar structure, which is further confirmed by nonlinear optical and piezoelectric test. Nonlinear optical activity tests of 1 and 3 show SHG signals 0.32 and 0.28 times that of KDP, while 2 has a piezoelectric coefficient d33 of 6.8 pC/N along polar axis. Magnetic studies reveal antiferromagnetic coupling between MII ions in 1 and 2. Spin canting was found only in 2 with anisotropic CoII ions, and 2 is a canted antiferromagnetically with TN = 5 K. Further field-induced spin flop was also found in 2 with a critical field 0.9 T.

Discovery of Robust Ferroelectricity in 2D Defective Semiconductor α-Ga2 Se3.

2D ferroelectrics with robust polar order in the atomic-scale thickness at room temperature are needed to miniaturize ferroelectric devices and tackle challenges imposed by traditional ferroelectrics. These materials usually have polar point group structure regarding as a prerequisite of ferroelectricity. Yet, to introduce polar structure into otherwise nonpolar 2D materials for producing ferroelectricity remains a challenge. Here, by combining first-principles calculations and experimental studies, it is reported that the native Ga vacancy-defects located in the asymmetrical sites in cubic defective semiconductor α-Ga2 Se3 can induce polar structure. Meanwhile, the induced polarization can be switched in a moderate energy barrier. The switched polarization is observed in 2D α-Ga2 Se3 nanoflakes of ≈4nm with a high switching temperature up to 450 K. Such polarization switching could arise from the displacement of Ga vacancy between neighboring asymmetrical sites by applying an electric field. This work removes the point group limit for ferroelectricity, expanding the range of 2D ferroelectrics into the native defective semiconductors.

Molecular Design Principles for Ferroelectrics: Ferroelectrochemistry.

Ferroelectric materials have a variety of technological applications, as transducers, capacitors, sensors, etc. Great interest in molecular ferroelectrics has emerged because of their structural flexibility, tunability, and homochirality. However, the discoveries of molecular ferroelectrics are not abundant. The lack of chemical design is the main challenge in realizing new molecular ferroelectrics. Consequently, chemical design approaches, including the ideas of introducing quasi-spherical theory, homochirality, and H/F substitution, have been developed recently. Through these advanced methodologies, a wide range of ferroelectrics were successfully synthesized, changing the blind search into a targeted chemical design. In this Perspective, we aim to provide insight into the fundamental chemistry and physics of molecular ferroelectrics and propose the concept of "ferroelectrochemistry", concerned with the targeted design and performance optimization of molecular ferroelectrics from the chemical point of view. We start with the basic theories used in the modification of chemical structures for new molecular ferroelectrics, such as the quasi-spherical theory. After that, we focus on the fundamentals of homochirality from the perspective of chemistry and advantages of introducing a homochiral molecule within the scope of ferroelectrics. Further, we explore another design strategy, H/F substitution, as an analogue of the H/D isotope effect. The introduction of a F atom usually does not change the polar point group but may induce a minor structural disruption that enhances physical properties such as Curie temperature and spontaneous polarization. We hope our comprehensive studies on the targeted design and performance optimization strategies for molecular ferroelectrics may build up and enrich the content of ferroelectrochemistry.