In 1880, the brothers Jacques and Pierre Curie were studying the phenomenon of pyroelectricity. This is the effect where crystals that both lack a center-of-symmetry (i.e., are “acentric”) and possess a unique axis of symmetry exhibit a separation of positive and negative Coulombic charge when subjected to a temperature change. They observed something quite unexpected, which was that the mineral α-quartz exhibited a pyroelectric effect. They were well-aware that this should not have been the case because, although α-quartz is acentric, it does not have a unique axis of symmetry. A careful analysis of their experimental conditions showed that that they were subjecting the quartz crystals to a temperature gradient. In doing this, they had subjected the crystals to thermally induced mechanical stresses, and they thus discovered piezoelectricity.1 This ability of acentric crystals to turn mechanical stresses into an electrical signal, and conversely to turn an electrical signal into mechanical strains, received enormous attention and development during World War 1 for use in submarine-detecting sonar equipment. Very large piezoelectric coefficients had been discovered in Rochelle salt (sodium potassium tartrate), and single crystals several centimeters in size were being grown at the time by General Electric (GE).2 As has been described by Cady3 and Valasek,4 Anderson and Cady studied the properties of Rochelle salt for underwater sonar applications around 1918 and had noted anomalous dielectric behavior. Josef Valasek at the University of Minnesota studied the piezoelectric and dielectric properties of GE-made Rochelle salt crystals. He found that between temperatures of −20 and +20 °C, the crystals not only became polar but the direction of the polar axis could also be switched between two opposite states by the application of an electric field of sufficient magnitude and that these states were stable when the field was removed,5 a characteristic that we now recognize as “ferroelectricity.” This work was later reproduced by Kobeko and Kurchatov in Leningrad.6 The effect was initially termed “Seignette electricity,” after Elie Seignette, who first prepared the compound in La Rochelle in the seventeenth century. Although the concept of “ferroelectricity” was first mooted as a theoretical possibility by Erwin Schrodinger in 1912,7 Valasek has indicated that his first awareness of the use of the term as applied to materials that possessed an electrically switchable spontaneous dielectric polarization was by Hans Mueller [Massachusetts Institute of Technology (MIT)] in 1940.8 At first, the effect was thought to be connected to the presence of polar water molecules and hydrogen bonding. However, as has been reviewed by Kanzig,9 a major step-forward toward the widespread use of ferroelectric materials came with the independent discovery of anomalous dielectric behavior and ferroelectricity in the perovskite oxide barium titanate (BaTiO3) during the 1940s in the USA, the UK, Russia, and Japan. This eventually led to the discovery of exceptional piezoelectric properties in the lead zirconate–lead titanate (PbZrO3–PbTiO3) ceramic solid solution system by Jaffe et al.10 at the National Bureau of Standards in the USA when the compositions were close to PbZr0.55Ti0.45O3. The Clevite Corporation developed a family of piezoelectric ceramics based on this discovery,11 which were doped with different elements to confer particular property improvements, such as Fe, for stability under high driving fields. (These were given the trademark “PZT,” and while this term is now almost universally applied by the community as a shorthand-term for ferroelectric materials based on the PbZrO3–PbTiO3 solid solution system, it is worth remembering where it came from).
The switchable spontaneous polarization of ferroelectric materials confers upon them many useful properties with an extraordinarily wide range of applicability; some examples include:
Piezoelectric coefficients that are very large in relation to non-ferroelectric piezoelectrics, meaning they are widely used in underwater sonar (military and civilian), medical ultrasound equipment, audible sound generators, nano-positioners for optics, magnetic disk memory read heads and ultraprecision equipment such as photolithographic steppers, motors, surface acoustic wave (SAW) and film bulk acoustic resonator (FBAR) devices, etc. The global piezoelectric materials market was US$1.35 × 109 in 2019, expected to rise to US$1.84 × 109 in 2027;12 the piezoelectric devices market was estimated at US$28.9 × 109 in 2020, expected to grow to US$34.7 × 109 by 2025, at a compound annual growth rate (CAGR) of 3.7%.13
Very high dielectric constants, leading to widespread use in ceramic capacitors. The global ceramic capacitors market is expected to reach a total market size of US$9.2 × 109 in 2023, rising from US$6.1 × 109 in 2017 at a CAGR of 6.9%.14
Very large pyroelectric coefficients, leading to the widespread application of ferroelectric materials to pyroelectric infra-red (PIR) sensors, which are used in intruder sensors, remote light switches, environmental monitors, medical instrumentation, and people sensing.15,16 PIR devices are expected to have a market of ca US$50 × 106 in 2020, reaching US$68 × 106 by 202517 or ∼10% of the total infrared detector market. Any pyroelectric material will also exhibit the electrocaloric effect (ECE). This is a change in temperature engendered by an applied electric field. Some of the first measurements on Rochelle salt were conducted by Wiseman and Kuebler.18 Thacher measured temperature changes (ΔT) of about 1 K at applied fields of up to 30 kV cm−1 in PZT-related materials,19 and Shebanov et al.20 showed similar values of ΔT in PbSc½Ta½O3. Very high electric fields, above the breakdown fields of most bulk materials, are needed to get useful values of ΔT. However, in 2006, Mischenko et al.21 showed that values of ΔT up to 12 K could be obtained in thin films of PbZr0.95Ti0.05O3 at fields of 480 kV cm−1. This has led to a resurgence of interest in the potential applications of the ECE in solid-state refrigeration.22–24
The switchable spontaneous polarization that has led to applications of ferroelectrics in non-volatile memories or ferroelectric random access memory (FRAM)25 based on thin films of, e.g., PZT, SrBi2Ta2O9 (SBT), or, more-recently, HfO2-based ferroelectric films.26 Global markets for FRAM are expected to reach ca US$340 × 106 by 2025.27
Ferroelectrics that exhibit strong electro-optic (EO) effects, which has led to their widespread use in applications such as photonic switches, EO modulators for fibre-optic communications, laser Q-switches, etc. It has been estimated that the global market for LiNbO3-based modulators will hit US$36.7 × 109 by 2026.28
Barium titanate ceramics that can be doped with, e.g., Nb or La, to make them semiconducting. These materials can show very strong positive temperature coefficients (PTCs) of resistance in the region of the Curie temperature TC.29 They are widely used in self-stabilizing heaters and devices for, e.g., electric motor protection. The global PTC thermistor market was valued at US$285.1 × 106 in 2020, and it is expected to reach US$343.2 × 106 by the end of 2027, growing at a CAGR of 2.7% during 2021–2027.
The above-mentioned markets for ferroelectric materials and devices alone are predicted to be worth about US$80 × 109 by 2027, with a much greater market-value for the systems using them, indicating that the science and technology of ferroelectric materials has developed into a field of very significant economic importance.
One of the fascinating aspects of ferroelectricity as a phenomenon is that it appears in a very wide range of different material types with diverse physical properties. This greatly increases the potential for applications of ferroelectricity. These include:
Hydrogen bonded crystals such as Rochelle salt itself (originally used in piezoelectric devices), potassium dihydrogen phosphate, or KDP (used in electro-optic switches, light modulators, and frequency doublers30), and triglycine sulfate, or TGS (used in PIR detectors31). Such crystals tend to be water soluble. This simplifies the growth of very large, high quality single crystals, such as the growth of large KDP crystals for EO modulator plates used in ultra-high-power laser systems but has the disadvantage that the resulting crystals tend to be water sensitive and need careful handling and encapsulation. Recently, crystals of complex molecules such as tetrathiafulvalene (TTF) with halogenated quinones (Q)32 in hydrogen-bonded networks have been shown to exhibit ferroelectric behavior with significant spontaneous polarizations33 (∼5 μC cm−2) and have attracted considerable interest. Simple organic salts such as diisopropylammonium chloride (DIPAC)34 and diisopropylammonium bromide (DIPAB)35 were found to show excellent ferroelectric properties comparable to those of BaTiO3, with high Curie temperatures (440 and 426 K) as well as a spontaneous polarization of 23 µC cm−2, making them competitive with inorganic and polymer ferroelectrics. Besides the above-mentioned salts, single component organic crystals with high Curie temperature have also emerged recently, such as R-/S-3-quinuclidinol and 2- (hydroxymethyl)-2-nitro-1,3-propanediol, with unique homochirality36 and functional properties.37 Ye et al. also reported a previously unknown family of all-organic perovskites, including the first metal-free three-dimensional chiral perovskite ferroelectrics.38
Ferroelectric oxide single crystals such as LiNbO3 and LiTaO3 that have for many years underpinned a host of applications, such as SAW and bulk acoustic wave (BAW) devices39 (used in, e.g., radio frequency or RF filters and sensors), electro-optic photonic devices,40 and PIR detectors.41 These crystals are (or are close to being) congruently melting and are thus relatively simple to grow using the well-known Czochralski method.42,43 The discovery by Kuwata et al.44 and others45 of very high piezoelectric coupling factors in single crystals of PbZn⅓Nb⅔O3–PbTiO3 (PZN-PT) and PbMg⅓Nb⅔O3–PbTiO3 (PMN-PT) and more recently in ternary crystals such as PbIn½Nb½O3–PbMg⅓Nb⅔O3–PbTiO3 (PIN-PMN-PT) and (Na½Bi½)TiO3–BaTiO3-(K½Na½)NbO3 (NBT-BT-KNN)46 has led to major improvements in non-destructive evaluation (NDE)47 and medical ultrasound equipment.48 These compositions are not congruently melting, so the crystals have to be grown from flux, which has entailed significant development work to obtain acceptable uniformity of composition throughout large crystals.49
The fact that ferroelectrics can be polarized by applying an electric field in the poling process, meaning that polycrystalline materials can be given a net spontaneous polarization. Ferroelectric oxide ceramics are much cheaper and easier to make with a very wide range of compositions than oxide single crystals, allowing the physical properties to be “tuned” for particular applications. Ceramics based on BaTiO3 and PbZrO3–PbTiO3 (PZT) solid solutions (see above) have been the main materials underpinning dielectric, piezoelectric, and pyroelectric applications since the 1950s, although there has been a growing interest in lead-free ceramic compositions for environmental reasons.50 Various multiaxial molecular ferroelectrics can also be used in polycrystalline form, exhibiting advantages of mechanical flexibility, low processing temperature,51 and simple thin-film fabrication,52 leading to new opportunities for practical applications.53
The principle of combining one material with another to form a multi-phase composite to “tune” properties for a particular application, or even to get new properties, which has been a principle long applied to materials for structural applications. This principle has also been applied to ferroelectrics, with some of the earliest work being by Van Den Boomgaard et al.54,55 who showed that BaTiO3-barium ferrite composites made by eutectic melting could show novel magnetoelectric properties. Newnham et al.56 were the first to apply this principle to the creation of new piezoelectric and pyroelectric materials by making PZT-polymer composites. They described and codified the different “connectivity” patterns, defining how the different phases might be structured in three-dimensions, and showed how important connectivity was in determining the properties of the final composite material.
Thin films of ferroelectrics that have been explored for many years for their potential for integrating non-volatile memories onto silicon (FRAM). The first patents were in the late 1950s,57,58 while some of the earliest practical work in the field was on the integration of Aurivillius-phase59 ferroelectric oxide bismuth titanate into the gate of a field effect transistor (FET), reported by Wu and Francombe60,61 in the early 1970s. The field really took-off in the 1980s, with work on PZT and other Aurivillius oxides such as strontium bismuth tantalate,25,62 leading to their ultimate commercialization. More-recently, there has been considerable interest in the discovery of ferroelectric behavior in thin films of HfO2 when in solid solution with SiO263 and ZrO2.64 The better compatibility of these oxides with silicon integrated circuit (IC) processing promises lowering the technological barriers to the large-scale integration of FRAM onto silicon.
The strong piezoelectricity that was reported in β-phase polyvinylidene fluoride (PVDF)65 after poling in 1976. The authors tentatively assigned this to ferroelectricity, although it took some time before this was conclusively proved.66 Since then, ferroelectricity has also been demonstrated in copolymers of PVDF with trifluoroethylene (TrFE)67 and tetrafluoroethylene.68 There has been extensive application of these materials to piezoelectric69,70 and pyroelectric71,72 devices. Ferroelectricity has also been observed in odd-numbered nylons such as nylon-11.73 Recently, there has been considerable interest in the potential application of the ECE in P(VDF-TrFE) copolymers74 and terpolymers of P(VDF-TrFE) with chlorofluoroethylene.75
It can be seen from the brief review mentioned above that there has been enormous progress over the last 100 years in the science and technology of ferroelectric materials for a huge range of applications since the discovery of the phenomenon by Valasek. This special topic is a collection of papers celebrating the 100th anniversary of that discovery, and it is an exciting glimpse into the future of where the field will move in the future. The papers in this collection are directly related to most of the fields and material-types referred to above.
The understanding of the fundamentals of the paraelectric-to-ferroelectric phase transition has been a subject of research for many years. The two main theories are the order-disorder transition in which polar groups in the structure are disordered at high temperature, moving to an ordered state at lower temperature due to cooperative dipole interactions. The other main description is the soft-mode theory first described by Cochran,76 in which the frequency of a zone-center phonon mode goes to zero, or “softens,” at the Curie temperature. In both Rochelle salt and KDP, the transition order-disorder is driven by the occupancy of the protons in the hydrogen bonds linking the ions. This was first demonstrated by Bacon and Pease77 using neutron diffraction. In this collection, we have a new neutron diffraction study by Francher et al.78 on the field-dependence of the crystallographic structure of KDP as it is cycled around the hysteresis loop. The favored theory for oxide ferroelectrics is the soft-mode theory, and here, we have a perspective by Kamba79 discussing the results of broadband dielectric, THz, and IR spectroscopic investigations of soft-mode phenomena in H-bonded ferroelectrics, BaTiO3, relaxors, and multiferroics.
While ceramics based on the PZT solid-solution system occupy the bulk of the application space for piezoelectrics, it has long been recognized that the Curie temperatures of PZT ceramics limits their operational temperature to between 150 and 290 °C. Bell et al.80 have reviewed how this limits the operational space for piezoelectrics and have shown how new ceramic compositions based on BiTiO3–PbTiO3–K½Bi½TiO3 solid solutions can work up to 400 or even 600 °C, while providing useful piezoelectric coefficients to enable diverse novel applications, such as applications in sensing deep within operating gas turbine engines or the atomization of liquid metals.
The need to harvest ambient sources of energy to power micro-devices for the “Internet-of-Things” (IoT) and flexible/wearable electronics has excited interest in vibrational81 and flexible piezoelectric energy harvesters.82 In this issue, Liu et al.83 demonstrate how multi-layer stacks of PIN-PMN-PT single crystal wafers can give power outputs which are ca 400× greater than those of piezoelectric energy harvesters based on conventional PZT ceramic cantilevers. Li et al.84 give us an interesting perspective on how solution-processed polymeric and composite ferroelectrics based on PVDF exploiting both the piezoelectric and pyroelectric effects can be used to generate useful amounts of energy for flexible electronics. The theme of exploiting PVDF-based materials in energy applications is also addressed by Jiang et al.,85 who address the use of ferroelectric polymer nanocomposites in dielectric energy storage applications, showing that energy storage densities as high as 35.4 J cm−3 can be achieved in alternating pure P(VDF-HFP) layers and P(VDF-HFP)/BaTiO3 nanocomposite layers. This is more than ten times the level achievable in current commercial polymer dielectric films and offers great potential for new supercapacitors for energy storage.
The ferroelectric/magnetoelectric composite field can reasonably be traced back to the early work on magnetoelectric composites by Van Den Boomgaard et al.54,55 referred to above. In this issue, we have an interesting discussion by Yang et al.86 of the properties of new magnetoelectric composites made using Metglas foils and PZT thick films, with potential applications as magnetic field sensors. Lee et al.87 review the area of ferroelectric nanocomposites and show that vertically aligned nanocomposites (VANs) offer great potential and new avenues for future research, including new magnetoelectric multiferroics. The intrinsic nanocomposite nature of Aurivillius oxide systems are proposed to offer some very interesting multiferroic properties, as has already been demonstrated by Keeney et al.88
The pyroelectric and electrocaloric effects continue to excite interest. In this issue, Neumann et al.89 have taken some of the PIN-PMN-PT single crystal materials originally developed for piezoelectric applications and explored their use in practical PIR detectors. They show that it is possible to obtain performance figures-of-merit that are 5× greater than those of comparable devices based on LiTaO3, one of the normally preferred materials for use in this application. This is a remarkable performance increase, although it also comes at significantly increased device fabrication costs. Velarde et al.90 reviewed the present and future status of ferroelectric thin films for use in a range of pyroelectric applications, including the recovery of electrical energy from waste heat sources—something that offers considerable promise. Berenov et al.91 present a detailed study of the pyroelectric properties of sputtered Nb-doped PZT thin films and show that photonic stimulation of the films with visible light can considerably increase the magnitudes of the measured pyroelectric coefficients, an effect that remains unexplained. Crossley et al.92 present a very detailed practical study of the thermodynamics of the ECE in PST ceramics under adiabatic and isothermal conditions, something that could be of real importance in potential future applications of this material. Barman et al.93 present measurements of a remarkably high ECE ΔT of 13.5 K at a field of 1000 kV cm−1 in a Ba0.85Ca0.15Ti0.9Zr0.1O3 thin film heterostructure.
Since the work60,61 in the early-1970s that incorporated bismuth titanate ferroelectric thin films into the gate of an FET to make the first FeFET, the possible use of ferroelectric thin films as active components in semiconductor devices has continued to attract great interest, especially as our understanding of the materials we can use and the technologies for their fabrication improve. Kim et al.94 review the current status of FeFET technology, discussing the use of a wide range of potential ferroelectrics into the gate, ranging from PZT, through Hf0.5Zr0.5O2 (HZO) and PVDF to the ferroelectric semiconductor In2Se3 and exploring their potential for use in novel devices, including artificial synapses to mimic the operation of brain cells. The phase structure of HfO2-based ferroelectric films has been of considerable interest since ferroelectricity was first discovered in them.63,64 Here, Onaya et al.95 provide a detailed study of the phase structure of Hf0.43Zr0.57O2 films using synchrotron-radiation based XRD, particularly evaluating the effects of the post-metallization annealing (PMA) temperature on the formation of the orthorhombic phase present in the films and how this correlates with “wake-up” effects and remanent polarization in the films. Spreitzer et al.96 provide a research update on the growth of ferroelectric oxide thin films on silicon, with a particular view toward their applications in piezoelectric micro electromechanical systems (MEMS), electro-optic devices for communications applications, and catalysis and a consideration of the complexities of the silicon–ferroelectric interface. It is clear from this review that PZT thin films are still very interesting for a range of applications, and Do et al.97 provide a detailed structural study of the mechanisms behind the switching fatigue effects that occur in PZT thin films on Pt electrodes, showing that there is a large increase in coercive field, caused by the formation of a non-ferroelectric layer of binary oxides and diffused Pt at the Pt/PZT interface. Hanrahan et al.98 give a report on the growth of antiferroelectric PbHfO3 films from 20 to 200 nm thickness by atomic layer deposition (ALD). The films possess high energy storage densities (16 J cm−3 @ 2 M V m−1) and excellent (221% @ 50 M V m−1) tunability of the dielectric constant. Das et al.99 and Tian et al.100 provide evaluations of the new future direction for ferroelectric thin films in the form of superlattice structures of PbTiO3/SrTiO3, which can form new topological structures such as skyrmion and vortex features, offering new potential functionalities for information storage, multiferroicity, and optoelectronics. The pyroelectric properties of doped HfO2 ferroelectric thin films have been attracting interest recently for their potential application in PIR sensing and energy recovery applications.101,102 In this collection, Mart et al.103 present measurements of pyroelectric coefficients of up to −142 μC m−2 K−1 at the morphotropic transition from the ferroelectric orthorhombic to the centrosymmetric phase in films doped with 4.8 at. % Si, when a bias field of ∼1 M V cm−1 is applied. This is significantly higher than the pyroelectric coefficients measured previously in doped HfO2 thin films, which are typically around −80 μC m−2 K−1 (in, e.g., a film doped with 8.9 at. % La104).
The regions with different polar directions in a ferroelectric (domains) and the walls at their interfaces have long been of great interest for the effects they have on the physical properties of the materials. This ranges from the earliest work by Matthias and von Hippel105 on the effects of domains on the dielectric properties of BaTiO3, by Kittel106 on domain wall energies, by Mitsui and Furuichi107 on domain wall velocities in Rochelle salt and KDP, to that of Arlt108 on the effects of domains on piezoelectric ceramics. More recently, the discovery by Seidel et al.109 that domain walls in ferroelectrics can form highly conducting paths with nanometre width in an otherwise-insulating matrix have led to the concept of domain wall nanoelectronics (Catalan et al.110) in which domain walls can form circuits and semiconducting components that can be actively rewritten. Improper ferroelectrics, in which dielectric polarization is not the order parameter, are of particular interest here. Examples in which charged domain walls show enhanced conductivity include hexagonal ErMnO3 and related rare-earth manganites111 and copper chlorine boracite (Cu3B7O13Cl).112,113 There are a number of papers relevant to this concept in this collection. Salje114 give a perspective on the use of ferroelectric and ferroelastic domain walls in neuromorphic computing, whereby walls take the role of mimicking synapses and defect cluster neurons. Roede et al.115 demonstrate the use of focused ion and electron beams to charge the surface and reversibly switched improper ferroelectric domains in ErMnO3. McCartan et al.116 discuss the properties of the improper ferroelectric CsNbW2O9 as being the first material outside the manganite family to show similar meandering six-fold domain wall patterns with the potential for charged domain walls that may ultimately show enhanced domain wall conduction. Moore et al.117 review the use of aberration-corrected, sub-atomic resolution STEM to study the changes in polarization, chemical composition, charge density, and strain at ferroelectric domain wall boundaries and vortices and mapped the 3D nature of ferroelectric polar skyrmions. Domains are also of considerable interest in antiferroelectric materials, and An et al.118 discuss the hierarchical nature of domains in PZT crystals with only 2 at. % Ti, close to the antiferroelectric PbZrO3 composition, showing how this forms in the intermediate ferroelectric state and have a significant impact on the materials’ physical properties. Also in the collection, crystallographic investigations of antiferroelectric crystals of PbZr1−xSnxO3 with 0.05 < x < 0.3 by Jankowska-Sumara et al.119 reveal new incommensurate phase structures and correlated disorder of the octahedral tilts.
The study of ferroelectricity started 100 years ago with Rochelle salt, sodium potassium tartrate, and continued with KDP and its isomorphs until the discovery of ferroelectricity in perovskite oxides and their relatives. While there has been a strong emphasis since the early 1950s on the science and technology of materials such as BaTiO3 and PZT, there has also been continuous interest in hybrid metal organic and molecular ferroelectrics, which are now undergoing something of a renaissance. Ferroelectricity was demonstrated at low temperatures in tetramethylammonium cadmium tribromide by Gesi in 1960120 and at ambient temperatures in tetramethylammonium mercury trihalides121,122 in 1962. Subsequently, there has been shown to be a wide range of metal organic complex ferroelectrics, as reviewed by Hang et al.123 In this collection, we have an overview by Yao et al.124 of the potential for bandgap engineering in hybrid organic-inorganic perovskite (HOIP) ferroelectrics such as (MDABCO)(NH4)I3 [MDABCO = N-methyl-N′-diazabicyclo(2.2.2)octonium], with the potential for linking the excellent characteristics of the new perovskite materials for solar cells, such as methylammonium lead iodide (MAPbI3),125 with the photovoltaic effects seen in ferroelectrics. Bie et al.126 report on theoretical predictions for how the Curie temperature can be increased to >500 K (from 448 K) in (MDABCO)(NH4)I3 by replacing the NH4+ ion with metal ions such as K+ or Rb+. The molecular ferroelectrics solid solution system, (TMFM)x(TMCM)1−xCdCl3 (TMFM = trimethylfluoromethyl ammonium; TMCM = trimethylchloromethyl ammonium, 0 ≤ x ≤ 1), has been shown by Liao et al.127 to possess a piezoelectric d33 > 1500 pC/N, which is much larger than PZT. Guo et al.128 described high pressure synchrotron XRD experiments on another HOIP ferroelectric, TMCM-CdCl3 (TMCM = trimethylchloromethyl ammonium), which indicate that while the material has piezoelectric coefficients comparable with that of BaTiO3,129 it has significantly lower acoustic impedance, indicating promise for energy recovery and sonar applications. Ye et al.130 describe two new molecular hexagonal perovskite ferroelectrics (R)-3-OH-(C4H9N)[CdCl3] and (R)-3-OH-(C4H9N)[CdCl3], both with significantly higher Curie temperatures than the parent compound, while Xiong et al.131 discussed two further new hybrid ferroelectric perovskites: [(CH3)2SO][RE(HCOO)3], with RE = Lu3+ and Y3+. Harada132 give a perspective on plastic molecular ferroelectrics such as quinuclidinium perrhenate, which have interesting spontaneous polarisation at room temperature. Wang et al.133 present the results of density functional theory studies of the ferroelectric properties of the Preyssler-type polyoxometalates, while Hu and Ren134 present their theoretical studies of the electro-resistance and electro-optic properties of molecular ferroelectrics. Recently, intriguing topological vortex domain structures have been observed in organic ferroelectrics and organic-inorganic hybrid perovskite ferroelectrics,37,135,136 showing great potential as a reconfigurable electronic element for soft robotics, flexible and wearable devices, and biomachines. Liu et al.137 had proposed the concept of “ferroelectrochemistry” that focuses on the chemical design and performance optimization of molecular ferroelectrics. In this collection, Mu et al.138 present a brief summary of the design strategies and phenomenological theories behind this concept, which set a new trend for rational chemical synthesis over the next 100 years for ferroelectrics.
The science and technology of ferroelectric materials has undergone enormous development in the last 100 years, leading to huge diversity of material forms and types, a wide variety of useful properties, and a world-market for materials and devices that is expected to reach close to US$100 × 109 in this decade. The papers in this collection offer a fascinating snapshot of the topic and an invaluable perspective on where the subject is going. The editors hope that the readers of this collection will agree that the topic of ferroelectric materials is as interesting and exciting as it has ever been over the last 100 years and shows no signs of running out of steam.
The guest editors would like to thank Professor Wei Li of Nankai University, Peoples Republic of China and Professor Judith Driscoll of the University of Cambridge, UK, who are associate editors of APL Materials and who conceived this special topic to celebrate the 100th anniversary of the discovery of ferroelectricity. We would also like to thank the editorial staff at APL Materials who supported us all behind the scenes, especially Emma Nicholson and Ania Bukowski. This collection of papers could not have been put together without their hard work and dedication.
