100 Years of Ferroelectricity-A Celebration

A Celebration Roger W. Whatmore, Yu-Meng You, Ren-Gen Xiong, Chang-Beom Eom 1 Department of Materials, Royal School of Mines, South Kensington Campus, Imperial College London, London SW7 2AZ, United Kingdom 2 Ordered Matter Science Research Center, Southeast University, Nanjing 211189, People’s Republic of China 3 Ordered Matter Science Research Center, Nanchang University, Nanchang 330031, People’s Republic of China 4 Department of Materials Science and Engineering, University of Wisconsin–Madison, Madison, WI 53706, USA a) Author to whom correspondence should be addressed: r.whatmore@imperial.ac.uk

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 electricallyswitchable 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 towards the widespread use of ferroelectric materials came with the independent discovery during the 1940's in the USA, UK, Russia and Japan of anomalous dielectric behaviour and ferroelectricity in the perovskite oxide barium titanate (BaTiO 3 ). This led eventually to the discovery by H. Jaffe and co workers 10  • 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 billion in 2023, rising from US$6.1 billion 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 This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. • The switchable spontaneous polarization 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, SrBi 2 Ta 2 O 9 (SBT) or, more-recently, HfO 2 -based ferroelectric films 26 . Global markets for FRAM are expected to reach ca US$340 million by 2025 27 .

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• Ferroelectrics exhibit strong electro-optic (EO) effects, which has led to their widespread use in applications such as photonic switches, EO modulators for fibreoptic communications, laser Q-switches etc. It has been estimated that the global market for LiNbO 3 -based modulators will hit US$36.7 billion by 2026 28 .
• Barium titanate ceramics can be doped with e.g. Nb or La to make them semiconducting. These materials can show very strong positive temperature coefficients (PTC) of resistance in the region of the Curie temperature T C 29 . These are widely used in self-stabilizing heaters and devices for e.g electric motor protection. The global PTC Thermistors market was valued at US$285.1 million in 2020 and it is expected to reach US$343.2 million by the end of 2027, growing at a CAGR of 2.7% during 2021-2027.
The above markets for ferroelectric materials and devices alone are predicted to be worth about US$80 billion by 2027, with a much greater market-value for the systems using them, indicating that the science and technology of ferroelectric materials has 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 electrooptic switches, light modulators and frequency doublers 30 ) and triglycine sulfate, or TGS (used in PIR detectors 31 ). 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 behaviour with significant spontaneous polarizations 33 (~5 Ccm -2 ) and are exciting considerable interest. Simple organic salts such as diisopropylammonium chloride (DIPAC) 34 and diisopropylammonium bromide (DIPAB) 35  • Ferroelectric oxide single crystals such as LiNbO 3 and LiTaO 3 have for many years underpinned a host of applications, such as SAW and bulk acoustic wave (BAW) devices 39 (used in e.g. radio frequency or RF filters and sensors), electrooptic 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 wellknown Czochralski method 42,43 . The discovery by Uchino et al 44 46 has led to major improvements in non-destructive evaluation (NDE) 47 and medical ultrasound equipment 48 . These compositions are not congruently melting and 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 .
This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI:10.1063/5.0059208
• The fact that ferroelectrics can be polarized by applying an electric field in the poling process means 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 BaTiO 3 and PbZrO 3 -PbTiO 3 (PZT) solid solutions (see above), have been the main materials underpinning dielectric, piezoelectric and pyroelectric applications since the 1950's, although for environmental reasons there has been a growing interest in lead-free ceramic compositions 50 . Various multiaxial molecular ferroelectrics can also be used in polycrystalline form, giving 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, has been a principle long applied to materials for structural applications. This principle has also been applied to ferroelectrics, with some of This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI:10.1063/5.0059208
• In 1976, strong piezoelectricity was reported in -phase polyvinylidene fluoride (PVDF) 65 after poling. 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 trifuoroethylene (TrFE) 67 and tetrafluoroethylene 68 . There has been extensive application of these materials to piezoelectric 69,70 and pyroelectric 71,72 devices.
Ferroelectricity has also been observed in the odd-numbered nylons such as nylon-11 73 . Recently, there has been considerable interest in the potential applications of the ECE in P(VDF-TrFE) copolymers 74  It can be seen from the brief review 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 100 th anniversary of that discovery and it is an exciting glimpse into the future of where the field is moving in the future. The papers in this collection are directly related to most of the fields and material-types referred to above.
The understanding the fundamentals of the paraelectric-to-ferroelectric phase transition has been a subject of research for many years. The two main theories are the orderdisorder 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  This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. 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 billion 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.