Editing Ferroelectricity (section)

==Materials==
The internal electric dipoles of a ferroelectric material are coupled to the material lattice so anything that changes the lattice will change the strength of the dipoles (in other words, a change in the spontaneous polarization).  The change in the spontaneous polarization results in a change in the surface charge.  This can cause current flow in the case of a ferroelectric capacitor even without the presence of an external voltage across the capacitor. Two stimuli that will change the lattice dimensions of a material are force and temperature. The generation of a surface charge in response to the application of an external stress to a material is called [[piezoelectricity]]. A change in the spontaneous polarization of a material in response to a change in temperature is called [[pyroelectricity]].

Generally, there are 230 [[space group]]s among which 32 [[Crystal system#Crystal classes|crystalline classes]] can be found in crystals. There are 21 non-centrosymmetric classes, within which 20 are [[piezoelectricity|piezoelectric]]. Among the piezoelectric classes, 10 have a spontaneous electric polarization which varies with temperature; thus they are [[pyroelectricity|pyroelectric]]. Ferroelectricity is a subset of pyroelectricity, which brings spontaneous electronic polarization to the material.<ref>{{Citation |last=Whatmore |first=R. W. |title=Piezoelectric and Pyroelectric Materials and Their Applications |date=1991 |url=https://doi.org/10.1007/978-1-4615-3818-9_19 |work=Electronic Materials: From Silicon to Organics |pages=283–290 |editor-last=Miller |editor-first=L. S. |place=Boston, MA |publisher=Springer US |language=en |doi=10.1007/978-1-4615-3818-9_19 |isbn=978-1-4615-3818-9 |access-date=2022-09-22 |editor2-last=Mullin |editor2-first=J. B.}}</ref>

{| class="wikitable centre" style="border:5px #E00;" width="90%"
! colspan="5" | 32 [[Crystallographic point group|Crystalline class]]es
|-
! colspan="4" |21 noncentrosymmetric
!11 [[Centrosymmetry|centrosymmetric]]
|-
! colspan="3" align="center" width="75%" | 20 classes [[Piezoelectricity|piezoelectric]] 
| rowspan="4" align="center" |
| rowspan="4" align="center" |non piezoelectric
|- 
!width="50%" colspan="2" align="center"| 10 classes [[pyroelectricity|pyroelectric]] 
| align="center" rowspan="2"| non pyroelectric
|-
!width="25%" align="center"| ferroelectric
| align="center" |non ferroelectric
|-
|e.g. : [[Lead zirconate titanate|PbZr/TiO<sub>3</sub>]], [[Barium titanate|BaTiO]]<sub>3</sub>, [[Lead titanate|PbTiO]]<sub>3</sub>, [[Aluminium nitride|AlN]]<ref>Wanlin Zhu, John Hayden, Fan He, Jung-In Yang, Pannawit Tipsawat, Mohammad D. Hossain, Jon-Paul Maria, and Susan Trolier-McKinstry, "Strongly temperature dependent ferroelectric switching in AlN, Al1-xScxN, and Al1-xBxN thin films", Appl. Phys. Lett. 119, 062901 (2021) https://doi.org/10.1063/5.0057869</ref>
|e.g. : [[Tourmaline]], [[Zinc oxide|ZnO]], 
|e.g. : [[Quartz]], [[Lanthanum gallium silicate|Langasite]]
|}

Ferroelectric phase transitions are often characterized as either displacive (such as BaTiO<sub>3</sub>) or order-disorder (such as NaNO<sub>2</sub>), though often phase transitions will demonstrate elements of both behaviors. In [[barium titanate]], a typical ferroelectric of the displacive type, the transition can be understood in terms of a polarization catastrophe, in which, if an ion is displaced from equilibrium slightly, the force from the local [[electric field]]s due to the ions in the crystal increases faster than the elastic-restoring [[force (physics)|force]]s. This leads to an asymmetrical shift in the equilibrium ion positions and hence to a permanent dipole moment. The ionic displacement in barium titanate concerns the relative position of the titanium ion within the oxygen octahedral cage.  In [[lead titanate]], another key ferroelectric material, although the structure is rather similar to barium titanate the driving force for ferroelectricity is more complex with interactions between the lead and oxygen ions also playing an important role. In an order-disorder ferroelectric, there is a dipole moment in each unit cell, but at high temperatures they are pointing in random directions. Upon lowering the temperature and going through the phase transition, the dipoles order, all pointing in the same direction within a domain.

An important ferroelectric material for applications is [[lead zirconate titanate]] (PZT), which is part of the solid solution formed between ferroelectric lead titanate and [[anti-ferroelectric]] lead zirconate. Different compositions are used for different applications; for memory applications, PZT closer in composition to lead titanate is preferred, whereas piezoelectric applications make use of the diverging piezoelectric coefficients associated with the morphotropic phase boundary that is found close to the 50/50 composition.

Ferroelectric [[crystals]] often show several [[transition temperature]]s and [[Hysteresis#Electrical hysteresis|domain structure hysteresis]], much as do [[ferromagnetism|ferromagnetic]] crystals. The nature of the [[phase transition]] in some ferroelectric crystals is still not well understood.

In 1974 R.B. Meyer used symmetry arguments to predict ferroelectric [[liquid crystals]],<ref name=Clark>{{cite journal |last1=Clark |first1=Noel A. |last2=Lagerwall |first2=Sven T. |title=Submicrosecond bistable electro‐optic switching in liquid crystals |journal=Applied Physics Letters |date=June 1980 |volume=36 |issue=11 |pages=899–901 |doi=10.1063/1.91359 |bibcode=1980ApPhL..36..899C }}</ref> and the prediction could immediately be verified by several observations of behavior connected to ferroelectricity in smectic liquid-crystal phases that are chiral and tilted. The technology allows the building of flat-screen monitors. Mass production between 1994 and 1999 was carried out by Canon. Ferroelectric liquid crystals are used in production of reflective [[LCoS]].

In 2010 [[David Field (astrophysicist)|David Field]] found that prosaic films of chemicals such as [[nitrous oxide]] or propane exhibited ferroelectric properties.<ref>{{Cite journal |last=Plekan |first=Oksana |date=2010 |title=Novel ferroelectric behaviour of N2O films: spontaneous potentials of up to 40 V. |url=https://pure.au.dk/portal/en/persons/richard-balog(31665d74-6b65-49f2-9bf1-ff81d3c50ef7)/publications/novel-ferroelectric-behaviour-of-n2o-films-spontaneous-potentials-of-up-to-40-v(137cba00-900d-11df-8c1a-000ea68e967b)/export.html |journal=Poster Session Presented at ECAMP 2010, Salamanca, Spain. |via=Aarhus University}}</ref> This new class of ferroelectric materials exhibit "[[spontelectrics|spontelectric]]" properties, and may have wide-ranging applications in device and nano-technology and also influence the electrical nature of dust in the interstellar medium.

Other ferroelectric materials used include [[triglycine sulfate]], [[polyvinylidene fluoride]] (PVDF) and [[lithium tantalate]].<ref name="Aggarwal">{{cite web |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110008068_2011008855.pdf |title=Pyroelectric Materials for Uncooled Infrared Detectors: Processing, Properties, and Applications |last=Aggarwal |first=M.D. |author2=A.K. Batra |author3=P. Guggilla |author4=M.E. Edwards |author5=B.G. Penn |author6=J.R. Currie Jr. |date=March 2010 |publisher=[[NASA]] 
|page=3 |access-date=26 July 2013}}</ref> A single atom thick ferroelectric monolayer can be created using pure [[bismuth]]. <ref>{{cite web |url=https://www.science.nus.edu.sg/blog/2023/04/06/discovery-of-ferroelectricity-in-an-elementary-substance/ |title=Discovery of ferroelectricity in an elementary substance|date=April 2023 |publisher=[[National University of Singapore]]  |access-date=10 April 2023}}</ref>

It should be possible to produce materials which combine both ferroelectric and metallic properties simultaneously, at room temperature.<ref>{{Cite web|url=https://www.rutgers.edu/news/rutgers-physicists-create-new-class-2d-artificial-materials|title = Rutgers Physicists Create New Class of 2D Artificial Materials}}</ref> According to research published in 2018 in ''Nature Communications'',<ref>{{cite journal |last1=Cao |first1=Yanwei |last2=Wang |first2=Zhen |last3=Park |first3=Se Young |last4=Yuan |first4=Yakun |last5=Liu |first5=Xiaoran |last6=Nikitin |first6=Sergey M. |last7=Akamatsu |first7=Hirofumi |last8=Kareev |first8=M. |last9=Middey |first9=S. |last10=Meyers |first10=D. |last11=Thompson |first11=P. |last12=Ryan |first12=P. J. |last13=Shafer |first13=Padraic |last14=N’Diaye |first14=A. |last15=Arenholz |first15=E. |last16=Gopalan |first16=Venkatraman |last17=Zhu |first17=Yimei |last18=Rabe |first18=Karin M.|author18-link= Karin M. Rabe |last19=Chakhalian |first19=J. |title=Artificial two-dimensional polar metal at room temperature |journal=Nature Communications |date=18 April 2018 |volume=9 |issue=1 |pages=1547 |doi=10.1038/s41467-018-03964-9 |pmid=29670098 |pmc=5906683 |arxiv=1804.05487 |bibcode=2018NatCo...9.1547C }}</ref> scientists were able to produce a two-dimensional sheet of material which was both ferroelectric (had a polar crystal structure) and which conducted electricity.