Editing Strontium titanate

{{Chembox
| Watchedfields  = changed
| verifiedrevid  = 470471659
| ImageFile      = Tausonite.jpg
| ImageFile_Ref  = {{chemboximage|correct|??}}
| ImageSize      = 244
| ImageName      = Sample of strontium titanite as tausonite
| OtherNames     = Strontium titanium oxide<br />
Tausonite
STO
| Section1       = {{Chembox Identifiers
| CASNo = 12060-59-2
| CASNo_Ref = {{cascite|correct|CAS}}
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = OLH4I98373
| PubChem = 82899
| ChemSpiderID = 74801
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| EINECS = 235-044-1
| MeSHName = Strontium+titanium+oxide
| SMILES = [Sr++].[O-][Ti]([O-])=O
| SMILES1 = [Sr+2].[O-][Ti]([O-])=O
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/3O.Sr.Ti/q;2*-1;+2;
| InChI = 1/3O.Sr.Ti/q;2*-1;+2;/rO3Ti.Sr/c1-4(2)3;/q-2;+2
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = VEALVRVVWBQVSL-UHFFFAOYSA-N
| InChIKey = VEALVRVVWBQVSL-VUHNDFTMAE
}}
| Section2       = {{Chembox Properties
| Formula = {{Chem|SrTiO|3}}
| MolarMass = 183.49 g/mol
| Appearance = White, opaque crystals
| Density = 5.11 g/cm<sup>3</sup>
| MeltingPtC = 2080
| Solubility = insoluble
| RefractIndex = 2.394
}}
| Section3       = {{Chembox Structure
| CrystalStruct = [[Cubic crystal system|Cubic]] [[Perovskite (structure)|Perovskite]]
| SpaceGroup = Pm{{overline|3}}m, [[List of space groups|No. 221]]
}}
}}
'''Strontium titanate''' is an [[oxide]] of [[strontium]] and [[titanium]] with the [[chemical formula]] [[strontium|Sr]][[titanium|Ti]][[oxygen|O]]<sub>3</sub>. At room temperature, it is a [[centrosymmetric]] [[paraelectricity|paraelectric]] material with a [[Perovskite (structure)|perovskite]] structure. At low temperatures it approaches a [[Ferroelectricity|ferroelectric]] phase transition with a very large [[dielectric constant]] ~10<sup>4</sup> but remains [[paraelectricity|paraelectric]] down to the lowest temperatures measured as a result of [[quantum fluctuation]]s, making it a quantum paraelectric.<ref>{{cite journal| title = SrTiO<sub>3</sub>: An intrinsic quantum paraelectric below 4 K| journal = Phys. Rev. B | volume = 19| pages = 3593–3602 | year =1979|author1=K. A. Muller  |author2=H. Burkard | doi =10.1103/PhysRevB.19.3593| issue = 7| bibcode=1979PhRvB..19.3593M}}</ref> It was long thought to be a wholly artificial material, until 1982 when its natural counterpart—discovered in [[Siberia]] and named [[tausonite]]—was recognised by the [[International Mineralogical Association|IMA]]. Tausonite remains an extremely rare mineral in nature, occurring as very tiny [[crystal]]s. Its most important application has been in its synthesized form wherein it is occasionally encountered as a [[diamond simulant]], in precision [[optics]], in [[varistor]]s, and in advanced [[ceramic]]s.

The name ''tausonite'' was given in honour of [[Lev Vladimirovich Tauson]] (1917–1989), a Russian [[geochemistry|geochemist]]. Disused trade names for the synthetic product include ''strontium mesotitanate'', ''Diagem'', and ''Marvelite''. This product is currently being marketed for its use in jewelry under the name ''Fabulite''.<ref>{{cite journal | first=Annibale |last=Mottana |title=Una brillante sintesi |journal=Scienza e Dossier |publisher=Giunti|volume=1|issue=  1| date=March 1986 |pages=9|language=it}}</ref>  Other than its type locality of the [[Murun Massif]] in the [[Sakha Republic]], natural tausonite is also found in [[Cerro Sarambi]], [[Concepción Department (Paraguay)|Concepción department]], [[Paraguay]]; and along the [[Kotaki River]] of [[Honshū]], [[Japan]].<ref name=webmineral>{{cite web|publisher= Webmineral. | title = Tausonite| access-date = 2009-06-06| url = http://webmineral.com/data/Tausonite.shtml}}</ref><ref name=mindat>{{cite web|publisher= Mindat | title = Tausonite| access-date = 2009-06-06| url = http://www.mindat.org/min-3895.html}}</ref>

== Properties ==
[[File:Stohrem.jpg|thumb|left|Atomic resolution image of SrTiO<sub>3</sub> acquired using a Scanning Transmission Electron Microscope (STEM) and a high angle annular dark field (HAADF) detector. Brighter spots are columns of atoms containing Sr, and darker spots contain Ti. Columns containing only O atoms are not visible.|alt=]]
[[File:Perovskite.jpg|thumb|left|Structure of SrTiO<sub>3</sub>. The red spheres are oxygens, blue are Ti<sup>4+</sup> cations, and the green ones are Sr<sup>2+</sup>.]]

SrTiO<sub>3</sub> has an indirect [[band gap]] of 3.25 eV and a direct gap of 3.75 eV <ref>{{cite journal| author=K. van Benthem, C. Elsässer and R. H. French
| title=Bulk electronic structure of SrTiO<sub>3</sub>: Experiment and theory
| journal=[[Journal of Applied Physics]]
| volume=90 | issue=12
|page=6156 |year=2001 |doi=10.1063/1.1415766 |bibcode=2001JAP....90.6156V| s2cid=54065614
}}</ref> in the typical range of [[semiconductor]]s.
Synthetic strontium titanate has a very large [[dielectric constant]] (300) at room temperature and low electric field. It has a specific resistivity of over 10<sup>9</sup> Ω-cm for very pure crystals.<ref>{{cite web|title=Strontium Titanate|work=ESPI Metals|publisher=ESPICorp|url=http://www.espimetals.com/index.php/technical-data/248-strontium-titanate|archive-url=https://web.archive.org/web/20150924002236/http://www.espimetals.com/index.php/technical-data/248-strontium-titanate|archive-date=2015-09-24}}</ref> It is also used in high-voltage capacitors.
Introducing mobile charge carriers by doping leads to [[Fermi liquid|Fermi-liquid]] metallic behavior already at very low charge carrier densities.<ref>{{cite journal| author=Xiao Lin, Benoît Fauqué, Kamran Behnia
| title=Scalable T<sup>2</sup> resistivity in a small single-component Fermi surface
| journal=[[Science (journal)|Science]]
| volume=349 | issue=6251
| pages=945–8
|year=2015 |doi=10.1126/science.aaa8655 | pmid=26315430
| arxiv=1508.07812 | bibcode=2015Sci...349..945L | s2cid=148360
}}</ref>
At high electron densities strontium titanate becomes [[Superconductivity|superconducting]] below 0.35 K and was the first insulator and oxide discovered to be superconductive.<ref>{{cite journal| title = Superconducting Transition Temperatures of Semiconducting SrTiO3| journal = Phys. Rev. | volume = 163| page = 380| year =1967| doi =10.1103/PhysRev.163.380| issue = 2 |bibcode=1967PhRv..163..380K| last1 = Koonce| first1 = C. S.| last2 = Cohen| first2 = Marvin L. }}</ref>

Strontium titanate is both much denser ([[specific gravity]] 4.88 for natural, 5.13 for synthetic) and much softer ([[Mohs scale of mineral hardness|Mohs hardness]] 5.5 for synthetic, 6–6.5 for natural) than [[diamond]]. Its [[crystal system]] is [[Cubic crystal system|cubic]] and its [[refractive index]] (2.410—as measured by [[sodium]] light, 589.3&nbsp;nm) is nearly identical to that of diamond (at 2.417), but the [[dispersion (optics)|dispersion]] (the optical property responsible for the "fire" of the cut gemstones) of strontium titanate is 4.3× that of diamond, at 0.190 (B–G interval). This results in a shocking display of fire compared to diamond and diamond simulants such as [[Yttrium aluminium garnet|YAG]], [[Gadolinium Aluminum Garnet|GAG]], [[Gadolinium gallium garnet|GGG]], [[Cubic Zirconia]], and [[Moissanite]].<ref name=webmineral/><ref name=mindat/>

Synthetics are usually transparent and colourless, but can be [[dopant|doped]] with certain [[Rare earth element|rare earth]] or [[transition metal]]s to give reds, yellows, browns, and blues. Natural tausonite is usually translucent to opaque, in shades of reddish brown, dark red, or grey. Both have an adamantine (diamond-like) [[Lustre (mineralogy)|lustre]]. Strontium titanate is considered extremely brittle with a [[conchoidal fracture]]; natural material is cubic or octahedral in [[crystal habit|habit]] and [[Mineral#Streak|streak]]s brown. Through a hand-held (direct vision) [[spectroscope]], doped synthetics will exhibit a rich [[absorption spectrum]] typical of doped stones. Synthetic material has a [[melting point]] of ca. 2080&nbsp;°C (3776&nbsp;°F) and is readily attacked by [[hydrofluoric acid]].<ref name=webmineral/><ref name=mindat/> Under extremely low oxygen partial pressure, strontium titanate decomposes via incongruent [[sublimation (phase transition)|sublimation]] of strontium well below the melting temperature.<ref>{{cite journal| title = Stability and Decomposition of Perovskite-Type Titanates upon High-Temperature Reduction| journal = Phys. Status Solidi RRL | volume = 11| page = 1700222| year =2017| doi =10.1002/pssr.201700222| issue = 9 | author1 = C. Rodenbücher| author2 = P. Meuffels | author3 = W. Speier | author4 = M. Ermrich | author5 = D. Wrana | author6 = F. Krok | author7 = K. Szot| bibcode = 2017PSSRR..1100222R | s2cid = 102882984 }}</ref>

At temperatures lower than 105 K, its cubic structure transforms to [[tetragonal]].<ref>{{cite journal| title = Electron Paramagnetic Resonance of Trivalent Gadolinium Ions in Strontium and Barium Titanates| journal = Phys. Rev. | volume = 127| page = 702 | year =1962|author1=L. Rimai  |author2=G. A. deMars | doi =10.1103/PhysRev.127.702| issue = 3 |bibcode = 1962PhRv..127..702R }}</ref>  Its monocrystals can be used as optical windows and high-quality [[sputter deposition]] targets.

[[File:SrTiO3 single crystal substrates.png|right|thumb|Strontium titanate single crystal substrates (5×5×0.5mm). The transparent substrate (left) is pure SrTiO<sub>3</sub> and the black substrate is [[doping (semiconductors)|doped]] with 0.5% (weight) of [[niobium]]]]
SrTiO<sub>3</sub> is an excellent substrate for [[epitaxial growth]] of [[high-temperature superconductor]]s and many oxide-based [[thin film]]s. It is particularly well known as the substrate for the growth of the [[lanthanum aluminate-strontium titanate interface]]. Doping strontium titanate with [[niobium]] makes it electrically conductive, being one of the only conductive commercially available single crystal substrates for the growth of [[perovskite (structure)|perovskite]] oxides. Its bulk lattice parameter of 3.905Å makes it suitable as the substrate for the growth of many other oxides, including the rare-earth manganites, titanates, [[lanthanum aluminate]] (LaAlO<sub>3</sub>), [[strontium ruthenate]] (SrRuO<sub>3</sub>) and many others. Oxygen [[vacancy defect|vacancies]] are fairly common in SrTiO<sub>3</sub> crystals and thin films. Oxygen vacancies induce free electrons in the conduction band of the material, making it more conductive and opaque. These vacancies can be caused by exposure to reducing conditions, such as high vacuum at elevated temperatures.

High-quality, epitaxial SrTiO<sub>3</sub> layers can also be grown on [[silicon]] without forming [[silicon dioxide]], thereby making SrTiO<sub>3</sub> an alternative gate dielectric material. This also enables the integration of other thin film perovskite oxides onto silicon.<ref>{{cite journal| title = Crystalline Oxides on Silicon: The First Five Monolayers| journal = Phys. Rev. Lett. | volume = 81| page = 3014 | year =1998|author1=R. A. McKee |author2=F. J. Walker |author3=M. F. Chisholm  | doi =10.1103/PhysRevLett.81.3014| bibcode=1998PhRvL..81.3014M| issue = 14|url=https://zenodo.org/record/1233911}}</ref>

SrTiO<sub>3</sub> can change its properties when it is exposed to light.<ref name="d1ma00906k">{{cite journal|title=Photoinduced electronic and ionic effects in strontium titanate|first1=Matthäus|last1=Siebenhofer|first2=Alexander|last2=Viernstein|first3=Maximilian|last3=Morgenbesser|first4=Jürgen|last4=Fleig|first5=Markus|last5=Kubicek|date=February 6, 2021|journal=Materials Advances|volume=2|issue=23|pages=7583–7619|doi=10.1039/D1MA00906K|pmid=34913036 |pmc=8628302}}</ref><ref name="pmid34913036">{{cite journal|title=Photoinduced electronic and ionic effects in strontium titanate |journal=Mater Adv |volume=2 |issue=23 |pages=7583–7619 |date=November 2021 |pmid=34913036 |pmc=8628302 |doi=10.1039/d1ma00906k |url= |last1=Siebenhofer |first1=Matthäus |last2=Viernstein |first2=Alexander |last3=Morgenbesser |first3=Maximilian |last4=Fleig |first4=Jürgen |last5=Kubicek |first5=Markus }}</ref> These changes depend on the temperature and the defects in the material.<ref name="pmid34913036"/><ref name="d1ma00906k"/> SrTiO<sub>3</sub> has been shown to possess [[persistent photoconductivity]] where exposing the crystal to light will increase its electrical conductivity by over 2 orders of magnitude. After the light is turned off, the enhanced conductivity persists for several days, with negligible decay.<ref name=WSU>{{cite journal|publisher= Department of Physics and Astronomy, Washington State University, Pullman, Washington. | title = Persistent Photoconductivity in Strontium Titanate| year = 2013| doi = 10.1103/PhysRevLett.111.187403| access-date = 2013-11-18| url = http://prl.aps.org/abstract/PRL/v111/i18/e187403| last1 = Tarun| first1 = Marianne C.| last2 = Selim| first2 = Farida A.| last3 = McCluskey| first3 = Matthew D.| journal = Physical Review Letters| volume = 111| issue = 18| page = 187403| pmid = 24237562| bibcode = 2013PhRvL.111r7403T| url-access = subscription}}</ref><ref name=nature>{{cite web|publisher= Nature World News | title = Light Exposure Increases Crystal's Electrical Conductivity 400-fold [VIDEO]| access-date = 2013-11-18| url = http://www.natureworldnews.com/home/news/services/print.php?article_id=4943}}</ref> At low temperatures, the main effects of light are electronic, meaning that they involve the creation, movement, and recombination of electrons and holes (positive charges) in the material.<ref name="pmid34913036"/><ref name="d1ma00906k"/> These effects include photoconductivity, photoluminescence, photovoltage, and photochromism. They are influenced by the defect chemistry of SrTiO<sub>3</sub>, which determines the energy levels, band gap, carrier concentration, and mobility of the material. At high temperatures (>200 °C), the main effects of light are photoionic, meaning that they involve the migration of oxygen vacancies (negative ions) in the material. These vacancies are the main ionic defects in SrTiO<sub>3</sub>, and they can alter the electronic structure, defect chemistry, and surface properties of the material. These effects include photoinduced phase transitions, photoinduced oxygen exchange, and photoinduced surface reconstruction. They are influenced by the oxygen pressure, the crystal structure, and the doping level of SrTiO<sub>3</sub>.<ref name="pmid34913036"/><ref name="d1ma00906k"/>

Due to the significant [[Ionic compound|ionic]] and [[electron]]ic [[Electrical conductor|conduction]] of SrTiO<sub>3</sub>, it is potent to be used as the [[mixed conductor]].<ref name="MPG">{{cite web | url=http://www.fkf.mpg.de/2698712/MixedConductors | title=Mixed conductors | publisher=Max Planck institute for solid state research | access-date=16 September 2016}}</ref>

== Synthesis ==
[[File:Stocrystal.jpg|thumb|upright|A plate cut out of synthetic SrTiO<sub>3</sub> crystal]]

Synthetic strontium titanate was one of several [[titanate]]s [[patent]]ed during the late 1940s and early 1950s; other titanates included [[barium titanate]] and [[calcium titanate]]. Research was conducted primarily at the [[National Lead Company]] (later renamed [[NL Industries]]) in the [[United States]], by [[Leon Merker]] and [[Langtry E. Lynd]]. Merker and Lynd first patented the growth process on February 10, 1953; a number of refinements were subsequently patented over the next four years, such as modifications to the feed powder and additions of colouring dopants.

A modification to the basic [[Verneuil process]] (also known as flame-fusion) is the favoured method of growth. An inverted oxy-hydrogen [[blowpipe (tool)|blowpipe]] is used, with feed powder mixed with [[oxygen]] carefully fed through the blowpipe in the typical fashion, but with the addition of a third pipe to deliver oxygen—creating a ''tricone'' burner. The extra oxygen is required for successful formation of strontium titanate, which would otherwise fail to oxidize completely due to the titanium component. The ratio is ca. 1.5 volumes of [[hydrogen]] for each volume of oxygen. The highly purified feed powder is derived by first producing titanyl double oxalate [[salt (chemistry)|salt]] (SrTiO([[carbon|C]]<sub>2</sub>O<sub>4</sub>)<sub>2</sub>{{hydrate|2}}) by reacting [[strontium chloride]] (Sr[[chlorine|Cl]]<sub>2</sub>) and [[oxalic acid]] ((COO[[hydrogen|H]])<sub>2</sub>{{hydrate|2|nolink=yes}}) with [[titanium tetrachloride]] (TiCl<sub>4</sub>). The salt is washed to eliminate [[chloride]], heated to 1000&nbsp;°C in order to produce a free-flowing granular
powder of the required composition, and is then ground and sieved to ensure all particles are between 0.2 and 0.5 [[micrometre]]s in size.<ref name=growth/>

The feed powder falls through the [[oxyhydrogen flame]], melts, and lands on a rotating and slowly descending pedestal below. The height of the pedestal is constantly adjusted to keep its top at the optimal position below the flame, and over a number of hours the molten powder cools and crystallises to form a single pedunculated pear or ''[[boule (crystal)|boule]]'' crystal. This boule is usually no larger than 2.5 centimetres in diameter and 10 centimetres long; it is an opaque black to begin with, requiring further [[Annealing (metallurgy)|annealing]] in an oxidizing atmosphere in order to make the crystal colourless and to relieve [[Strain (chemistry)|strain]]. This is done at over 1000&nbsp;°C for 12 hours.<ref name=growth>{{cite book| page = [https://archive.org/details/crystalgrowthtec00sche_031/page/n438 431]| title= Crystal growth technology: from fundamentals and simulation to large-scale production| url = https://archive.org/details/crystalgrowthtec00sche_031| url-access = limited|author1=H. J. Scheel  |author2=P. Capper | publisher= Wiley-VCH| year = 2008
| isbn =978-3-527-31762-2}}</ref>

Thin films of SrTiO<sub>3</sub> can be grown epitaxially by various methods, including [[pulsed laser deposition]], [[molecular beam epitaxy]], [[sputter deposition|RF sputtering]] and [[atomic layer deposition]]. As in most thin films, different growth methods can result in significantly different defect and impurity densities and crystalline quality, resulting in a large variation of the electronic and optical properties.

== Use as a diamond simulant ==
Its cubic structure and high dispersion once made synthetic strontium titanate a prime candidate for [[diamond simulants|simulating diamond]]. Beginning {{Circa|1955}}, large quantities of strontium titanate were manufactured for this sole purpose. Strontium titanate was in competition with synthetic [[rutile]] ("titania") at the time, and had the advantage of lacking the unfortunate yellow tinge and strong [[birefringence]] inherent to the latter material. While it was softer, it was significantly closer to diamond in likeness. Eventually, however, both would fall into disuse, being eclipsed by the creation of "better" simulants: first by [[yttrium aluminium garnet]] (YAG) and followed shortly after by [[gadolinium gallium garnet]] (GGG); and finally by the (to date) ultimate simulant in terms of diamond-likeness and cost-effectiveness, [[cubic zirconia]].<ref>{{cite book| title = Jewelrymaking through history: an encyclopedia |author = R. W. Hesse| publisher = Greenwood Publishing Group| year = 2007| isbn =  978-0-313-33507-5| page = 73}}</ref>

Despite being outmoded, strontium titanate is still manufactured and periodically encountered in jewellery. It is one of the most costly of diamond simulants, and due to its rarity collectors may pay a premium for large i.e. >2 [[Carat (unit)|carat]] (400&nbsp;mg) specimens. As a diamond simulant, strontium titanate is most deceptive when mingled with melée i.e. <0.20 carat (40&nbsp;mg) stones and when it is used as the base material for a composite or ''doublet'' stone (with, e.g., synthetic [[corundum]] as the crown or top of the stone). Under the [[microscope]], [[gemology|gemmologist]]s distinguish strontium titanate from diamond by the former's softness—manifested by surface abrasions—and excess dispersion (to the trained eye), and occasional gas bubbles which are remnants of synthesis. Doublets can be detected by a join line at the girdle ("waist" of the stone) and flattened air bubbles or glue visible within the stone at the point of bonding.<ref>{{cite book| author = Nassau, K. | year =1980 | title = Gems made by man |pages=  214–221| publisher = Gemological Institute of America|location =  Santa Monica, California| isbn = 0-87311-016-1}}</ref><ref>{{cite book| author = O'Donoghue, M. | year =2002| title = Synthetic, imitation & treated gemstones |pages=  34, 65| publisher =  Elsevier Butterworth-Heinemann|location =  Great Britain| isbn = 0-7506-3173-2}}</ref><ref>{{cite book| author = Read, P. G. | year =1999 | title = Gemmology, second edition |pages=  173, 176, 177, 293| publisher =  Butterworth-Heinemann|location =  Great Britain | isbn = 0-7506-4411-7}}</ref>

==Use in radioisotope thermoelectric generators==
Due to its high melting point and insolubility in water, strontium titanate has been used as a [[strontium-90]]-containing material in [[radioisotope thermoelectric generator]]s (RTGs), such as the US Sentinel and Soviet Beta-M series.<ref name=OTA94>{{cite web |title = Power Sources for Remote Arctic Applications |date = June 1994 |location = Washington, DC |publisher = U.S. Congress, Office of Technology Assessment
| url = http://govinfo.library.unt.edu/ota/Ota_1/DATA/1994/9423.PDF |id = OTA-BP-ETI-129 }}</ref><ref>{{citation |title=Assessment of environmental, health and safety consequences of decommissioning radioisotope thermal generators (RTGs) in Northwest Russia |url=http://www.nrpa.no/dav/c600d1d288.pdf |number=StrålevernRapport 2005:4 |year=2005 |publisher=[[Norwegian Radiation Protection Authority]] |location=Østerås |last1=Standring |first1=WJF |last2=Selnæs |first2=ØG |last3=Sneve |first3=M |last4=Finne |first4=IE |last5=Hosseini |first5=A |last6=Amundsen |first6=I |last7=Strand |first7=P |access-date=2013-12-04 |archive-date=2016-03-03 |archive-url=https://web.archive.org/web/20160303210325/http://www.nrpa.no/dav/c600d1d288.pdf |url-status=dead }}</ref> As strontium-90 has a high [[fission product yield]] and is easily extracted from [[spent nuclear fuel]], Sr-90 based RTGs can in principle be produced cheaper than those based on [[plutonium-238]] or other radionuclides which have to be produced in dedicated facilities. However, due to the lower [[power density]] (~0.45W<sub> thermal</sub> per gram of Strontium-90-Titanate) and half life, space based applications, which put a particular premium on low weight, high reliability and longevity prefer [[Plutonium-238]]. Terrestrial [[off-grid]] applications of RTGs meanwhile have been largely phased out due to concern over [[orphan source]]s and the decreasing price and increasing availability of solar panels, small wind turbines, chemical battery storage and other off-grid power solutions.

==Use in solid oxide fuel cells==
Strontium titanate's mixed conductivity has attracted attention for use in [[solid oxide fuel cell]]s (SOFCs). It demonstrates both electronic and ionic conductivity which is useful for SOFC electrodes because there is an exchange of gas and oxygen ions in the material and electrons on both sides of the cell.

:{{Chem2 |  H2 + O(2-) -> H2O + 2 e- }}{{spaces|4}}(anode)
:{{Chem2 | ½ O2 + 2 e- -> O(2-) }}{{spaces|4}}(cathode)

Strontium titanate is doped with different materials for use on different sides of a fuel cell. On the fuel side (anode), where the first reaction occurs, it is often doped with lanthanum to form lanthanum-doped strontium titanate (LST). In this case, the A-site, or position in the unit cell where strontium usually sits, is sometimes filled by lanthanum instead, this causes the material to exhibit n-type semiconductor properties, including electronic conductivity. It also shows oxygen ion conduction due to the [[perovskite]] structure tolerance for oxygen vacancies. This material has a [[thermal expansion|thermal coefficient of expansion]] similar to that of the common electrolyte [[yttria-stabilized zirconia]] (YSZ), chemical stability during the reactions which occur at fuel cell electrodes, and electronic conductivity of up to 360 S/cm under SOFC operating conditions.<ref>{{cite journal |last1=Marina |first1=O |title=Thermal, electrical, and electrocatalytical properties of lanthanum-doped strontium titanate |journal=Solid State Ionics |date=2002 |volume=149 |issue=1–2 |pages=21–28 |doi=10.1016/S0167-2738(02)00140-6 }}</ref> Another key advantage of these LST is that it shows a resistance to sulfur poisoning, which is an issue with the currently used nickel - ceramic ([[cermet]]) anodes.<ref>{{cite journal |last1=Gong |first1=Mingyang |last2=Liu |first2=Xingbo |last3=Trembly |first3=Jason |last4=Johnson |first4=Christopher |title=Sulfur-tolerant anode materials for solid oxide fuel cell application |journal=Journal of Power Sources |date=2007 |volume=168 |issue=2 |pages=289–298 |doi=10.1016/j.jpowsour.2007.03.026 |bibcode=2007JPS...168..289G }}</ref>

Another related compound is strontium titanium ferrite (STF) which is used as a cathode (oxygen-side) material in SOFCs. This material also shows [[mixed conductor|mixed ionic and electronic conductivity]] which is important as it means the reduction reaction which happens at the cathode can occur over a wider area.<ref>{{cite journal |last1=Jung |first1=WooChul |last2=Tuller |first2=Harry L. |title=Impedance study of SrTi1−xFexO3−δ (x=0.05 to 0.80) mixed ionic-electronic conducting model cathode |journal=Solid State Ionics |date=2009 |volume=180 |issue=11–13 |pages=843–847 |doi=10.1016/j.ssi.2009.02.008 }}</ref> Building on this material by adding cobalt on the B-site (replacing titanium) as well as iron, we have the material STFC, or cobalt-substituted STF, which shows remarkable stability as a cathode material as well as lower polarization resistance than other common cathode materials such as [[lanthanum strontium cobalt ferrite]]. These cathodes also have the advantage of not containing [[rare-earth element|rare earth metals]] which make them cheaper than many of the alternatives.<ref>{{cite journal |last1=Zhang |first1=Shan-Lin |last2=Wang |first2=Hongqian |last3=Lu |first3=Matthew Y. |last4=Zhang |first4=Ai-Ping |last5=Mogni |first5=Liliana V. |last6=Liu |first6=Qinyuan |last7=Li |first7=Cheng-Xin |last8=Li |first8=Chang-Jiu |last9=Barnett |first9=Scott A. |title=Cobalt-substituted SrTi <sub>0.3</sub> Fe <sub>0.7</sub> O <sub>3−δ</sub> : a stable high-performance oxygen electrode material for intermediate-temperature solid oxide electrochemical cells |journal=Energy & Environmental Science |date=2018 |volume=11 |issue=7 |pages=1870–1879 |doi=10.1039/C8EE00449H |hdl=11336/99985 |hdl-access=free }}</ref>

== See also ==
* [[Calcium copper titanate]]

== References ==
{{reflist}}

== External links ==
* [http://deyoung.famsf.org/about/gerhard-richter-strontium-2005 An electron micrograph of strontium titanate, as artwork entitled "Strontium" at the DeYoung Museum in San Francisco] {{Webarchive|url=https://web.archive.org/web/20131022152321/http://deyoung.famsf.org/about/gerhard-richter-strontium-2005 |date=2013-10-22 }}

{{Strontium compounds}}
{{Titanium compounds}}
{{Titanates}}

{{Authority control}}

[[Category:Titanates]]
[[Category:Strontium compounds]]
[[Category:Gemstones]]
[[Category:Ceramic materials]]
[[Category:Transition metal oxides]]
[[Category:Diamond simulants]]
[[Category:Perovskites]]