vanadium(IV) oxide

{{stack|File:VO2 structure.png}}

At temperatures below T_c = {{convert|340|K|C}}, {{chem|VO|2}} has a monoclinic (space group P2₁/c) crystal structure. Above T_c, the structure is tetragonal, like rutile {{chem|TiO|2}}. In the monoclinic phase, the V⁴⁺ ions form pairs along the c axis, leading to alternate short and long V-V distances of 2.65 Å and 3.12 Å. In comparison, in the rutile phase the V⁴⁺ ions are separated by a fixed distance of 2.96 Å. As a result, the number of V⁴⁺ ions in the crystallographic unit cell doubles from the rutile to the monoclinic phase.{{cite journal|last1=Morin|first1=F. J.|title=Oxides Which Show a Metal-to-Insulator Transition at the Neel Temperature|journal=Physical Review Letters|volume=3|issue=1|year=1959|pages=34–36|doi=10.1103/PhysRevLett.3.34|bibcode=1959PhRvL...3...34M}}

The equilibrium morphology of rutile {{chem|VO|2}} particles is acicular, laterally confined by (110) surfaces, which are the most stable termination planes.{{cite journal|last1=Mellan|first1=Thomas A.|last2=Grau-Crespo|first2=Ricardo|title=Density functional theory study of rutile VO₂ surfaces|journal=The Journal of Chemical Physics|volume=137|issue=15|year=2012|pages=154706|doi=10.1063/1.4758319|pmid=23083183|arxiv=1209.6177|bibcode=2012JChPh.137o4706M|s2cid=29006673}} The surface tends to be oxidized with respect to the stoichiometric composition, with the oxygen adsorbed on the (110) surface forming vanadyl species. The presence of V⁵⁺ ions at the surface of {{chem|VO|2}} films has been confirmed by X-ray photoelectron spectroscopy.{{cite journal|last1=Manning|first1=Troy D.|last2=Parkin|first2=Ivan P.|last3=Pemble|first3=Martyn E.|last4=Sheel|first4=David|last5=Vernardou|first5=Dimitra|title=Intelligent Window Coatings: Atmospheric Pressure Chemical Vapor Deposition of Tungsten-Doped Vanadium Dioxide|journal=Chemistry of Materials|volume=16|issue=4|year=2004|pages=744–749|doi=10.1021/cm034905y}}

In 2022, a to date unique and unknown feature of the material was reported – it can "remember" previous external stimuli{{clarify|date=November 2022}} (via structural rather than electronic states), with potential for e.g. data storage and processing, potentially including in neuromorphic computing.{{cite news |title=Researchers discover a material that can learn like the brain |url=https://techxplore.com/news/2022-08-material-brain.html |access-date=15 September 2022 |work=Ecole Polytechnique Federale de Lausanne |language=en}}{{cite journal |last1=Samizadeh Nikoo |first1=Mohammad |last2=Soleimanzadeh |first2=Reza |last3=Krammer |first3=Anna |last4=Migliato Marega |first4=Guilherme |last5=Park |first5=Yunkyu |last6=Son |first6=Junwoo |last7=Schueler |first7=Andreas |last8=Kis |first8=Andras |last9=Moll |first9=Philip J. W. |last10=Matioli |first10=Elison |title=Electrical control of glass-like dynamics in vanadium dioxide for data storage and processing |journal=Nature Electronics |date=22 August 2022 |volume=5 |issue=9 |pages=596–603 |doi=10.1038/s41928-022-00812-z |s2cid=251759964 |language=en |issn=2520-1131|url=https://www.nature.com/articles/s41928-022-00812-z|url-access=}}

At the rutile to monoclinic transition temperature ({{convert|67|C|K}}), {{chem|VO|2}} also exhibits a metal to semiconductor transition in its electronic structure: the rutile phase is metallic while the monoclinic phase is semiconducting.{{Cite journal| last = Goodenough| first = John B.| date = 1971-11-01| title = The two components of the crystallographic transition in VO₂| journal = Journal of Solid State Chemistry| volume = 3| issue = 4| pages = 490–500| doi = 10.1016/0022-4596(71)90091-0| bibcode = 1971JSSCh...3..490G}} The optical band gap of VO₂ in the low-temperature monoclinic phase is about 0.7 eV.{{Cite journal| last1 = Shin| first1 = S.| last2 = Suga| first2 = S.| last3 = Taniguchi| first3 = M.| last4 = Fujisawa| first4 = M.| last5 = Kanzaki| first5 = H.| last6 = Fujimori| first6 = A.| last7 = Daimon| first7 = H.| last8 = Ueda| first8 = Y.| last9 = Kosuge| first9 = K.| title = Vacuum-ultraviolet reflectance and photoemission study of the metal-insulator phase transitions in VO₂, V₆O₁₃, and V₂O₃| journal = Physical Review B| volume = 41| issue = 8| pages = 4993–5009| doi = 10.1103/physrevb.41.4993| pmid = 9994356| year = 1990| bibcode = 1990PhRvB..41.4993S}}

Metallic VO₂ contradicts the Wiedemann–Franz law that holds that the ratio of the electronic contribution of the thermal conductivity (κ) to the electrical conductivity (σ) of a metal is proportional to the temperature. The thermal conductivity that could be attributed to electron movement was 10% of the amount predicted by the Wiedemann–Franz law. The reason for this appears to be the fluidic way that the electrons move through the material, reducing the typical random electron motion.{{Cite news|url=http://www.sciencealert.com/physicists-have-found-a-metal-that-conducts-electricity-but-not-heat|title=Physicists Have Found a Metal That Conducts Electricity but Not Heat |last=MacDonald |first=Fiona|newspaper=ScienceAlert|date=2017-01-28}} Thermal conductivity ~ 0.2 W/m⋅K, electrical conductivity ~ 8.0 ×10^5 S/m.{{Cite journal |last1=Lee|first1=Sangwook |last2=Hippalgaonkar|first2=Kedar |last3=Yang|first3=Fan |last4=Hong|first4=Jiawang |last5=Ko|first5=Changhyun |last6=Suh|first6=Joonki|last7=Liu|first7=Kai|last8=Wang|first8=Kevin|last9=Urban|first9=Jeffrey J.|date=2017-01-27|title=Anomalously low electronic thermal conductivity in metallic vanadium dioxide|journal=Science|volume=355|issue=6323|pages=371–374|doi=10.1126/science.aag0410|pmid=28126811|bibcode=2017Sci...355..371L|s2cid=206650639 |url=http://wu.mse.berkeley.edu/publications/Lee-Science2017.pdf}}

Potential applications include converting waste heat from engines and appliances into electricity,{{cite news |title=Scientists discover material that conducts electricity but no heat |url=https://indianexpress.com/article/technology/science/scientists-discover-material-that-conducts-electricity-but-no-heat-4497636/ |access-date=29 July 2022 |work=The Indian Express |date=29 January 2017 |language=en}} and windows or window coverings that keep buildings cool. Thermal conductivity varied when VO₂ was mixed with other materials. At a low temperature it could act as an insulator, while conducting heat at a higher temperature.

{{stack|Image:Nanostars-it1302.jpg}}

Following the method described by Berzelius, {{chem|VO|2}} is prepared by comproportionation of vanadium(III) oxide and vanadium(V) oxide:Brauer, G. ed. (1963) Handbook of Preparative Inorganic Chemistry, 2nd Ed. Academic Press. NY. Vol. 1. p. 1267.

: {{chem|V|2|O|5}} + {{chem|V|2|O|3}} → 4 {{chem|VO|2}}

At room temperature VO₂ has a distorted rutile structure with shorter distances between pairs of V atoms indicating metal-metal bonding. Above {{convert|68|C|K}}, the structure changes to an undistorted rutile structure and the metal-metal bonds are broken causing an increase in electrical conductivity and magnetic susceptibility as the bonding electrons are "released". The origin of this insulator to metal transition remains controversial and is of interest both for condensed matter physics[http://phys.org/news/2015-04-insulator-to-metal-transition-vanadium-dioxide.html New studies explain insulator-to-metal transition of vanadium dioxide], PhysOrg. April 11, 2015. and practical applications, such as electrical switches, tunable electrical filters, power limiters, nano-oscillators,{{cite journal|doi=10.1088/1468-6996/11/6/065002|pmid=27877369|pmc=5090451|title=Voltage- and current-activated metal–insulator transition in VO₂-based electrical switches: A lifetime operation analysis|journal=Science and Technology of Advanced Materials|volume=11|issue=6|pages=065002|year=2010|last1=Crunteanu|first1=Aurelian|last2=Givernaud|first2=Julien|last3=Leroy|first3=Jonathan|last4=Mardivirin|first4=David|last5=Champeaux|first5=Corinne|last6=Orlianges|first6=Jean-Christophe|last7=Catherinot|first7=Alain|last8=Blondy|first8=Pierre|bibcode=2010STAdM..11f5002C}} memristors, field-effect transistors and metamaterials.{{cite journal|doi=10.1080/14686996.2018.1521249|title=Electrical oscillation generation with current-induced resistivity switching in VO₂ micro-channel devices|journal=Science and Technology of Advanced Materials|volume=19|issue=1|pages=693–701|year=2018|last1=Pattanayak|first1=Milinda|last2=Hoque|first2=Md Nadim F.|last3=Fan|first3=Zhaoyang|last4=Bernussi|first4=Ayrton A.|bibcode=2018STAdM..19..693P|doi-access=free|hdl=2346/95263|hdl-access=free}}{{open access}}{{cite journal |last1=Driscoll |first1=T. |last2=Palit |first2=S. |last3=Qazilbash |first3=M. M. |last4=Brehm |first4=M. |title=Dynamic tuning of an infrared hybrid-metamaterial resonance using vanadium dioxide |journal=Applied Physics Letters |date=2008 |volume=93 |doi=10.1063/1.2956675|display-authors=3|page=024101|issue=2|bibcode=2008ApPhL..93b4101D }}{{cite journal |last1=Kats |first1=Mikhail A. |last2=Blanchard |first2=Romain |last3=Zhang |first3=Shuyan |last4=Genevet |first4=Patrice |title=Vanadium Dioxide as a Natural Disordered Metamaterial: Perfect Thermal Emission and Large Broadband Negative Differential Thermal Emittance |journal=Physical Review X |date=21 October 2013 |volume=3 |issue=4 |page=041004 |doi=10.1103/PhysRevX.3.041004|arxiv=1305.0033 |bibcode=2013PhRvX...3d1004K |display-authors=3|doi-access=free}}{{open access}}

{{stack|File:VO2-SiO2 transmittance.png}}

{{Chem|VO|2}} expresses temperature-dependent reflective properties. When heated from room temperature to {{convert|80|C|K}}, the material's thermal radiation rises normally until {{convert|74|C|K}}, before suddenly appearing to drop to around {{convert|20|C|K}}. At room temperature, {{Chem|VO|2}} is almost transparent to infrared light. As its temperature rises it gradually changes to reflective. At intermediate temperatures it behaves as a highly absorbing dielectric.{{cite web|url=http://physicsworld.com/cws/article/news/2013/oct/25/natural-metamaterial-looks-cooler-when-heated |title=Natural metamaterial looks cooler when heated |publisher=physicsworld.com |date= 2013-10-25|access-date=2014-01-01}}{{Cite journal | last1 = Kats | first1 = M. A. | last2 = Blanchard | first2 = R. | last3 = Zhang | first3 = S. | last4 = Genevet | first4 = P. | last5 = Ko | first5 = C. | last6 = Ramanathan | first6 = S. | last7 = Capasso | first7 = F. | title = Vanadium Dioxide as a Natural Disordered Metamaterial: Perfect Thermal Emission and Large Broadband Negative Differential Thermal Emittance | doi = 10.1103/PhysRevX.3.041004 | journal = Physical Review X | volume = 3 | issue = 4 | pages = 041004 | year = 2013 | arxiv = 1305.0033 | bibcode = 2013PhRvX...3d1004K | s2cid = 53496680 }}

A thin film of vanadium oxide on a highly reflecting substrate (for specific infrared wavelengths) such as sapphire is either absorbing or reflecting, dependent on temperature. Its emissivity varies considerably with temperature. When the vanadium oxide transitions with increased temperature, the structure undergoes a sudden decrease in emissivity – looking colder to infrared cameras than it really is.{{cite journal|doi=10.1080/14686996.2017.1360752|pmid=28970866|pmc=5613921|title=New intelligent multifunctional SiO₂/VO₂ composite films with enhanced infrared light regulation performance, solar modulation capability, and superhydrophobicity|journal=Science and Technology of Advanced Materials|volume=18|issue=1|pages=563–573|year=2017|last1=Wang|first1=Chao|last2=Zhao|first2=Li|last3=Liang|first3=Zihui|last4=Dong|first4=Binghai|last5=Wan|first5=Li|last6=Wang|first6=Shimin|bibcode=2017STAdM..18..563W}}

Varying the substrate materials (e.g., to indium tin oxide), as well as modifying the vanadium oxide coating using doping, straining, or other processes, alters the wavelengths and temperature ranges at which the thermal effects are observed.

Nanoscale structures that appear naturally in the materials' transition region can suppress thermal radiation as the temperature rises. Doping the coating with tungsten lowers the effect's thermal range to room temperature.

Undoped and tungsten-doped vanadium dioxide films can act as "spectrally-selective" coatings to block infrared transmission and reduce the loss of building interior heat through windows.Guzman, G. [https://web.archive.org/web/20180325084333/http://www.solgel.com:80/articles/august00/thermo/guzman.htm Vanadium dioxide as infrared active coating]. solgel.com{{cite web|url=http://www.azom.com/details.asp?ArticleID=2587 |title=Intelligent Window Coatings that Allow Light In but Keep Heat Out - News Item |publisher=Azom.com |date= 2004-08-12|access-date=2012-09-12}} Varying the amount of tungsten allows regulating the phase transition temperature at a rate of {{convert|20|C-change|K-change}} per 1 atomic percent of tungsten. The coating has a slight yellow-green color.{{cite web|url=http://oemagazine.com/fromthemagazine/nov04/eyeontech.html |title=Intelligent Window Coating Reflects Heat, Not Light|author=Espinasse, Phillip |publisher=oe magazine |date=2009-11-03 |archive-url=https://web.archive.org/web/20050524080100/http://oemagazine.com/fromthemagazine/nov04/eyeontech.html|access-date=2012-09-12|archive-date=2005-05-24}} The performance of energy-saving smart windows can be enhanced by combining VO2 with antireflection layers.{{Cite journal |last=Houska |first=Jiri |date=2022-03-21 |title=Design and reactive magnetron sputtering of thermochromic coatings |url=https://aip.scitation.org/doi/10.1063/5.0084792 |journal=Journal of Applied Physics |volume=131 |issue=11 |pages=110901 |doi=10.1063/5.0084792 |bibcode=2022JAP...131k0901H |issn=0021-8979|hdl=11025/47644 |s2cid=247568375 |hdl-access=free }} The technology of low-temperature preparation of V_1−xW_xO₂-based multilayers has been scaled up to industrial dimensions.{{Cite journal |last1=Rezek |first1=Jiří |last2=Szelwicka |first2=Jolanta |last3=Vlček |first3=Jaroslav |last4=Čerstvý |first4=Radomír |last5=Houška |first5=Jiří |last6=Fahland |first6=Matthias |last7=Fahlteich |first7=John |date=July 2022 |title=Transfer of the sputter technique for deposition of strongly thermochromic VO2-based coatings on ultrathin flexible glass to large-scale roll-to-roll device |url=https://linkinghub.elsevier.com/retrieve/pii/S0257897222001943 |journal=Surface and Coatings Technology |language=en |volume=442 |pages=128273 |doi=10.1016/j.surfcoat.2022.128273|hdl=11025/49623 |s2cid=247121490 |hdl-access=free }}

Other potential applications of its thermal properties include passive camouflage, thermal beacons, communication, or to deliberately speed up or slow down cooling. These applications could be useful for a variety of structures from homes to satellites.

Vanadium dioxide can act as extremely fast optical modulators, infrared modulators for missile guidance systems, cameras, data storage, and other applications. The thermochromic phase transition between the transparent semiconductive and reflective conductive phase, occurring at {{convert|68|C|K}}, can happen in times as short as 100 femtoseconds.{{cite web|url=http://www.physorg.com/news3629.html |title=Timing nature's fastest optical shutter |publisher=Physorg.com |date=2005-04-07 }}

Vanadium dioxide is essential to achieving temperature-based 'switchable' cooling and heating effects for passive daytime radiative cooling surfaces without additional energy input. Temperature-based switching can be essential to mitigate potential "overcooling" effects of radiative cooling devices in urban environments, especially those with hot summers and cool winters, making it possible for radiative coolers to also function as passive heating devices when necessary.{{Cite journal |last1=Chen |first1=Meijie |last2=Pang |first2=Dan |last3=Chen |first3=Xingyu |last4=Yan |first4=Hongjie |last5=Yang |first5=Yuan |title=Passive daytime radiative cooling: Fundamentals, material designs, and applications |journal=EcoMat |year=2022 |volume=4 |doi=10.1002/eom2.12153 |s2cid=240331557 |doi-access=free }}{{Cite journal |last1=Wang |first1=Zhaochen |last2=Kim |first2=Sun-Kyung |last3=Hu |first3=Run |date=March 2022 |title=Self-switchable radiative cooling |journal=Matter |volume=5 |issue=3 |pages=780–782 |doi=10.1016/j.matt.2022.01.018 |doi-access=free }}

The insulator-metal phase transition in VO₂ can be manipulated at the nanoscale using a biased conducting atomic force microscope tip,{{cite journal|author1=Jeehoon Kim|title=Nanoscale imaging and control of resistance switching in VO₂ at room temperature|journal=Applied Physics Letters|doi=10.1063/1.3435466 |year=2010|volume=96|issue=21|pages=213106|last2=Ko|first2=Changhyun|last3=Frenzel|first3=Alex|last4=Ramanathan|first4=Shriram|last5=Hoffman|first5=Jennifer E. |bibcode=2010ApPhL..96u3106K|s2cid=122696544 |url=https://dash.harvard.edu/bitstream/handle/1/4342532/Hoffman_NanoscaleImaging.pdf?sequence=1}} suggesting applications in computing and information storage.{{Cite journal|title = Mott Memory and Neuromorphic Devices|journal = Proceedings of the IEEE|date = 2015-08-01|pages = 1289–1310|volume = 103|issue = 8|doi = 10.1109/JPROC.2015.2431914|first1 = You|last1 = Zhou|first2 = S.|last2 = Ramanathan|s2cid = 11347598|url = https://zenodo.org/record/895565}}

• Vanadium redox battery

{{reflist}}

{{Commons category|Vanadium(IV) oxide}}

• {{cite book | editor= Haynes, William M. | year = 2011 | title = CRC Handbook of Chemistry and Physics | edition = 92nd | publisher = CRC Press | isbn = 978-1-4398-5511-9| title-link = CRC Handbook of Chemistry and Physics }}

{{Vanadium compounds}}

{{Oxides}}

Category:Vanadium(IV) compounds

Category:Transition metal oxides