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{{Infobox thulium}}
'''Thulium''' is a [[chemical element]]; it has [[Symbol (chemistry)|symbol]] '''Tm''' and [[atomic number]] 69. It is the thirteenth element in the [[lanthanide]] series of [[metal]]s. It is the second-least abundant lanthanide in the Earth's crust, after radioactively unstable [[promethium]]. It is an easily workable metal with a bright silvery-gray luster. It is fairly soft and slowly [[tarnish]]es in air. Despite its high price and rarity, thulium is used as a [[dopant]] in [[solid-state laser]]s. It has no significant biological role and is not particularly toxic. Artificial radioactive [[isotopes of thulium]] are used as [[#X-ray source|radiation sources]] in some portable [[X-ray]] devices.

In 1879, the Swedish chemist [[Per Teodor Cleve]] separated two previously unknown components, which he called [[holmium(III) oxide|holmia]] and [[thulium(III) oxide|thulia]], from the [[rare-earth]] mineral [[erbium(III) oxide|erbia]]; these were the oxides of [[holmium]] and thulium, respectively. His example of thulium oxide contained impurities of ytterbium oxide. A relatively pure sample of thulium oxide was first obtained in 1911. The metal itself was first obtained in 1936 by [[Wilhelm Klemm]] and Heinrich Bommer.<ref>{{cite journal|author1=W. Klemm|author2=H. Bommer|title=Zur Kenntnis der Metalle der seltenen Erden.|lang=de|journal=Zeitschrift für anorganische und allgemeine Chemie|volume=231|year=1937|issue=1–2 |pages=138–171|doi=10.1002/zaac.19372310115 |bibcode=1937ZAACh.231..138K |url=https://pbc.gda.pl/dlibra/publication/edition/36/content }}.</ref>

Like the other lanthanides, its most common [[oxidation state]] is +3, seen in its oxide, halides and other compounds. In [[aqueous solution]], like compounds of other late lanthanides, soluble thulium compounds form [[coordination complex]]es with nine water molecules.

== Properties ==

=== Physical properties ===
Pure thulium metal has a bright, silvery luster, which tarnishes on exposure to air. The metal can be cut with a knife,<ref name="history" /> as it has a [[Mohs hardness]] of 2 to 3; it is malleable and ductile.<ref name="CRC" /> Thulium is [[ferromagnetic]] below 32{{nbsp}}K, [[antiferromagnetic]] between 32 and 56{{nbsp}}K, and [[paramagnetic]] above 56{{nbsp}}K.<ref>{{cite journal |author= Jackson, M. |title= Magnetism of Rare Earth |url= http://www.irm.umn.edu/quarterly/irmq10-3.pdf |journal= The IRM Quarterly |volume= 10 |issue= 3 |page= 1 |date= 2000}}</ref>

Thulium has two major [[allotrope]]s: the [[tetragonal crystal system|tetragonal]] α-Tm and the more stable [[hexagonal crystal system|hexagonal]] β-Tm.<ref name="CRC" />

=== Chemical properties ===
Thulium tarnishes slowly in air and burns readily at 150{{nbsp}}[[Celsius|°C]] to form [[thulium(III) oxide]]:<ref name="InorgChem">
{{cite book
| title = Inorganic Chemistry, 3rd Edition
| chapter = Chapter 25: The ''f''-block metals: lanthanoids and actinoids
| author1 = Catherine E. Housecroft
| author2 = Alan G. Sharpe
| publisher = Pearson
| year = 2008
| isbn = 978-0-13-175553-6
| page = 864
}}</ref>
: {{chem2|4Tm + 3O2 → 2Tm2O3}}

Thulium [[dust]] can cause [[explosion]]s and [[fires]].<ref>{{cite web |author1=Thermo Fisher Scientific Chemicals, Inc. |title=SAFETY DATA SHEET |url=https://www.fishersci.com/store/msds?partNumber=AA4416803&productDescription=THULIUM+PWR+-40+MESH+99.9%25+1G&vendorId=VN00024248&countryCode=US&language=en |website=fisher scientific |access-date=1 June 2024 |location=Section: 5. Fire-fighting measures |date=28 March 2024}}</ref>

Thulium is quite [[electropositive]] and reacts slowly with cold water and quite quickly with hot water to form thulium hydroxide:
: {{chem2|2Tm_{(s)} + 6 H2O_{(l)} → 2Tm(OH)3_{(aq)} + 3H2_{(g)}|}}

Thulium reacts with all the [[halogen]]s. Reactions are slow at room temperature, but are vigorous above 200{{nbsp}}°C:
: {{chem2|2Tm_{(s)} + 3F2_{(g)} → 2TmF3_{(s)}|}} (white)
: {{chem2|2Tm_{(s)} + 3Cl2_{(g)} → 2TmCl3_{(s)}|}} (yellow)
: {{chem2|2Tm_{(s)} + 3Br2_{(g)} → 2TmBr3_{(s)}|}} (white)
: {{chem2|2Tm_{(s)} + 3I2_{(g)} → 2TmI3_{(s)}|}} (yellow)

Thulium dissolves readily in dilute [[sulfuric acid]] to form [[Solution (chemistry)|solution]]s containing the pale green Tm(III) ions, which exist as {{chem2|[Tm(OH2)9](3+)}} complexes:<ref>{{cite web |url= https://www.webelements.com/thulium/chemistry.html |title= Chemical reactions of Thulium |publisher=Webelements |access-date=2009-06-06}}</ref>
: {{chem2|2Tm_{(s)} + 3H2SO4_{(aq)} → 2Tm(3+)_{(aq)} + 3SO4(2-)_{(aq)} + 3H2_{(aq)}|}}

Thulium reacts with various metallic and non-metallic elements forming a range of binary compounds, including {{chem2|TmN}}, {{chem2|TmS}}, {{chem2|TmC2}}, {{chem2|Tm2C3}}, {{chem2|TmH2}}, {{chem2|TmH3}}, {{chem2|TmSi2}}, {{chem2|TmGe3}}, {{chem2|TmB4}}, {{chem2|TmB6}} and {{chem2|TmB12}}.{{citation needed|date=March 2014}} Like most lanthanides, the +3 state is most common and is the only state observed in thulium solutions.<ref name="patnaik">{{cite book |last= Patnaik |first= Pradyot |date= 2003 |title= Handbook of Inorganic Chemical Compounds |publisher= McGraw-Hill |page= 934 |isbn= 0-07-049439-8 |url= https://books.google.com/books?id=Xqj-TTzkvTEC&pg=PA934}}</ref> Thulium exists as a {{chem2|Tm(3+)}} ion in solution. In this state, the thulium ion is surrounded by nine molecules of water.<ref name="history" /> {{chem2|Tm(3+)}} ions exhibit a bright blue luminescence.<ref name="history" /> Because it occurs late in the [[lanthanide series|series]], the +2 oxidation state can also exist, stabilized by the nearly full 4f [[electron shell]], but occurs only in solids.{{citation needed|date=April 2022}}

Thulium's only known oxide is [[thulium oxide|{{chem2|Tm2O3}}]]. This oxide is sometimes called "thulia".<ref name= "hist and use">{{cite book |url= https://books.google.com/books?id=yb9xTj72vNAC&pg=PA300 |title= The History and Use of Our Earth's Chemical Elements: A Reference Guide |isbn= 978-0-313-33438-2 |author= Krebs, Robert E |date= 2006|publisher= Greenwood Publishing }}</ref> Reddish-purple thulium(II) compounds can be made by the [[chemical reduction|reduction]] of thulium(III) compounds. Examples of thulium(II) compounds include the halides (except the fluoride). Some hydrated thulium compounds, such as {{chem2|TmCl3*7H2O}} and {{chem2|Tm2(C2O4)3*6H2O}} are green or greenish-white.<ref name= "concise encyclopedia">{{cite book |url=https://books.google.com/books?id=Owuv-c9L_IMC&pg=PA1105 |title=Concise Encyclopedia Chemistry |isbn=978-3-11-011451-5 |last1=Eagleson |first1=Mary |date=1994|publisher=Walter de Gruyter|page=1105}}</ref> Thulium dichloride reacts very vigorously with [[water]]. This reaction results in [[hydrogen]] gas and [[thulium(III) hydroxide|{{chem2|Tm(OH)3}}]] exhibiting a fading reddish color.{{citation needed|date=March 2014}} Combination of thulium and [[chalcogen]]s results in thulium [[chalcogenide]]s.<ref>{{cite book |url=https://books.google.com/books?id=es-Pu2hI5swC |title=Advances in Inorganic Chemistry and Radiochemistry |isbn=978-0-08-057869-9 |last1=Emeléus |first1=H. J. |last2=Sharpe |first2=A. G. |date=1977|publisher=Academic Press}}</ref>

Thulium reacts with [[hydrogen chloride]] to produce hydrogen gas and thulium chloride. With [[nitric acid]] it yields thulium nitrate, {{chem2|Tm(NO3)3}}.<ref name="cool">{{Cite web|title=Thulium|url=http://www.chemicool.com/elements/thulium.html|access-date=2023-03-10|website=www.chemicool.com}}</ref>

=== Isotopes ===
{{Main|Isotopes of thulium}}

Natural thulium consists of the single stable isotope thulium-169 (it is predicted to undergo [[alpha decay]] to [[holmium]]-165 with a very long half-life.<ref name="history" /><ref name="bellidecay">{{cite journal |last1=Belli |first1=P. |last2=Bernabei |first2=R. |last3=Danevich |first3=F. A. |last4=Incicchitti |first4=A. |last5=Tretyak |first5=V. I. |display-authors=3 |title=Experimental searches for rare alpha and beta decays |journal=European Physical Journal A |date=2019 |volume=55 |issue=8 |pages=140–1–140–7 |doi=10.1140/epja/i2019-12823-2 |issn=1434-601X |arxiv=1908.11458|bibcode=2019EPJA...55..140B |s2cid=201664098 }}</ref>). Known isotopes of thulium range from {{chem2|^{144}Tm}} to {{chem2|^{183}Tm}}.{{NUBASE2020|ref}}<ref name=PRL132.7>{{cite journal |first1=O. B. |last1=Tarasov |first2=A. |last2=Gade |first3=K. |last3=Fukushima |display-authors=et al. |title=Observation of New Isotopes in the Fragmentation of <sup>198</sup>Pt at FRIB |journal=Physical Review Letters |volume=132 |number=72501 |date=2024 |article-number=072501 |doi=10.1103/PhysRevLett.132.072501|pmid=38427880 |bibcode=2024PhRvL.132g2501T |osti=2309727 }}</ref>

The primary [[decay mode]] before the stable isotope, {{chem2|^{169}Tm}}, is [[electron capture]] to [[Isotopes of erbium|erbium]] isotopes, and the primary mode after is [[beta emission]] to [[Isotopes of ytterbium|ytterbium]] isotopes. The longest-lived radioisotopes are thulium-171, which has a [[half-life]] of 1.92 years, and [[thulium-170]], which has a half-life of 128.6 days. Most other isotopes have half-lives under 10 minutes.{{NUBASE2020|ref}}

== History ==
[[File:Per Teodor Cleve c1885.jpg|thumb|upright|Per Teodor Cleve, the scientist who discovered thulium as well as [[holmium]].]]
Thulium was [[discovery of the chemical elements|discovered]] by Swedish chemist [[Per Teodor Cleve]] in 1879 by looking for impurities in the [[oxide]]s of other rare earth elements. This was the same method [[Carl Gustaf Mosander]] earlier used to discover some other rare earth elements.<ref>See:
* {{cite journal|last1=Cleve|first1=P. T.|title=Sur deux nouveaux éléments dans l'erbine|journal=Comptes rendus|date=1879|volume=89|pages=478–480|url=https://babel.hathitrust.org/cgi/pt?id=umn.31951d00008409h;view=1up;seq=484|trans-title=Two new elements in the oxide of erbium|language=fr}} Cleve named thulium on p. 480: "''Pour le radical de l'oxyde placé entre l'ytterbine et l'erbine, qui est caractérisé par la bande ''x'' dans la partie rouge du spectre, je propose la nom de ''thulium'', dérivé de Thulé, le plus ancien nom de la Scandinavie.''" (For the radical of the oxide located between the oxides of ytterbium and erbium, which is characterized by the ''x'' band in the red part of the spectrum, I propose the name of "thulium", [which is] derived from ''Thule'', the oldest name of Scandinavia.)
* {{cite journal|last1=Cleve|first1=P. T.|title=Sur l'erbine|journal=Comptes rendus|date=1879|volume=89|pages=708–709|url=https://babel.hathitrust.org/cgi/pt?id=umn.31951d00008409h;view=1up;seq=714|trans-title=On the oxide of erbium|language=fr}}
* {{cite journal|last1=Cleve|first1=P. T.|title=Sur le thulium|journal=Comptes rendus|date=1880|volume=91|pages=328–329|url=https://babel.hathitrust.org/cgi/pt?id=umn.31951d00008411u;view=1up;seq=332|trans-title=On thulium|language=fr}}</ref> Cleve started by removing all of the known contaminants of [[erbia]] ({{chem2|[[erbium|Er]]2[[oxygen|O]]3}}). Upon additional processing, he obtained two new substances; one brown and one green. The brown substance was the oxide of the element [[holmium]] and was named holmia by Cleve, and the green substance was the oxide of an unknown element. Cleve named the oxide [[thulia]] and its element thulium after [[Thule]], an [[Ancient Greek]] place name associated with Scandinavia or [[Iceland]]. Thulium's atomic symbol was initially Tu, but later{{when|date=April 2022}} changed to Tm.{{why|date=April 2022}}<ref name="history" /><ref>{{cite book |url=https://books.google.com/books?id=Owuv-c9L_IMC&pg=PA1061|page=1061 |title=Concise Encyclopedia Chemistry |isbn=978-3-11-011451-5 |last1=Eagleson |first1=Mary |date=1994|publisher=Walter de Gruyter}}</ref><ref name="Weeks">{{cite book |last1=Weeks |first1=Mary Elvira |title=The discovery of the elements |date=1956 |publisher=Journal of Chemical Education |location=Easton, PA |url=https://archive.org/details/discoveryoftheel002045mbp |edition=6th }}</ref><ref name="XVI">{{cite journal | author = Weeks, Mary Elvira |author-link=Mary Elvira Weeks| title = The discovery of the elements: XVI. The rare earth elements | journal = Journal of Chemical Education | year = 1932 | volume = 9 | issue = 10 | pages = 1751&ndash;1773 | doi = 10.1021/ed009p1751 | bibcode=1932JChEd...9.1751W}}</ref><ref name="Virginia">{{cite journal |last1=Marshall |first1=James L. Marshall |last2=Marshall |first2=Virginia R. Marshall |title=Rediscovery of the elements: The Rare Earths–The Confusing Years |journal=The Hexagon |date=2015 |pages=72–77 |url=http://www.chem.unt.edu/~jimm/REDISCOVERY%207-09-2018/Hexagon%20Articles/rare%20earths%20II.pdf |access-date=30 December 2019}}</ref><ref>{{Cite journal|last=Piguet|first=Claude|year=2014|title=Extricating erbium|journal=Nature Chemistry|volume=6|issue=4|page=370|doi=10.1038/nchem.1908|bibcode=2014NatCh...6..370P|pmid=24651207|doi-access=free}}</ref><ref name="RSThulium">{{cite web |title=Thulium |url=https://www.rsc.org/periodic-table/element/69/thulium |website=Royal Society of Chemistry|date= 2020 |access-date=4 January 2020}}</ref>

Thulium was so rare that none of the early workers had enough of it to purify sufficiently to actually see the green color; they had to be content with [[spectroscopy|spectroscopically]] observing the strengthening of the two characteristic absorption bands, as erbium was progressively removed. The first researcher to obtain nearly pure thulium was [[Charles James (chemist)|Charles James]], a British expatriate working on a large scale at [[University of New Hampshire|New Hampshire College]] in [[Durham, New Hampshire|Durham]], USA. In 1911 he reported his results, having used his discovered method of bromate fractional crystallization to do the purification. He famously needed 15,000 purification operations to establish that the material was homogeneous.<ref>{{cite journal|last=James|first=Charles|date=1911|title=Thulium I|journal=Journal of the American Chemical Society|volume=33|issue=8|pages=1332–1344|url=https://babel.hathitrust.org/cgi/pt?id=hvd.32044103069787;view=1up;seq=314|doi=10.1021/ja02221a007|bibcode=1911JAChS..33.1332J }}</ref>

High-purity thulium oxide was first offered commercially in the late 1950s, as a result of the adoption of [[ion exchange|ion-exchange]] separation technology. Lindsay Chemical Division of American Potash & Chemical Corporation offered it in grades of 99% and 99.9% purity. The price per kilogram oscillated between US$4,600 and $13,300 in the period from 1959 to 1998 for 99.9% purity, and it was the second highest for the lanthanides behind [[lutetium]].<ref>{{cite news |publisher= U.S. Geological Survey |title= Rare-Earth Metals |author= Hedrick, James B. |access-date= 2009-06-06 |url= https://minerals.usgs.gov/minerals/pubs/commodity/rare_earths/740798.pdf}}</ref><ref>{{cite news |title= Rare Earth Elements |author= Castor, Stephen B. |author2= Hedrick, James B. |name-list-style= amp |access-date= 2009-06-06 |url= http://www.rareelementresources.com/i/pdf/RareEarths-CastorHedrickIMAR7.pdf}}</ref>

== Occurrence ==
[[File:Monazit - Madagaskar.jpg|thumb|Thulium is found in the mineral monazite]]
The element is never found in nature in pure form, but it is found in small quantities in [[mineral]]s with other rare earths. Thulium is often found with minerals containing [[yttrium]] and [[gadolinium]]. In particular, thulium occurs in the mineral [[gadolinite]].<ref name= "handbook of metals">{{cite book|author=Walker, Perrin|author2=Tarn, William H.|name-list-style=amp |title=CRC Handbook of Metal Etchants|url=https://books.google.com/books?id=-2ObmTZTq2QC&pg=PA1241|date=2010|publisher=CRC Press|isbn=978-1-4398-2253-1|pages=1241–}}</ref> Like many other [[lanthanides]], thulium also occurs in the minerals [[monazite]], [[xenotime]], and [[euxenite]]. Thulium has not been found in prevalence over the other rare earths in any mineral yet.<ref>{{cite web |url=https://www.mindat.org/ |title=Mindat.org |author=Hudson Institute of Mineralogy |date=1993–2018 |website=www.mindat.org |access-date=14 January 2018}}</ref> Its [[Abundance of elements in Earth's crust |abundance in the Earth's crust]] is 0.5&nbsp;mg/kg by weight.<ref name=CRCAbundanceTable>ABUNDANCE OF ELEMENTS IN THE EARTH'S CRUST AND IN THE SEA, ''CRC Handbook of Chemistry and Physics'', 97th edition (2016–2017), p. 14–17</ref>

Thulium makes up approximately 0.5 parts per million of [[soil]], although this value can range from 0.4 to 0.8 parts per million. Thulium makes up 250 parts per quadrillion of [[seawater]].<ref name="history">{{cite book |pages=442–443 |url= https://books.google.com/books?id=Yhi5X7OwuGkC&pg=PA442 |title= Nature's building blocks: an A-Z guide to the elements |author= Emsley, John |publisher=Oxford University Press |location= US |date= 2001 |isbn= 0-19-850341-5}}</ref> In the [[Solar System]], thulium exists in concentrations of 200 parts per trillion by weight and 1&nbsp;part per trillion by moles.<ref name="cool" /> Thulium ore occurs most commonly in [[China]]. [[Australia]], [[Brazil]], [[Greenland]], [[India]], [[Tanzania]], and the [[United States]] also have large reserves of thulium. In 2001, the total world reserves of thulium were approximately 100,000 [[tonne]]s. Thulium is the least abundant [[lanthanide]] on Earth except for the radioactive [[promethium]].<ref name="history" />

== Production ==
Thulium is principally extracted from [[monazite]] ores (~0.007% thulium) found in river sands, through [[ion exchange]]. Newer ion-exchange and solvent-extraction techniques have led to easier separation of the rare earths, which has yielded much lower costs for thulium production. The principal sources today are the ion [[adsorption]] clays of southern China. In these, where about two-thirds of the total rare-earth content is yttrium, thulium is about 0.5% (or about tied with [[lutetium]] for rarity).<ref name="history" />

The metal can be isolated through [[redox|reduction]] of its oxide with [[lanthanum]] metal or by [[calcium]] reduction in a closed container. None of thulium's natural [[chemical compound|compounds]] are commercially important. In 2001, approximately 50 tonnes per year of thulium oxide were produced.<ref name="history" /> In 1996, thulium oxide cost US$20 per gram, and in 2005, 99%-pure thulium metal powder cost US$70 per gram.<ref name="CRC">{{cite book |author= Hammond, C. R. |chapter=The Elements|title= Handbook of Chemistry and Physics|edition= 81st |publisher= CRC press |date= 2000 |isbn= 0-8493-0481-4}}</ref>

== Applications ==

=== Lasers ===
[[Holmium]]-[[chromium]]-thulium triple-doped [[yttrium aluminium garnet]] ({{chem2|Ho:Cr:Tm:YAG}}, or {{chem2|Ho,Cr,Tm:YAG}}) is an active laser medium material with high efficiency. It lases at 2080&nbsp;nm in the infrared and is widely used in military applications, medicine, and meteorology. Single-element thulium-doped YAG (Tm:YAG) lasers operate at 2010&nbsp;nm.<ref>{{cite book |page=49|url= https://books.google.com/books?id=8yM4yF_B72QC&pg=PA49 |title= Solid-state laser engineering |author= Koechner, Walter |publisher= Springer |date= 2006 |isbn= 0-387-29094-X}}</ref> The wavelength of thulium-based lasers is very efficient for superficial ablation of tissue, with minimal coagulation depth in air or in water. This makes thulium lasers attractive for laser-based surgery.<ref>{{cite book |page=214 |url=https://books.google.com/books?id=FCDPZ7e0PEgC&pg=PA214 |title= Tunable laser applications |author= Duarte, Frank J. |publisher= CRC Press |date= 2008 |isbn=978-1-4200-6009-6 |author-link= F. J. Duarte}}</ref>

=== X-ray source ===
Despite its high cost, portable X-ray devices use thulium that has been bombarded with neutrons in a [[nuclear reactor]] to produce the isotope thulium-170, having a half-life of 128.6 days and five major emission lines of comparable intensity (at 7.4, 51.354, 52.389, 59.4 and 84.253&nbsp;keV). These [[radioactive source]]s have a useful life of about one year, as tools in medical and dental diagnosis, as well as to detect defects in inaccessible mechanical and electronic components. Such sources do not need extensive radiation protection{{snd}}only a small cup of lead.<ref name="appl">{{cite book |page= 32 |url= https://books.google.com/books?id=F0Bte_XhzoAC&pg=PA32 |title= Extractive metallurgy of rare earths |author= Gupta, C. K. |author2= Krishnamurthy, Nagaiyar |name-list-style= amp |publisher= CRC Press |date= 2004 |isbn= 0-415-33340-7}}</ref>
They are among the most popular radiation sources for use in [[industrial radiography]].<ref>{{cite book |url=https://books.google.com/books?id=lOCjakwiRWAC&pg=PA55 |title=Practical Radiography |isbn=978-1-84265-188-9 |last1=Raj |first1=Baldev |last2=Venkataraman |first2=Balu |date=2004|publisher=Alpha Science Int'l }}</ref>
Thulium-170 is gaining popularity as an X-ray source for cancer treatment via [[brachytherapy]] (sealed source radiation therapy).<ref>
{{cite journal
|last= Krishnamurthy
|first= Devan
|author2= Vivian Weinberg
|author3= J. Adam M. Cunha
|author4= I-Chow Hsu
|author5= Jean Pouliot
|title= Comparison of high–dose rate prostate brachytherapy dose distributions with iridium-192, ytterbium-169, and thulium-170 sources
|journal= Brachytherapy
|volume= 10
|issue= 6
|pages= 461–465
|date= 2011
|doi= 10.1016/j.brachy.2011.01.012
|pmid= 21397569}}</ref><ref>{{cite book |last1=Ayoub |first1=Amal |chapter=Development of New Radioactive Seeds Tm-170 for Brachytherapy |date=2009 |chapter-url=http://link.springer.com/10.1007/978-3-642-03474-9_1 |title=World Congress on Medical Physics and Biomedical Engineering, September 7 - 12, 2009, Munich, Germany |volume=25/1 |pages=1–4 |editor-last=Dössel |editor-first=Olaf |access-date=2023-04-01 |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |doi=10.1007/978-3-642-03474-9_1 |isbn=978-3-642-03472-5 |last2=Shani |first2=Gad |series=IFMBE Proceedings |editor2-last=Schlegel |editor2-first=Wolfgang C.}}</ref>

=== Others ===
Thulium has been used in [[high-temperature superconductor]]s similarly to [[yttrium]]. Thulium potentially has use in [[ferrite (magnet)|ferrite]]s, ceramic magnetic materials that are used in [[microwave]] equipment.<ref name="appl" /> Thulium is also similar to [[scandium]] in that it is used in arc lighting for its unusual spectrum, in this case, its green emission lines, which are not covered by other elements.<ref>{{cite book |page=[https://archive.org/details/elementsvisualex0000gray/page/159 159] |url=https://archive.org/details/elementsvisualex0000gray/page/159 |title=The Elements: A Visual Exploration of Every Known Atom In The Universe |author=Gray, Theodore W. |author2=Mann, Nick |name-list-style=amp |publisher=Black Dog & Leventhal Publishers |date=2009 |isbn=978-1-57912-814-2 }}</ref> Because thulium [[fluoresce]]s with a blue color when exposed to [[ultraviolet|ultraviolet light]], thulium is put into [[euro]] [[banknotes]] as a measure against [[counterfeit]]ing.<ref name="Photochemistry">{{cite book |url=https://books.google.com/books?id=XAjIWgENf5UC&pg=PA75 |page=75 |title=Principles and Applications of Photochemistry |isbn=978-0-470-71013-5 |last1=Wardle |first1=Brian |date=2009-11-06|publisher=John Wiley & Sons}}</ref>

The blue fluorescence of Tm-doped calcium sulfate has been used in personal dosimeters for visual monitoring of radiation.<ref name="history" /> Tm-doped halides in which Tm is in its 2+ oxidation state are luminescent materials that are proposed for electric power generating windows based on the principle of a [[luminescent solar concentrator]].<ref name="Richards 2023">{{cite journal | last1=Richards | first1=Bryce S. | last2=Howard | first2=Ian A. | title=Luminescent solar concentrators for building integrated photovoltaics: opportunities and challenges | journal=Energy & Environmental Science | volume=16 | issue=8 | date=2023 | issn=1754-5692 | doi=10.1039/D3EE00331K | pages=3214–3239| doi-access=free | bibcode=2023EnEnS..16.3214R }}</ref>

== Biological role ==
Soluble thulium salts are mildly [[toxicity|toxic]], but insoluble thulium salts are completely [[nontoxic]].<ref name="history" /> When injected, thulium can cause degeneration of the [[liver]] and [[spleen]] and can also cause [[hemoglobin]] concentration to fluctuate. Liver damage from thulium is more prevalent in male [[Mouse|mice]] than female mice. Despite this, thulium has a low level of toxicity.<ref>{{Cite book |last=Ayres |first=D. C. |title=Dictionary of environmentally important chemicals |publisher=CRC Press |others=Desmond Hellier |date=15 February 2022 |isbn=978-1-315-14115-2 |edition=1st |location=United States |page=299 |language=English |oclc=1301431003}}</ref><ref>{{Cite book |last=Jha |first=A. R. |title=Rare Earth Materials: Properties and Applications. |date=2014 |publisher=CRC Press |isbn=978-1-4665-6403-9 |location=Boca Raton |page=63 |oclc=880825396}}</ref><!--The following refs don't exactly support the statement here, which needs to be rewritten slightly: <ref>{{cite journal |pmid=1141999 |year=1975 |last1=Hutcheson |first1=D. P. |last2=Gray |first2=D. H. |last3=Venugopal |first3=B. |last4=Luckey |first4=T. D. |title=Studies of nutritional safety of some heavy metals in mice |volume=105 |issue=6 |pages=670–5 |journal=The Journal of Nutrition|doi=10.1093/jn/105.6.670}}</ref><ref>{{cite journal |doi=10.1016/0041-008X(63)90014-0 |title=Pharmacology and toxicology of terbium, thulium, and ytterbium chlorides |year=1963 |last1=Haley |first1=Thomas J. |last2=Komesu |first2=N. |last3=Flesher |first3=A. M. |last4=Mavis |first4=L. |last5=Cawthorne |first5=J. |last6=Upham |first6=H. C. |journal=Toxicology and Applied Pharmacology |volume=5 |issue=4 |pages=427–436}}</ref><ref>{{cite journal |doi=10.1002/jps.2600540502 |title=Pharmacology and toxicology of the rare earth elements |year=1965 |last1=Haley |first1=Thomas J. |journal=Journal of Pharmaceutical Sciences |volume=54 |issue=5 |pages=663–70 |pmid=5321124}}</ref><ref>{{cite journal |doi=10.1016/0041-008X(63)90067-X |title=The acute mammalian toxicity of rare earth nitrates and oxides |year=1963 |last1=Bruce |first1=David W. |last2=Hietbrink |first2=Bernard E. |last3=Dubois |first3=Kenneth P. |journal=Toxicology and Applied Pharmacology |volume=5 |issue=6 |pages=750–759 |pmid=14082480 |bibcode=1963ToxAP...5..750B}}</ref>-->

In humans, thulium occurs in the highest amounts in the [[liver]], [[kidney]]s, and [[bone]]s. Humans typically consume several micrograms of thulium per year. The roots of [[plant]]s do not take up thulium, and the [[dry matter]] of vegetables usually contains one [[parts per billion|part per billion]] of thulium.<ref name="history" /> Thulium metal has low to moderate toxicity.<ref>{{cite web |author1=<!--not stated--> |title=Thulium |url=https://periodic.lanl.gov/69.shtml |website=Los Alamos National Laboratory |publisher=Triad National Security, LLC |access-date=1 June 2024}}</ref>
== See also ==
* {{Category link|Thulium compounds}}

== References ==
{{reflist|35em}}

== External links ==
{{Commons|Thulium}}
{{Wiktionary|thulium}}
* {{cite book|author1-link=Charles P. Poole|author=Poole, Charles P. Jr.|title=Encyclopedic Dictionary of Condensed Matter Physics|url=https://books.google.com/books?id=CXwrqM2hU0EC&pg=PA1395|date=2004|publisher=Academic Press|isbn=978-0-08-054523-3|page=1395}}

{{Thulium compounds}}
{{Periodic table (navbox)}}

{{Authority control}}
[[Category:Thulium| ]]
[[Category:Chemical elements]]
[[Category:Chemical elements with hexagonal close-packed structure]]
[[Category:Lanthanides]]
[[Category:Metals]]
[[Category:Reducing agents]]