yttrium hydride

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|Section1 = {{Chembox Identifiers

| index1_label = trihydride

| PubChem1 = 166870

| ChemSpiderID1 = 146001

| CASNo = 13598-57-7

| EINECS = 237-074-0

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Yttrium hydride is a compound of hydrogen and yttrium. It is considered to be a part of the class of rare-earth metal hydrides. It exists in several forms, the most common being a metallic compound with formula YH₂. YH₂ has a face-centred cubic structure, and is a metallic compound. Under great pressure, extra hydrogen can combine to yield an insulator with a hexagonal structure, with a formula close to YH₃.{{cite journal|last1=Kume|first1=Tetsuji|last2=Ohura|first2=Hiroyuki|last3=Takeichi|first3=Tomoo|last4=Ohmura|first4=Ayako|last5=Machida|first5=Akihiko|last6=Watanuki|first6=Tetsu|last7=Aoki|first7=Katsutoshi|last8=Sasaki|first8=Shigeo|last9=Shimizu|first9=Hiroyasu|last10=Takemura|first10=Kenichi|title=High-pressure study of ScH3: Raman, infrared, and visible absorption spectroscopy|journal=Physical Review B|date=31 August 2011|volume=84|issue=6|pages=064132|doi=10.1103/PhysRevB.84.064132|bibcode=2011PhRvB..84f4132K}} Hexagonal YH₃ has a band gap of 2.6 eV. Under pressure of 12 GPa YH₃ transforms to an intermediate state, and when the pressure increases to 22 GPa another metallic face-centred cubic phase is formed.{{cite web|last1=Machida|first1=Akihiko|title=Unique Structures in Yttrium Trihydride at High Pressure|url=http://www.spring8.or.jp/pdf/en/res_fro/07/058-059.pdf|publisher=SPring 8|access-date=1 December 2015|pages=58–59|work=Research Frontiers|date=2007}}

In 1996, it was shown that the metal-insulator transition when going from YH₂ to YH₃ can be used to change the optical state of windows from non-transparent to transparent.{{cite journal|last1=Huiberts|first1=J. N.|last2=Griessen|first2=R.|last3=Rector|first3=J. H.|last4=Wijngaarden|first4=R. J.|last5=Dekker|first5=J. P.|last6=de Groot|first6=Koeman|last7=N J|title=Yttrium and lanthanum hydride films with switchable optical properties|journal=Nature|date=1996|volume=380|issue=6571|page=231|doi=10.1038/380231a0|bibcode=1996Natur.380..231H|s2cid=4228469}} This report spurred a wave of research on metal hydride-based chromogenic materials and smart windows; gasochromic windows reacting to hydrogen gas and electrochromic structures where the transparency can be regulated by applying an external voltage.{{cite journal|last1=van der Sluis|first1=P.|last2=Mercier|first2=V. M. M.|title=Solid state Gd-Mg electrochromic devices with ZrO₂H_x electrolyte|journal=Electrochimica Acta|date=2001|volume=46|issue=13–14|page=2167|doi=10.1016/S0013-4686(01)00375-9}} When containing a substantial amount of oxygen, yttrium hydride is also found to exhibit reversible photochromic properties.{{cite journal|last1=Mongstad|first1=T|last2=Plazer-Björkman|first2=C.|last3=Maehlen|first3=J. P.|last4=Mooij|first4=L.|last5=Pivak|first5=Y.|last6=Dam|first6=B.|last7=Marstein|first7=E.|last8=Hauback|first8=B.|last9=Karazhanov|first9=S. Zh.|title=A new thin film photochromic material: Oxygen-containing yttrium hydride|journal=Solar Energy Materials and Solar Cells|date=2011|volume=95|issue=12|page=3596|doi=10.1016/j.solmat.2011.08.018|arxiv=1109.2872|bibcode=2011SEMSC..95.3596M|s2cid=55961818}} This switchable optical property enables their utilization in many technological applications, such as sensors, goggles, and medical devices in addition to the smart windows. According to a research results, the strength of the photochromic response is found to decrease with increasing oxygen concentration in the film accompanied by an optical band gap widening. {{Cite journal|last1=Moldarev|first1=Dmitrii|last2=Moro|first2=Marcos V.|last3=You|first3=Chang C.|last4=Baba|first4=Elbruz M.|last5=Karazhanov|first5=Smagul Zh.|last6=Wolff|first6=Max|last7=Primetzhofer|first7=Daniel|date=2018-11-26|title=Yttrium oxyhydrides for photochromic applications: Correlating composition and optical response|url=https://link.aps.org/doi/10.1103/PhysRevMaterials.2.115203|journal=Physical Review Materials|volume=2|issue=11|pages=115203|doi=10.1103/PhysRevMaterials.2.115203|bibcode=2018PhRvM...2k5203M|s2cid=139290764 }}

Yttrium hydride is being looked at as a high temperature superconductor.{{Cite web|date=2021-03-12|title=Scientists Synthesize New High-Temperature Superconductor|url=https://interestingengineering.com/scientists-synthesize-new-high-temperature-superconductor|access-date=2021-06-29|website=interestingengineering.com|language=en-US}}

Yttrium hydride is being looked at as a neutron moderator{{Cite web|title=ORNL developing 3D-printed nuclear microreactor : New Nuclear - World Nuclear News|url=https://www.world-nuclear-news.org/Articles/ORNL-developing-3D-printed-nuclear-microreactor|access-date=2021-06-29|website=www.world-nuclear-news.org}} for use in new nuclear reactor designs.