Yttrium hydride is a compound of

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

and

yttrium Yttrium is a chemical element with the symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and has often been classified as a " rare-earth element". Yttrium is almost always found in co ...

. It is considered to be a part of the class of

rare-earth The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...

metal hydrides In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride of ...

. It exists in several forms, the most common being a metallic compound with formula YH₂. YH₂ has a

face-centred cubic In crystallography, the cubic (or isometric) crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals. There are three main varieties of ...

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. Hexagonal YH₃ has a

band gap In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference ( ...

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

phase is formed. In 1996, it was shown that the

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

- insulator transition when going from YH₂ to YH₃ can be used to change the optical state of windows from non-transparent to transparent. This report spurred a wave of research on

metal hydride In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride of ...

-based

chromogenic In chemistry, the term chromogen refers to a colourless (or faintly coloured) chemical compound that can be converted by chemical reaction into a compound which can be described as "coloured". There is no universally agreed definition of the term. ...

materials and smart windows; gasochromic windows reacting to hydrogen gas and

electrochromic Electrochromism is a phenomenon in which a material displays changes in color or opacity in response to an electrical stimulus. In this way, a smart window made of an electrochromic material can block specific wavelengths of ultraviolet, vis ...

structures where the transparency can be regulated by applying an external voltage. When containing a substantial amount of oxygen, yttrium hydride is also found to exhibit reversible

photochromic Photochromism is the reversible transformation of a chemical species ( photoswitch) between two forms by the absorption of electromagnetic radiation ( photoisomerization), where the two forms have different absorption spectra. In plain language, ...

properties. 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. Yttrium hydride is being looked at as a high temperature superconductor. Yttrium hydride is being looked at as a neutron moderator for use in new nuclear reactor designs.

References

Yttrium compounds Metal hydrides High-temperature superconductors {{inorganic-chemistry-stub