Yttrium hydride is a compound of

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

and

yttrium Yttrium is a chemical element; it has Symbol (chemistry), 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 a ...

. 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 rare earths, and sometimes the lanthanides or lanthanoids (although scandium and yttrium, which do not belong to this series, are usually included as rare earths), are a set of ...

metal hydrides In chemistry, a hydride is formally the anion of hydrogen (H−), a hydrogen ion with two electrons. In modern usage, this is typically only used for ionic bonds, but it is sometimes (and has been more frequently in the past) applied to all co ...

. 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 o ...

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 and solid-state chemistry, a band gap, also called a bandgap or energy gap, is an energy range in a solid where no electronic states exist. In graphs of the electronic band structure of solids, the band gap refers to t ...

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 () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...

- 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 A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. These properties are all associated with having electrons available at the Fermi level, as against non ...

-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" (a chromophore). There is no universally agreed definiti ...

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 change of color upon exposure to light. It is a transformation of a chemical species (photoswitch) between two forms through the absorption of electromagnetic radiation (photoisomerization), where each form has a d ...

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 In solid-state physics and solid-state chemistry, a band gap, also called a bandgap or energy gap, is an energy range in a solid where no electronic states exist. In graphs of the electronic band structure of solids, the band gap refers to the ...

widening. Yttrium hydride is being looked at as a high temperature superconductor. Yttrium hydride is being looked at as a

neutron moderator In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, ideally without capturing any, leaving them as thermal neutrons with only minimal (thermal) kinetic energy. These thermal neutrons are immensely ...

for use in new nuclear reactor designs.

References

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