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. 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.
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. 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. When containing a substantial amount of oxygen, yttrium hydride is also found to exhibit photochromic properties.
- Kume, Tetsuji; Ohura, Hiroyuki; Takeichi, Tomoo; Ohmura, Ayako; Machida, Akihiko; Watanuki, Tetsu; Aoki, Katsutoshi; Sasaki, Shigeo; Shimizu, Hiroyasu; Takemura, Kenichi (31 August 2011). "High-pressure study of ScH3: Raman, infrared, and visible absorption spectroscopy". Physical Review B. 84 (6). doi:10.1103/PhysRevB.84.064132.
- Machida, Akihiko (2007). "Unique Structures in Yttrium Trihydride at High Pressure" (PDF). Research Frontiers. SPring 8. pp. 58–59. Retrieved 1 December 2015.
- Huiberts, J N; Griessen, R; Rector, J H; Wijngaarden, R J; Dekker, J P; de Groot, Koeman; N J (1996). "Yttrium and lanthanum hydride films with switchable optical properties". Nature. 380: 231. doi:10.1038/380231a0.
- van der Sluis, P; Mercier, V M M (2001). "Solid state Gd-Mg electrochromic devices with ZrO2Hx electrolyte". Electrochimica Acta. 46: 2167. doi:10.1016/S0013-4686(01)00375-9.
- Mongstad, T; Plazer-Björkman, C; Maehlen, J P; Mooij, L; Pivak, Y; Dam, B; Marstein, E; Hauback, B; Karazhanov, S Zh (2011). "A new thin film photochromic material: Oxygen-containing yttrium hydride". Solar Energy Materials and Solar Cells. 95: 3596. doi:10.1016/j.solmat.2011.08.018.