Under great pressure, extra hydrogen can combine to yield an insulator with a hexagonal structure, with a formula close to YH3.

[2] 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.

[4] When containing a substantial amount of oxygen, yttrium hydride is also found to exhibit reversible photochromic properties.

[5] 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.