Transition-Metal Hydride Electrochromics
A new type of electrochromic hydride material has interesting and unusual properties. Thin Ni-Mg films, for example, are mirror-like in appearance and have very low visible transmittance. On exposure to hydrogen gas or on reduction in alkaline electrolyte, the films become transparent. The transition is believed to result from formation of nickel magnesium hydride, Mg2NiH4.

Switchable mirrors based on rare earth hydrides were discovered in 1996 at Vrije University in the Netherlands,  Rare earth-magnesium alloy films were subsequently found to be superior to the pure lanthanides in maximum transparency and mirror-state reflectivity by Philips Laboratories. The newer transition-metal types which use less expensive and less reactive materials were discovered at LBNL. This has now become a very active area of study with a network of researchers.

Unlike conventional oxide electrochromics, hydrides become reflective rather than absorbing. The dynamic range  is typically 10 times greater than for absorbing materials. This has potential advantages in terms of energy performance, temperature stability, glare control and privacy.  The films were deposited using cosputtering from separate targets of Ni and Mg to facilitate variations in composition. Single-target d.c. magnetron sputtering could be used eventually which would be relatively simple compared to deposition of electrochromic oxides .