Surface sensitivity of Mg2NiH4 leading to a profound color change

Verysmallamounts of air have been observedtooxidise the surface of the ternaryhydrideMg2NiH4.This leads to a profound color change fromorange to brownand to a sealing of the surfacewith respect to desorption, which can be mistaken for a stability increase of the bulk hydride. An unoxidised sample starts to desorb significant amounts of hydrogen when it is heated above 450 K, as is expected fromthe van’t Hoff relation for the hydride. A sample with a slightly oxidised surface may need to be heated above 700 K before hydrogen is released. The subsequent hydrogen desorption is, on the other hand, very rapid at this elevated temperature. # 2004 Elsevier B.V. All rights reserved