Insulating fcc YH3-delta stabilized by MgH2 - art. no. 235116

We study the structural, optical, and electrical properties of MgzY1-z swit chable mirrors upon hydrogenation. It is found that the alloys disproportio nate into essentially pure YH3-delta and MgH2 with the crystal structure of YH3-delta dependent on the Mg concentration z. For 0<z<0.1, both hcp and f ee YH3-delta are observed, whereas for z greater than or equal to 0.1 only cubic YH3-delta is present. Interestingly, cubic YH3-delta is expanded comp ared to YH2, in disagreement with theoretical predictions. From optical and electrical measurements we conclude that cubic YH3-delta is a transparent insulator with properties similar to hexagonal YH3-delta Our results are in consistent with calculations predicting fee YH3-delta to be metallic, but t hey are in good agreement with recent GW calculations on both hcp and fee Y H3. Finally, we find an increase in the effective band gap of the hydrided MgzY1-z alloys with increasing z. Possibly this is due to quantum confineme nt effects in the small YH3 clusters.