Insulating fcc YH3-δ stabilized by MgH2 -: art. no. 235116

We study the structural, optical, and electrical properties of MgzY1-z switchable mirrors upon hydrogenation. It is found that the alloys disproportionate into essentially pure YH3-delta and MgH2 with the crystal structure of YH3-delta dependent on the Mg concentration z. For 0<0.1, both hcp and fee 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 compared 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 inconsistent with calculations predicting fee YH3-delta to be metallic, but they are in good agreement with recent GW calculations on both hcp and fee YH3. 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 confinement effects in the small YH3 clusters.