Insulating fcc YH_3-δ stabilized by MgH₂

We study the structural, optical, and electrical properties of Mg_zY_1-z switchable mirrors upon hydrogenation. It is found that the alloys disproportionate into essentially pure YH_3-δ and MgH₂ with the crystal structure of YH_3-δ dependent on the Mg concentration z. For 0<z<0.1, both hcp and fcc YH_3-δ are observed, whereas for z>~0.1 only cubic YH_3-δ is present. Interestingly, cubic YH_3-δ is expanded compared to YH₂, in disagreement with theoretical predictions. From optical and electrical measurements we conclude that cubic YH_3-δ is a transparent insulator with properties similar to hexagonal YH_3-δ. Our results are inconsistent with calculations predicting fcc YH_3-δ to be metallic, but they are in good agreement with recent GW calculations on both hcp and fcc YH₃. Finally, we find an increase in the effective band gap of the hydrided Mg_zY_1-z alloys with increasing z. Possibly this is due to quantum confinement effects in the small YH₃ clusters.