Insulating fcc YH

We study the structural, optical, and electrical properties of Mg{sub z}Y{sub 1{minus}z} switchable mirrors upon hydrogenation. It is found that the alloys disproportionate into essentially pure YH{sub 3{minus}{delta}} and MgH{sub 2} with the crystal structure of YH{sub 3{minus}{delta}} dependent on the Mg concentration z. For 0{lt}z{lt}0.1, both hcp and fcc YH{sub 3{minus}{delta}} are observed, whereas for z{ge}0.1 only cubic YH{sub 3{minus}{delta}} is present. Interestingly, cubic YH{sub 3{minus}{delta}} is expanded compared to YH{sub 2}, in disagreement with theoretical predictions. From optical and electrical measurements we conclude that cubic YH{sub 3{minus}{delta}} is a transparent insulator with properties similar to hexagonal YH{sub 3{minus}{delta}}. Our results are inconsistent with calculations predicting fcc YH{sub 3{minus}{delta}} to be metallic, but they are in good agreement with recent GW calculations on both hcp and fcc YH{sub 3}. Finally, we find an increase in the effective band gap of the hydrided Mg{sub z}Y{sub 1{minus}z} alloys with increasing z. Possibly this is due to quantum confinement effects in the small YH{sub 3} clusters.