Towards a metallic YH3 phase at high pressure

The switchable mirror compound YH3 has a high-pressure behavior at variance with current band-structure models. It remains transparent at least up to 25 GPa. At high pressure, the optical gap decreases linearly extrapolating to zero for 55±8 GPa, above which pressure a metallic state is expected. This is in disagreement with a theoretically predicted insulator-tometal phase transition at 1.5 GPa. In-situ structural studies using synchrotron radiation reveal a 2% drop in the c-lattice vector at 4 GPa, while both above and below the phase transition, the spectra are consistent with a hcp structure. Hence this phase transition is probably due to a rearrangement of the hydrogen lattice positions only and the predicted hcp to fcc transition is not observed.