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 Cpa. At high pressure, the optical gap decreases linearly extrapolating to zero for 55+/-8 Cpa, above which pressure a metallic state is expected. This is in disagreement with a theoretically predicted insulator-to-metal p hase transition at 1.5 GPa. In-situ structural studies using synchrotron ra diation reveal a 2% drop in the c-lattice vector at 4 Cpa, while both above and below the phase transition, the spectra are consistent with a hcp stru cture. Hence this phase transition is probably due to a rearrangement of th e hydrogen lattice positions only and the predicted hcp to fee transition i s not observed. (C) 2000 Elsevier Science S.A. All rights reserved.