High-pressure structure and electronic properties of YbD 2 to 34 GPa

International audienceYtterbium dihydride (YbH2) shows a well-known transition at ≈ 16 GPa from a CaH2-type structure to a high-pressure (high-P) phase with Yb at hcp sites and unknown H-positions. Here, we report its complete structure determination by neutron diffraction at 34 GPa. Hydrogen (deuterium) is located at 2a and 2d positions of space group P 63/mmc, thus forming a high-symmetry "collapsed" close-packed lattice. The transition is sluggish and can be seen as a transfer of 1/2 of the hydrogen atoms from strongly corrugated H-layers to interstitial sites of the Yb-lattice. We demonstrate by first-principles calculations that the transition is related to a change from a completely filled f-electron configuration to a fractional f-hole (≈ 0.25 h) occupation in the high-P phase. The f →d charge transfer closes the gap at the transition and leads to a metallic ground state with sizeable electron-phonon interaction involving out-of-plane vibrational modes of interstitial hydrogen