Ytterbium dihydride (YbH$_2$) shows a well-known transition at 16 GPa from a CaH$_2$-type structure to a high-pressure (high-P) phase with Yb at hcp sites and H(D)-positions, determined for the first time in this work by neutron diffraction up to 34 GPa. It is the highest pressure at which a structure was ever solved by neutron diffraction. This gives us the chance to compare the accuracy of the most used theories in the geometry prediction of hydrides. We demonstrate by first-principles DFT 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.
|Choix de session parallèle||1.2 La supraconductivité par couplage électron-phonon dans les composé à éléments légers: vers la température ambiante?|