High-pressure structure and electronic properties ofYbD2to 34 GPa

Ytterbium dihydride (${\mathrm{YbH}}_{2}$) shows a well-known transition at $\ensuremath{\approx}16\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ from a ${\mathrm{CaH}}_{2}$-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 $P{6}_{3}/mmc$, 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 ($\ensuremath{\approx}0.25\phantom{\rule{0.16em}{0ex}}\mathrm{h}$) occupation in the high-$P$ phase. The $f\ensuremath{\rightarrow}d$ charge transfer closes the gap at the transition and leads to a metallic ground state with a sizable electron-phonon interaction involving out-of-plane vibrational modes of interstitial hydrogen.