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High-pressure structure and electronic properties of YbD$$_{2}$$ to 34 GPa

Klotz, S.*; Casula, M.*; Komatsu, Kazuki*; Machida, Shinichi*; Hattori, Takanori

Ytterbium dihydride (YbH$$_{2}$$) shows a well-known transition at $$approx$$16 GPa from a 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 2$$a$$ and 2$$d$$ positions of space group $$P6_{3}/mmc$$, 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 ($$approx$$0.25 h) occupation in the high-$$P$$ phase. The $$f$$ $$to$$ $$d$$ 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.

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