Synthesis, Structure, and Electric Conductivity of Higher Hydrides of Ytterbium at High Pressure

While most of the rare-earth metals readily form trihydrides, due to increased stability of the filled 4f electronic shell for Yb(II), only YbH_2.67, formally corresponding to Yb^II(Yb^IIIH₄)₂ (or Yb₃H₈), remains the highest hydride of ytterbium. Utilizing the diamond anvil cell methodology and synchrotron powder X-ray diffraction, we have attempted to push this limit further via hydrogenation of metallic Yb and Yb₃H₈. Compression of the latter has also been investigated in a neutral pressure-transmitting medium (PTM). While the in situ heating of Yb facilitates the formation of YbH₂₊ hydrides, we have not observed clear qualitative differences between the systems compressed in H₂ and He or Ne PTM. In all of these cases, a sequence of phase transitions occurred within ca. 13-18 GPa (P3̅1m-I4/m phase) and around 27 GPa (to the I4/mmm phase). The molecular volume of the systems compressed in H₂ PTM is ca. 1.5% larger than of those compressed in inert gases, suggesting a small hydrogen uptake. Nevertheless, hydrogenation toward YbH₃ is incomplete, and polyhydrides do not form up to the highest pressure studied here (ca. 75 GPa). As pointed out by electronic transport measurements, the mixed-valence Yb₃H₈ retains its semiconducting character up to >50 GPa, although the very low remnant activation energy of conduction (<5 meV) suggests that metallization under further compression should be achievable. Finally, we provide a theoretical description of a hypothetical stoichiometric YbH₃.