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 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+x 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 (P3??1m-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.

Automated summary

Researchers attempted to investigate hydrogenation of ytterbium using a diamond anvil cell and synchrotron powder X-ray diffraction. The results showed that although YbH2+x hydrides could form, there were no clear differences in the hydrogenation effects between systems compressed in H2 and inert gases. Electronic transport measurements revealed that Yb3H8 maintained its semiconducting character under high pressure.