Coexistence of metallic and insulating channels in compressed YbB6

It remains controversial whether compressed ${\mathrm{YbB}}₆$ material is a topological insulator or a Kondo topological insulator. We performed high-pressure transport, x-ray diffraction (XRD), x-ray absorption spectroscopy, and Raman-scattering measurements on ${\mathrm{YbB}}₆$ samples in search for its topological Kondo phase. Both high-pressure powder XRD and Raman measurements show no trace of structural phase transitions in ${\mathrm{YbB}}₆$ up to 50 GPa. The nonmagnetic ${\mathrm{Yb}}²⁺$ gradually change to magnetic ${\mathrm{Yb}}³⁺$ above 18 GPa concomitantly with the increase in resistivity. However, the transition to the insulating state occurs only around 30 GPa, accompanied by the increase in the shear stress, and anomalies in the pressure dependence of the Raman ${\mathrm{T}}_2g$ mode and in the B atomic position. The resistivity at high pressures can be described by a model taking into account coexisting insulating and metallic channels with the activation energy for the insulating channel about 30 meV. We argue that ${\mathrm{YbB}}₆$ may become a topological Kondo insulator at high pressures above 35 GPa.