Pressure-induced large valence transitions in Yb-Cu binary intermetallic systems

Electronic and crystal structures of $\mathrm{YbCu}, {\mathrm{YbCu}}₂$, and ${\mathrm{YbCu}}_6.5$ under pressure were studied by high-resolution x-ray absorption spectroscopy (XAS) and x-ray diffraction. We found pressure-induced large valence transitions on the order of 0.3 in these Yb compounds. YbCu exhibited a first-order transition with a sudden change in the Yb valence at 4 GPa, at which a structural transition was not accompanied. With further increasing pressure, YbCu shows a structural transition from the FeB-type to the cubic CsCl-type structure around 10 GPa, where the Yb valence increases only gradually. In $\text{Yb}{\text{Cu}}₂$, a structural transition was not observed up to 11.8 GPa, while the Yb valence shows a rapid increase around 4--7 GPa, indicating a significant change in the electronic structure. Anomalous reentrant valence transition to a lower valence state was found at low pressures of 3--4 GPa in $\text{Yb}{\text{Cu}}_6.5$ without the structural phase transition. It is suggested that the anomaly in $\text{Yb}{\text{Cu}}_6.5$ is possibly correlated to the electronic structure of the Cu $d$ band. On the other hand, we found that these anomalous large changes in the Yb valences were linked to anomalies in crystal structures although there were no structural phase transitions. Complementary to the measurements of XAS at the Yb ${L}₃$ absorption edge, we also performed XAS at the Cu-${L}₃$ absorption edge and measured the valence band spectra at ambient pressure.