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.

Figure 1

(a)–(c) Pressure dependence of the PFY-XAS spectra for YbCu, $\text{Yb}{\text{Cu}}_2$, and $\text{Yb}{\text{Cu}}_{6.5}$, respectively. (d) A fit example of the PFY-XAS spectra of YbCu at 4.8 GPa. (e)–(g) Pressure dependence of the Yb valence of YbCu, $\text{Yb}{\text{Cu}}_2$, and $\text{Yb}{\text{Cu}}_{6.5}$, respectively. In panel (e) two independent measurements for YbCu are shown as closed and open circles. In panels (e)–(g) shaded areas correspond to the pressure range where the valence transitions occur.

Figure 2

(a)–(c) Pressure dependence of the XRD patterns for YbCu, $\text{Yb}{\text{Cu}}_2$, and $\text{Yb}{\text{Cu}}_{6.5}$, respectively.

Figure 3

(a) A schematic view of the YbCu crystal structure at low pressures before the structural phase transition. (b) Pressure dependence of the volume at a low-pressure phase. A shaded area corresponds to the pressure range of the valence transition in Fig. 1. (c) Pressure dependence of the normalized lattice parameters at a low-pressure phase. (d) Pressure dependence of the ratios of the lattice constants at a low-pressure phase. (e) Crystal structure at a high-pressure phase. (f) Pressure dependence of the volume at a high-pressure phase. (g) Pressure dependence of the lattice parameter at a high-pressure phase. For the data where the error is not visible, the error is less than or equal to the symbol size.

Figure 4

(a) A schematic view of the $\text{Yb}{\text{Cu}}_2$ crystal structure. (b) Pressure dependence of the volume. A shaded area around 4–8 GPa corresponds to the pressure range of the valence transition in Fig. 1. (c) Pressure dependence of the normalized lattice parameters. (d) Pressure dependence of the ratios of the lattice constants. For the data where the error is not visible, the error is less than or equal to the symbol size.

Figure 5

(a) A schematic view of the $\text{Yb}{\text{Cu}}_{6.5}$ crystal structure. Here, it is noted that Ref. [13] suggested that 18% of the Yb site was randomly occupied by Cu-Cu dimers (not shown in the figure). (b) Pressure dependence of the volume. A shaded area corresponds to the pressure range of the valence transition in Fig. 1. (c) Pressure dependence of the normalized lattice parameters. (d) Pressure dependence of the ratio $c/a$ of the lattice constants. (e) Pressure dependence of the normalized atomic spacing, where each distance is defined as shown in panel (a). For the data where the error is not visible, the error is less than or equal to the symbol size.

Figure 6

(a) XAS spectra of YbCu, $\text{Yb}{\text{Cu}}_2, \text{Yb}{\text{Cu}}_{4.5}, \text{Yb}{\text{Cu}}_{6.5}$, and $\text{Yb}\text{In}{\text{Cu}}_4$ at 10 K with the spectrum of ${\text{Cu}}_2\text{O}$ at 300 K. (b) Temperature dependence of the XAS spectra of $\text{Yb}{\text{Cu}}_{4.5}$.