Pressure-induced valence change of YbNiGe3 investigated by resonant x-ray emission spectroscopy at the Yb L3 edge

Pressure dependence of the Yb valence in YbNiGe3 has been investigated up to 15.6 GPa at 300 K and up to 7.7 GPa at 17 K by means of x-ray absorption spectroscopy in L alpha(1) partial fluorescence yield mode and resonant x-ray emission spectroscopy around the Yb L-3 absorption edge. The Yb valence in YbNiGe3 at ambient pressure strongly fluctuates with the mean valence of v = 2.52 +/- 0.01 at 300 K. The Yb valence rapidly changes toward a trivalent state with pressure up to similar to 5 GPa, slowly increases up to similar to 10 GPa, and then reaches a saturated value of v similar to 2.87 at 15.6 GPa. The Yb valence at 17 K slightly decreases compared to that at 300 K; v similar to 2.45 at ambient pressure and similar to 2.72 at 7.7 GPa. We found that the pressure-induced change in the intensity of a quadrupole component in the x-ray absorption spectra shows the same trend as the Yb valence in YbNiGe3. In contrast to YbNiGe3, the Yb valence in YbNiSi3 is nearly 3 at ambient pressure with almost no temperature dependence.