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Pressure dependence of the Yb valence in YbNiGe 3 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 Lα 1 partial fluorescence yield mode and resonant x-ray emission spectroscopy around the Yb L 3 absorption edge. The Yb valence in YbNiGe 3 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 ∼5 GPa, slowly increases up to ∼10 GPa, and then reaches a saturated value of v ∼ 2.87 at 15.6 GPa. The Yb valence at 17 K slightly decreases compared to that at 300 K; v ∼ 2.45 at ambient pressure and ∼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 YbNiGe 3. In contrast to YbNiGe 3 , the Yb valence in YbNiSi 3 is nearly 3 at ambient pressure with almost no temperature dependence.

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