Anomalous pressure dependence of the bulk modulus and Yb valence in cubic YbPd

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Abstract
We investigate the Yb valence instabilities in the strongly correlated YbPd compound using resonant x-ray emission spectroscopy as a function of pressure across the charge-order (CO) transition. At a low temperature ( T = 30 K ) in the CO phase, the Yb 4 f valence remains nearly constant up to a pressure P L = 1.5 ± 0.2 GPa , and then increases gradually at higher pressures. In contrast, at room temperature in the normal phase, an anomalous decrease of the Yb 4 f valence is observed, without any accompanying structural phase transition. This behavior is corroborated by a systematic pressure-dependent decrease of the unit-cell volume. Based on a Birch-Murnaghan analysis, the compressibility indicates hardening of the lattice with applied pressure up to a distinct kink seen at P K = 1.6 ± 0.2 GPa . In contrast, for P > P K , the Yb 4 f valence saturates, and the compressibility reveals a counterintuitive pressure-induced softening. The results show a minimum in the compressibility of YbPd (with f 0 – f 1 hole-type mixed valence), reminiscent of the maximum in compressibility seen in the γ – α first-order isostructural phase transition in cerium (with f 0 – f 1 electron-type mixed valence).
Funding Information
  • French Synchrotron facility SOLEIL (20241507)

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