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

TL;DR

This study reveals an unusual pressure-dependent behavior of the bulk modulus and Yb valence in YbPd, showing a transition from lattice hardening to softening without structural change, linked to valence fluctuations.

Contribution

It uncovers anomalous pressure effects on Yb valence and compressibility in YbPd, highlighting a softening behavior beyond a critical pressure without structural phase transition.

Findings

Yb valence remains constant up to 1.5 GPa and then increases

Anomalous decrease of Yb valence at room temperature without structural change

Compressibility shows a maximum at a critical pressure, indicating softening

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 $4f$ valence remains nearly constant up to a pressure P$_L$ = 1.5 $\pm {0.2}$ GPa, and then increases gradually at higher pressures. In contrast, at room temperature in the normal phase, an anomalous decrease of the Yb $4f$ 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 $\pm {0.2}$ GPa. In contrast, for P $>$ P$_K$, the Yb $4f$ 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 $\gamma$-$\alpha$ first-order isostructural phase transition in cerium (with $f^{0}$-$f^{1}$ electron-type mixed-valence).