Pressure-driven Valence evolution Coupled Hardening-to-Softening transition in YbPd

TL;DR

This study investigates pressure-driven Yb valence instabilities in YbPd across a charge-order transition using high-resolution RXES and HERFD-XAS at the Yb $L_3$ edge, complemented by X-ray diffraction. At $T=30$ K, the Yb valence remains fixed up to $P_K = 1.5$ GPa and then increases gradually, whereas at $T=300$ K a pronounced valence drop from $v\approx 2.87$ to $2.58$ occurs for $P\le P_K$ before saturating for $P> P_K$. XRD reveals a kink in the unit-cell volume at $P_K$, and a Birch-Murnaghan analysis shows a sign change in $K'_0$ across $P_K$, signaling a hardening-to-softening transition linked to a minimum in compressibility. The Kondo temperature $T_K$ exhibits a rapid increase to $\approx 768$ K at $P_K$ and then plateaus, suggesting a Kondo-volume-collapse–like mechanism in this mixed-valence system, with parallels to Ce’s $\eta$-like transitions yet distinct compression behavior. Overall, the work connects electronic valence fluctuations to lattice responses, advancing understanding of isostructural valence transitions in correlated materials.

Abstract

We investigate the Yb valence instabilities in the strongly correlated YbPd compound using resonant X-ray emission spectroscopy under pressure across the charge-ordering (CO) transition. At a low temperature (T = 30 K) in the CO ordered phase, the Yb $4f$ valence remains nearly constant up to a pressure P$_K$ = 1.5 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$_K$ = 1.5 GPa. In contrast, for P $>$ P$_K$, the Yb $4f$ valency saturates and the compressibility reveals a counterintuitive pressure-induced softening. The results show a minimum in the compressibility of YbPd (with $f^{13}$-$f^{14}$ hole-type mixed-valence) and is 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).

Figures (5)

• Figure 1: Crystal structure of YbPd at ambient pressure above (a) $T_1$ = 130 K (cubic phase) and below $T_1$ (tetragonal phase). YbPd shows only one-type of Yb-site in the normal cubic phase, but shows 2 types of Yb-sites in the tetragonal CO phase. P-T phase diagram reported in Ref Miyake2012.
• Figure 2: Yb L$_3$ XAS and RXES spectra of YbPd as a function of temperature at (a) $T$ = 30 K and (b) $T$ = 300 K.
• Figure 3: Yb L$_3$ RXES spectra of YbPd as a function of temperature at (a) $T$ = 30 K and (b) $T$ = 300 K. The incident photon energy was set to the maximum of the Yb$^{2+}$ resonance at the Yb L$_3$ absorption edge and the spectra were normalized to the areaa under the curve.
• Figure 4: (a) Evolution of the Yb valence as a function of pressure at T = 30 K. Evolution of the Yb valence (b), the compressibility (b) and the Kondo temperature (c) as a function of pressure at room temperature.
• Figure 5: (a) Contour map of X-ray diffraction intensities collected in the pressure range 0–3 GPa. (b) XRD patterns at P = 0.2 GPa and 2 GPa. (c) Pressure dependence of the volume. The blue and red curves are the to third-order Birch- Murnahgan’s equation of state for P $\leq$ P$_K$ and P $\geq$ P$_K$ respecively. The arrow indicates the presure P$_K$ where the kink is observed.