Two stage pressure-induced Yb valence change in the Hexagonal Laves Phase YbAg$_2$: Investigation by time differential perturbed angular $\gamma$-$\gamma$ correlation spectroscopy method and density functional calculations

TL;DR

This study investigates the pressure-induced valence change of ytterbium in YbAg$_2$ using advanced spectroscopy and density functional calculations, revealing a two-stage transition from valence 2.8 to 3.0 up to 19 GPa.

Contribution

It provides the first combined experimental and theoretical analysis of Yb valence change under pressure in the hexagonal Laves phase, identifying a two-stage transition mechanism.

Findings

Yb valence undergoes a two-stage transition from 2.8 to 3.0 under pressure.

Distinct quadrupole frequencies increase with pressure and saturate at specific pressures.

Silver 4d-states participate in metal bonding, influencing phase stability.

Abstract

We have studied the C14 hexagonal Laves phase of YbAg$_2$ at normal conditions and under external pressure up to 19 GPa by the time-differential perturbed angular $\gamma-\gamma$ correlation spectroscopy (TDPAC) using $^{111}$Cd probe nuclei. Under pressure the valence of Yb undergoes a two stage transition from 2.8 to 3. The two stage scenario is characterized by two distinct quadrupole frequencies of $^{111}$Cd probes in silver sublattice, monotonically increasing with pressure and saturating at 8 and 16 GPa. Our experimental data are compared with the density functional studies of the electron band structure of YbAg$_2$, whose results are used for discussion and interpretation of these experiments. We have found that there are two different electric field gradients at inequivalent silver sites and that $4d$-states of silver participate in metal bonding, allowing for the formation of the hexagonal Laves phase.