Pressurizing strategically selected compositions of the $\mathrm{EuC}{\mathrm{u}}_2{\left(\mathrm{G}{\mathrm{e}}_{1-x}\mathrm{S}{\mathrm{i}}_x\right)}_2$ series affords an opportunity for gaining microscopic insight into the ground-state properties and interplay between magnetism and valence fluctuations across a quantum critical point. This is investigated by way of systematic ${}^{151}\mathrm{Eu}$ Mössbauer spectroscopy measurements on $x=0$ and $x=0.5$ compositions in the series, pressurized up to 7 GPa including variable temperature scans in the range 300–4.2 K. In $\mathrm{EuC}{\mathrm{u}}_2\mathrm{G}{\mathrm{e}}_2$ the temperature and pressure dependences of the hyperfine interaction parameters indicate that both the magnetic and divalent state, $\mathrm{E}{\mathrm{u}}^{\nu +}$ where $\nu =2$, are stable up to 6–7 GPa, thus serving as a useful reference. Whereas in the $x=0.5$ composition which initially involves $\mathrm{E}{\mathrm{u}}^{2+}$, collapse of the magnetically ordered state is onset at ∼1.3 GPa and there is emergence of a nonmagnetic intermediate valence state coexisting with the magnetically ordered state. This regime of mixed states is a precursor of a quantum phase transition to a nonmagnetic homogeneous intermediate valence state $\nu \sim 2.45$, across a quantum critical point at 3.6 GPa, suggesting a first-order phase transition. X-ray-diffraction pressure studies at 300 K up to 6 GPa of the $x=0.5$ composition indicate there is no change in lattice symmetry from the tetragonal $\mathrm{ThC}{\mathrm{r}}_2\mathrm{S}{\mathrm{i}}_2$-type structure. There are also no obvious discontinuities in pressure dependences of the lattice parameters upon evolving through the quantum critical point at 3.6 GPa. Increasing pressure changes the starting $\mathrm{E}{\mathrm{u}}^{2+}$ valence monotonically, until the mean valence attains its largest value $\nu \sim 2.45$ indicative of enhanced charge fluctuations at the quantum critical point and plateaus thereafter. High-pressure resistance measurements at low temperatures down to 40 mK near the quantum critical point reveal no evidence for superconductivity.

• Received 6 March 2020
• Revised 23 April 2020
• Accepted 23 April 2020

DOI:https://doi.org/10.1103/PhysRevB.101.205127