We present results of the magnetic susceptibility on high-quality single crystals of EuPd2(Si1−xGex )2 for Ge concentrations 0 ≤ x ≤ 0.105 performed under varying hydrostatic (He-gas) pressure 0 ≤ p ≤ 0.5 GPa. The work extends recent studies at ambient pressure demonstrating the drastic change in the magnetic response from valence-crossover behavior for x = 0 and 0.058, to long-range antiferromagnetic (AFM) order below T_N = 47 K for x = 0.105. The valence-crossover temperature T_V shows an extraordinarily strong pressure dependence of dT'_V/dp= +(80 ± 10) K/GPa. In contrast, a very small pressure dependence of dT_N/dp ≤ +(1 ± 0.5) K/GPa is found for the AFM order upon pressurizing the x = 0.105 crystal from p = 0 to 0.05 GPa. Remarkably, by further increasing the pressure to 0.1 GPa, a drastic change in the ground state from AFM order to valencecrossover behavior is observed. Estimates of the electronic entropy related to the Eu 4f electrons, derived from analyzing susceptibility data at varying pressures, indicate that the boundary between AFM order and valence crossover represents a first-order phase transition. Our results suggest a particular type of second-order critical end point of the first-order transition for x = 0.105 at p_cr ≈ 0.06 GPa and T_cr ≈ 39 K where intriguing strong-coupling effects between fluctuating charge, spin, and lattice degrees of freedom can be expected.

Data from the figures of the publication B. Wolf et al., Phys. Rev. B 107, 245147 (2023)