Pressure study on the interplay between magnetic order and valence crossover in EuPd2(Si1-xGex)2

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 TN = 47 K for x = 0.105. The valence-crossover temperature TV ? shows an extraordinarily strong pressure dependence of dTV ?/dp = +(80 & PLUSMN; 10) K/GPa. In contrast, a very small pressure dependence of dTN/dp +(1 & PLUSMN; 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 4 f 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 pcr & AP; 0.06 GPa and Tcr & AP; 39 K where intriguing strongcoupling effects between fluctuating charge, spin, and lattice degrees of freedom can be expected.