Collapse of antiferromagnetism in CeRh₂Si₂: Volume versus entropy

The thermal expansion of the heavy fermion compound CeRh₂Si ₂ has been measured under pressure as a function of temperature using strain gages. A large anomaly associated with the Néel temperature has been detected even above the suspected critical pressure P_c∼1. 05GPa where no indication of antiferromagnetism has been observed in calorimetry experiments sensitive to the entropy change. The volume change at T _N1 proves the first-order nature of the collapse of antiferromagnetism. An unexpected feature is the weak pressure dependence of the antiferromagnetic-paramagnetic transition in comparison to the fast pressure collapse predicted for a homogeneous first-order quantum phase transition with one single pressure singularity at P_c. A large pressure dependence is observed in the anisotropy of the thermal expansion measured parallel or perpendicular to the c-axis of this tetragonal crystal. This points out the collapse of the efficiency of the crystal field effects at P_v which indicates the entrance into the intermediate valence regime.