Anomalous elastic properties across the γ to α volume collapse in cerium

The behavior of the f -electrons in the lanthanides and actinides governs important macroscopic properties but their pressure and temperature dependence is not fully explored. Cerium with nominally just one 4 f electron offers a case study with its iso-structural volume collapse from the γ-phase to the α-phase ending in a critical point ( p C , V C , T C ), unique among the elements, whose mechanism remains controversial. Here, we present longitudinal ( c L ) and transverse sound speeds ( c T ) versus pressure from higher than room temperature to T C for the first time. While c L experiences a non-linear dip at the volume collapse, c T shows a step-like change. This produces very peculiar macroscopic properties: the minimum in the bulk modulus becomes more pronounced, the step-like increase of the shear modulus diminishes and the Poisson’s ratio becomes negative—meaning that cerium becomes auxetic. At the critical point itself cerium lacks any compressive strength but offers resistance to shear.