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 4f 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.