Cerium losing compressive strength but displaying resistance to shear across the γ-α volume collapse

Figure caption: Ultrasonic longitudinal velocities at high temperatures as a function of pressure. The inset shows the same data without offsets. Uncertainty in pressure (<0.05 GPa) and speed (<1%) is within symbol size.
Figure caption: Ultrasonic longitudinal velocities at high temperatures as a function of pressure. The inset shows the same data without offsets. Uncertainty in pressure (<0.05 GPa) and speed (<1%) is within symbol size.

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. For example, the one f-electron in Ce plays an important role in the volume dependence under pressure which contracts abruptly by ~15% at room temperature when the pressure reaches ~0.75 GPa. The crystallographic symmetry remains across the γ-α volume collapse, even though the two fcc phases are quite different. The iso-structural volume collapse from the γ- to the α-phase ends at high temperatures in a critical point (pC, VC, TC), unique among the elements. A team from Lawrence Livermore National Laboratory and the Carnegie Institute of Washington has measured the longitudinal (cL) and transverse sound speeds (cT) vs pressure from room temperature to TC for the first time. While cL experiences a non-linear dip at the volume collapse (see Figure), cT 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 still offers resistance to shear. More in M. J. Lipp, et al, Nat. Comm. Oct 31 2017.