Strength and Debye temperature measurements of cerium across the γ → α volume collapse: the lattice contribution

Pressure-Driven Sequential Lattice Collapse and Magnetic Collapse in Transition-Metal-Intercalated Compounds FexNbS2

Volume collapse under high pressure is an intriguing phenomenon involving subtle interplay between lattice, spin, and charge. The two most important causes of volume collapse are lattice collapse (low-density to high-density) and magnetic collapse (high-spin to low-spin). Herein we report the pressure-driven sequential volume collapses in partially intercalated Fe_xNbS₂ (x = 1/4, 1/3, 1/2, 2/3). Because of the distinct interlayer atomic occupancy, the low-iron-content samples exhibit both lattice and magnetic collapses under compression, whereas the high-iron-content samples exhibit only one magnetic collapse. Theoretical calculations indicate that the low-pressure volume collapses for x = 1/4 and x = 1/3 are lattice collapses, and the high-pressure volume collapses for all four samples are magnetic collapses. The magnetic collapse involving the high-spin to low-spin crossover of Fe²⁺ has also been verified by in situ X-ray emission measurements. Integrating two distinct volume collapses into one material provides a rare playground of lattice, spin, and charge.

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.

The origin of the volume collapse of cerium, the only elemental metal with a critical point in the solid phase, remains elusive. Here the authors show that, near the critical point, the f-electrons make cerium lose its compressive strength while maintaining a finite shear strength—which makes cerium unexpectedly auxetic.

X-ray imaging for studying behavior of liquids at high pressures and high temperatures using Paris-Edinburgh press

Several X-ray techniques for studying structure, elastic properties, viscosity, and immiscibility of liquids at high pressures have been integrated using a Paris-Edinburgh press at the 16-BM-B beamline of the Advanced Photon Source. Here, we report the development of X-ray imaging techniques suitable for studying behavior of liquids at high pressures and high temperatures. White X-ray radiography allows for imaging phase separation and immiscibility of melts at high pressures, identified not only by density contrast but also by phase contrast imaging in particular for low density contrast liquids such as silicate and carbonate melts. In addition, ultrafast X-ray imaging, at frame rates up to ∼10(5) frames/second (fps) in air and up to ∼10(4) fps in Paris-Edinburgh press, enables us to investigate dynamics of liquids at high pressures. Very low viscosities of melts similar to that of water can be reliably measured. These high-pressure X-ray imaging techniques provide useful tools for understanding behavior of liquids or melts at high pressures and high temperatures.