Non-existence of the s-f transition in structures of solid gadolinium at pressure

TL;DR

This study uses density functional theory and X-ray diffraction to show that gadolinium does not undergo a significant volume collapse or f-electron transition under high pressure, contrary to previous beliefs.

Contribution

The paper demonstrates that the high-pressure phase of gadolinium involves no volume collapse and can be explained by standard DFT without special f-electron treatment.

Findings

No significant volume collapse observed in Gadolinium under high pressure.

High-pressure phase transitions follow standard stacking sequences.

No evidence of f-electron bonding or complex f-electron physics at high pressure.

Abstract

Gadolinium has long been believed to undergo a high pressure phase transition with a volume collapse around 5%. Theoretical explanations have focused on the idea of electrons transferring from the extended s-orbital to the compact f-orbital. However, experimental measurement has been unable to detect any associated change in the magnetic properties of the f-electrons. Here we resolve this discrepancy by showing that there is no significant volume collapse, beyond what is typical in high pressure phase transformations. We present density functional theory calculations of solid gadolinium under high pressure using a range of methods, and revisit the experimental situation using X-ray diffraction (XRD). The standard lanthanide pressure-transformation sequence involving different stackings of close-packed planes: hcp to 9R to dhcp to fcc to d-fcc is reproduced. The so-called "volume collapsed" high-pressure phase is shown to be an unusual stacking of close-packed planes, with Fddd symmetry and a density change less than 2%. The distorted fcc (d-fcc) structure is revealed to arise as a consequence of antiferromagnetism. The theoretical results are shown to be remarkably robust to various treatments of the f-electrons. The key result is that there is no XRD evidence for volume collapse in Gadolinium. The sequence of phase transitions is well described by standard DFT. There is no need for special treatment of the f-electrons or evidence of f-electron bonding. Noting previous spectroscopic evidence is that there is no change in the f-electrons we conclude that high pressure Gadolinium has no complicated f-electron physics such as Mott-Hubbard, Kondo or valence transition.