Electronic Transitions in f-electron Metals at High Pressures:

Abstract

This study was to investigate unusual phase transitions driven by electron correlation effects that occur in many f-band transition metals and are often accompanied by large volume changes: ~ 20% at the δ−α transition in Pu and 5-15% for analogous transitions in Ce, Pr, and Gd. The exact nature of these transitions has not been well understood, including the short-range correlation effects themselves, their relation to long-range crystalline order, the possible existence of remnants of the transitions in the liquid, the role of magnetic moments and order, the critical behavior, and dynamics of the transitions, among other issues. Many of these questions represent forefront physics challenges central to Stockpile materials and are also important in understanding the high-pressure behavior of other f-and d-band transition metal compounds including 3d-magnetic transition monoxide (TMO, TM=Mn, Fe, Co, Ni).The overarching goal of this study was, therefore, to understand the relationships between crystal structure and electronic structure of transition metals at high pressures, by using the nation's brightest third-generation synchrotron x-ray at the Advanced Photon Source (APS). Significant progresses have been made, including new discoveries of the Mott transition in MnO at 105 GPa and Kondo-like 4f-electron dehybridization and new developments of high-pressure resonance inelastic x-ray spectroscopy and x-ray emission spectroscopy. These scientific discoveries and technology developments provide new insights and enabling tools to understand scientific challenges in stockpile materials. The project has broader impacts in training two SEGRF graduate students and developing an university collaboration (funded through SSAAP). Scientific BackgroundThe f-electron metals, actinides and lanthanides alike, exhibit a profound change in character, both for individual metals as a function of compression, or across the series as a whole at ambient pressure. It is believed that this behavior is driven by strong 5f-electron correlation [1]. As we proceed through the series of the actinides, for example, two distinct parts of the series are discovered as illustrated in Fig. 1. The early part, from Th-Pu, shows a parabolic behavior of the equilibrium molar volume reminiscent of the d-transition metals, whereas the later part shows a more constant behavior of the specific volume as a function of atomic number. The similarity between the light actinides and the d-transition metals in this regard is governed by the fact that the light actinides and the d-transition metals both have delocalized electrons, f and d respectively. For the heavier actinides from americium and on, however, the 5f electrons are localized similar to the lanthanide series and their properties become quite different from that of the earlier actinides. The itinerant 5f electrons also show a distinguished difference compared to the 4d and 5d electrons; that is, their bandwidths are much narrower and in turn this leads to distortions in crystal structures of...

Mott transition of MnO under pressure: A comparison of correlated band theories

“…Resistance measurements 3 provided the first evidence of the Mott transition in MnO near 100 GPa. Recent x-ray diffraction and emission spectroscopy measurements of the crystal structure and magnetic moment by Yoo et al 4 have clarified the behavior. Around 90 GPa there is a structural transformation from the distorted B1 ͑rocksalt͒ phase to the B8 ͑NiAs͒ structure.…”