Changes in ionization potential at high-density drive changes in material transport properties and, often, crystal structure and equation of state. Current models used to describe and predict these effects have not been tested in regimes much beyond ~200 GPa because of limitations in experimental techniques. Now, however, high-energy laser facilities with flexible pulse shaping capabilities have made it possible to reach pressures in the 1,000 GPa range while maintaining temperatures below 1 eV.
In this project, we used these new methods to ramp-compress cobalt and magnesium. We measured shifts in the K-shell emission lines to infer changes in ionization stage and x-ray diffraction to measure changes in crystal structure. In the case of cobalt, commonly used, detailed models for effective ionization based on isolated-atom electronic structure give the wrong answer, while self-consistent density-functional-theory-based models reproduce the data well. In the case of magnesium, zero-kelvin density functional theory predictions of density-driven electron localization in lattice interstitials (so-called electride phases) at high pressure are found to be too simple to describe material evolution along a laser ramp-compression pathway. Instead, by a combination of experiment, first-principles theory, and evolutionary structure-searching algorithms, we discovered several unexpected, lower-symmetry metastable structures. First principles results find that the density of localized interstitial charge in the high-pressure magnesium phases is too low to be measurable with x-ray diffraction, and the body of theory work which has predicted high pressure electride phases for many systems, based on analysis of electron localization function, needs to be re-examined.
This project supports the NNSA goal to manage the nation's nuclear stockpile as well as the Department of Energy's strategic objective to maintain the safety, security, and effectiveness of the nation's nuclear deterrent without nuclear testing. Our research leverages Lawrence Livermore National Laboratory's core competency in high-energy-density science.
Our measurements provide key experimental benchmarking data for theoretical models forming the basis for predictive simulations at the foundation of assessing the effectiveness and reliability of the U.S. nuclear weapons stockpile and for internal confinement fusion design. A highly efficient platform for exciting x-ray fluorescence using fast electrons has been fully developed and demonstrated to provide data of sufficient precision to discriminate between the many existing models for ionization potential depression. Zero-kelvin evolutionary structure-search methods coupled with first-principles molecular dynamics to reach high temperatures have been demonstrated as an effective tool for identifying likely metastable phases in the energy landscape, to aid in structure solution where experimental data is incomplete.
Lawrence Livermore National Laboratory • 7000 East Avenue • Livermore, CA 94550
Operated by Lawrence Livermore National Security, LLC, for the Department of Energy's National Nuclear Security Administration.