Solid Solubility in Metallic Hydrogen

TL;DR

This study explores the potential for various elements to dissolve in solid metallic hydrogen at high pressures, revealing broad solubility across different bonding types, which could influence planetary science and material synthesis.

Contribution

It provides the first detailed theoretical analysis of the solubility of diverse elements in solid metallic hydrogen under extreme conditions.

Findings

All elements studied are soluble in metallic hydrogen.

Solubility occurs in sites substituting for multiple hydrogen atoms.

Elements with different bonding types show broad solubility.

Abstract

Hydrogen in its metallic form is the most common material in our solar system, found under the extreme pressure and temperature conditions found in giant planets. Such conditions are inaccessible to experiment and consequently, theoretical work has typically led experiment. Many remarkable properties are proposed for metallic hydrogen, which is expected to exist in solid form down to low temperatures. Relevant here is that solid metallic hydrogen is not expected to have a close-packed structure, even at extreme pressures. The predicted crystal structure has a packing fraction of only 0.44 compared with 0.74 for fcc. Alloying with other metallic elements can form compounds with atomic hydrogen and much reduced metallization pressure. Here, we investigate the possible solid solubility of a representative range of elements (Be, B, Mg, S, Fe, La) in metallic atomic hydrogen near 500GPa. Zero point energy is highly significant in all cases. The metallic elements Be, Mg, Fe, and La have a well-defined clathrate-style first neighbour shell at 300K, while the non-metallic B and S have more open arrangements. All the elements considered are soluble, in sites substituting for more than one hydrogen atom. The solid solubility of such a wide range of ambient bonding types: simple metals, covalent materials, transition metals, lanthanides, which gives a strong indication that most elements will dissolve in solid metallic hydrogen.