Creating Binary Cu-Bi Compounds via High-Pressure Synthesis: A Combined Experimental and Theoretical Study

Exploration beyond the known phase space of thermodynamically stable compounds into the realm of metastable materials is a frontier of materials chemistry. The application of high pressure in experiment and theory provides a powerful vector by which to explore this uncharted phase space, allowing discovery of complex new structures and bonding in the solid state. We harnessed this approach for the Cu-Bi system, where the realization of new phases offers potential for exotic properties such as superconductivity. This potential is due to the presence of bismuth, which, by virtue of its status as one of the heaviest stable elements, forms a critical component in emergent materials such as superconductors and topological insulators. To fully investigate and understand the Cu-Bi system, we welded theoretical predictions with experiment to probe the Cu-Bi system under high pressures. By employing the powerful approach of in situ X-ray diffraction in a laser-heated diamond anvil cell (LHDAC), we thoroughly explored the high-pressure and high-temperature (high-PT) phase space to gain insight into the formation of intermetallic compounds at these conditions. We employed density functional theory (DFT) calculations to calculate a pressure versus temperature phase diagram, which correctly predicts that CuBi is stabilized at lower pressures than Cu₁₁Bi₇, and allows us to uncover the thermodynamic contributions responsible for the stability of each phase. Detailed comparisons between the NiAs structure type and the two high-pressure Cu-Bi phases, Cu₁₁Bi₇ and CuBi, reveal the preference for elemental segregation within the Cu-Bi phases, and highlight the unique channels and layers formed by ordered Cu vacancies. The electron localization function from DFT calculations account for the presence of these "voids" as a manifestation of the lone pair orientation on the Bi atoms. Our study demonstrates the power of joint experimental-computational work in exploring the chemistry occurring at high-PT conditions. The existence of multiple high-pressure-stabilized phases in the Cu-Bi binary system, which can be readily identified with in situ techniques, offers promise for other systems in which no ambient pressure phases are known to exist.