Most non-elemental materials that we know have been synthesized using chemical processes that occur at conditions close to what we call “ambient”, meaning with temperatures within a few hundred °C and at atmospheric pressures. At these conditions, many elements react with each other and form compounds. However, there are some exceptional elements that simply will not form any compounds when they are combined. Such systems are referred to as immiscible.

For reasons which are still not completely understood, bismuth exhibits severe immiscibility with several elements from the periodic table. In particular, there is not a single intermetallic compound that can be formed at ambient conditions in the Cu-Bi systems. In the early 1960s, experiments were performed by B. Matthias to force copper and bismuth to react with each other by applying extreme pressures on them, essentially trying to squeeze the atoms closer to each other and hoping that they would form bonds. Although these experiments were successful, Matthias never completely characterized the new materials that he had created, and his findings were forgotten for decades.

Recently, in a collaboration with experimental researchers at Northwestern University, we revisited the Cu-Bi system and tried to recreate the experiments conducted more than 50 years ago. To our surprise, we found that copper and bismuth not only react with each other at high pressures, but that they can form different materials depending on the synthesis conditions. Using a combination of in-situ XRD at high pressure and temperature with computational modeling, we were able to demonstrate that at least two different materials can be formed at sufficiently high pressures. Both phases are superconducting, and exhibit peculiar low-dimensional voids. Although these voids appear to be empty at first sight, our density functional theory results show that they actually provide space for the Bi lone electron pairs, giving rise to their unique crystal structures and properties.

This joint experimental and theoretical work was recently published in Chemistry of Materials.