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Could Negative Pressures Turn Bismuth into a Metal? The Case of the Expanded

Materials may behave in non-expected ways when subject to unexpected conditions. For example, when Bi was turned into an amorphous phase (\textit{a}-Bi) unexpectedly it became a superconductor at temperatures below $10$ K. We provided an explanation as to why \textit{a}-Bi superconducts and the crystalline (\textit{c}-Bi) had not been found to do so: we computer calculated their electronic properties and found that \textit{a}-Bi has a larger electron density of states, eDoS, at the Fermi surface than \textit{c}-Bi and this explained the phenomenon. We even predicted an upper limit for the superconducting $T_c$ of the crystalline phase, which was experimentally corroborated within the following year. We now decided to investigate what happens to crystalline (Wyckoff structure) and amorphous Bi when pressures below the atmospheric are applied (expansion). Here we show that when expanded, \textit{c}-Bi becomes more metallic, since the eDoS increases when the volume increases for the Wyckoff structure, while the amorphous eDoS decreases. If the crystalline structure is maintained its $T_c$ would rise under expansion, whereas it would diminish for the \textit{a}-Bi. Expansion can be obtained in the laboratory by chemically etching Bi-based alloys, a process also known as dealloying, for example.