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.