  The search for new phases is an important direction in materials science. The
phase transition of sulfides results in significant changes in catalytic
performance, such as MoS2 and WS2. Cubic pentlandite [cPn, (Fe, Ni)9S8] can be
a functional material in batteries, solar cells, and catalytic fields. However,
no report about the material properties of other phases of pentlandite exists.
In this study, the unit-cell parameters of a new phase of pentlandite,
sulfur-vacancy enriched hexagonal pentlandite (hPn), and the phase boundary
between cPn and hPn were determined for the first time. Compared to cPn, the
hPn shows a high coordination number, more sulfur vacancies, and high
conductivity, which result in significantly higher hydrogen evolution
performance of hPn than that of cPn and make the non-nano rock catalyst hPn
superior to other most known nanosulfide catalysts. The increase of sulfur
vacancies during phase transition provides a new approach to designing
functional materials.
