Phase Stability in Nickel Phosphides at High Pressures

Phosphorus
is one of the potential light elements of the core of
the Earth and other planets. The high-pressure behavior of phosphorus
compounds with nickel and iron attracts considerable attention due
to their abundance in iron meteorites. In the present work, with modern
methods of crystal structure prediction, we investigate the structures
and stability of compounds in the Ni–P system at pressures
of 100–400 GPa. As a result, a homologous series of discrete
compounds (Ni, P), consisting of Ni14P, Ni12P, Ni10 P, Ni8P, Ni7P, Ni5P, and Ni3P was found. Phosphorus shows sufficient solubility
in the face-centered cubic (fcc) structure of Ni, and up to 25 mol
% of this element can be dissolved at low temperatures. Based on the
comparison of compounds in the Ni–P and Fe–P systems,
we suggest that at high pressures Ni facilitates phosphorus dissolution
in the closed-packed structure of d-metals, and dissolution of P in
the (Ni, P) alloy will be higher than that in pure Fe. For the Ni3P compound, a new high-pressure phase with the Cmca symmetry is predicted. This structure can be described as deformed
fcc packing and also belongs to the ordered representatives of the
series of (Ni, P) solid solutions. The transition from the low-pressure
phase of Ni3P-I4̅ to the Cmca phase occurs at a pressure of 62 GPa, regardless of
the external temperature. Ni2P is stabilized at a pressure
above 200 GPa in the form of an allabogdanite structure. The transition
from transjordanite to allabogdanite occurs at 78–88 GPa and
0–2000 K.