Pressure-induced development of bonding in NiAs type compounds and polymorphism of NiP

A reversible, displacive, pressure-induced structural phase transition has been found to occur in nickel monophosphide NiP at approximately 3.5 GPa by means of in situ synchrotron single-crystal X-ray diffraction. The new phase, with Pearson symbol oC56, assumes an orthorhombic structure with Cmc2₁ space group and unit cell parameters a=23.801(2) Å, b=5.9238(6) Å, and c=4.8479(4) Å at 5.79 GPa. The high-pressure phase is a superstructure of the ambient, oP16 phase with multiplicity of 3.5. The phosphorous sublattice gradually converts from the net of isolated P₂ dimers found in the ambient NiP, towards zig-zag polymeric P_∞ chains found in MnP-type structures. The transformation involves development of triatomic phosphorous clusters and interconnected Ni slabs with diamondoid topology. The high-pressure phase, which represents intermediate polymerization step, is a commensurately modulated superstructure of the NiAs aristotype. The phase transformation in NiP bears resemblance to the effect of successive substitution of Si or Ge in place of P found in the series of stoichiometric inhomogeneous linear structures in ternary NiP_1−xSi_x and NiP_1−xGe_x systems.