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) A, b=5.9238(6) A, and c=4.8479(4) A 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 NiP1-xSix and NiP1-xGex systems. - Graphical Abstract: A reversible, pressure-induced phase transition has been found in nickel monophosphide NiP at 3.5 GPa by means of in situ single-crystal X-ray diffraction. The high-pressure phase is a commensurately modulated superstructure of the NiAs aristotype involving gradual conversion from the net of isolated P2 dimers found in the ambient NiP, towards zig-zag polymeric P∞ chains found in MnP. Highlights: → NiP undergoes a phase transition at 3.5 GPa leading to high-pressure oC56 phase. → The oC56 phase exhibits transformation of isolated P2 dimers into P3 trimers. → The phase transition resembles substitution of Si or Ge in place of P.