X-ray powder diffraction and electron diffraction studies of the Ni _1±yGe_1-xP_x system

The wide-range of non-stoichiometric NiAs-type solid solution Ni _1±yGe_1-xP_x has been studied by means of X-ray powder and electron diffraction. The incommensurately modulated structure of Ni(Ge, P) has been found to exist over a wide compositional range which is limited by the end points ≈NiGe _0.8P_0.2 and NiGe_0.3P_0.7 so that the general stoichiometry might be referred to as NiGe_{1- x}P_x with 0.2≤x≤0.7. The modulation wave vector is of the type q=γ[112̄0]* and its modulus is strongly composition dependent. A possible interpretation is given as a "soft transition", via an incommensurately modulated structure, between the MnP and the NiP structure types, based on the almost purely displacive origin of the distortion. Further, the crystal structures of Ni₅Ge₂P₃ and Ni₂GeP seem to be commensurate approximations of the incommensurate modulated structure of Ni(Ge, P).