Several phosphides with the chemical formula M2P crystallize in a hexagonal structure: Mn2P, Fe2P, and Ni2P. The structure of Co2P is orthorhombic. These two structures are related via an identical elementary subcell consisting of a tetrahedral-site and a pyramidal-site M-atom pair. This investigation shows that solid solutions between two hexagonal end members, such as Fe2P or Ni2P with Mn2P, may exhibit orthorhombic structures at intermediate compositions. Fe2P shows complete solid solubility with Ni2P. Curiously, hexagonal and orthorhombic symmetries alternate with decreasing number of 3d electrons, Ni2P(hex) -Co2P(ortho) -Fe2P(hex) -FeMnP(ortho) -Mn2P(hex). Lattice parameter variations with composition and Mössbauer studies reveal atomic ordering in the mixed systems, Mn and Cr substituting for pyramidal-site iron in FeMnP and FeCrP while Ni and Co substitute preferentially for tetrahedralsite iron in FeCoP and FeNiP. While neither Co2P nor Mn2P are ferromagnetic, intermediate phases are, the Curie temperature and magnetization reaching the maximum values 310°C and 3.03 μB/molecule for MnCoP. Metamagnetism appears for a range of compositions about (Mn0.7Co0.3)2P. The Co2P-Fe2P system is also ferromagnetic with a maximum Curie temperature near FeCoP. This system shows a sharp discontinuity in magnetization, but only a small discontinuity in Curie temperature, across the phase transition. Sensitivity to stoichiometry in the magnetization of Fe2-εP is attributed to an electron/atom ratio near that for the appearance of metamagnetism.

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