In view of the interest that (Fe,Co)2(P,Si) compounds have as potential permanent magnets, their structural and magnetic phase diagrams are explored focusing on establishing the range where the hexagonal Fe2P-type structure is observed. In Fe1.93-xCoxP1-ySiy, the highest Si content prior entering a mixed phase domain is y ≈ 0.5. At high Si content but low Co for Fe substitutions, a structural distortion leading to a body-centered orthorhombic structure occurs. At high Co contents, when the Fe2P unit cell reaches a critical volume of about 102.4 Å3, the samples crystallize in a Co2P-type orthorhombic structure. Within the Fe2P-type structural range, the evolution of the unit-cell volume appears to follow the Vegard's law, but this hides strongly anisotropic changes. Simultaneous Co for Fe and Si for P substitutions increase the range where the hexagonal structure is observed in comparison to ternary Fe2(P,Si) and (Fe,Co)2P. The samples are ferromagnetic, but with Curie temperatures showing an unusual evolution, uncorrelated to the c/a ratio of the lattice parameters. At low Si content, TC increases with Co for Fe substitutions. For y = 0.2, the evolution is not significant, while at high Si content TC systematically decreases with the increase in Co. Large Si and Co substitutions lead to a swift weakening of the magnetocrystalline anisotropy until the easy axis anisotropy turns from the c axis toward the a-b plane. This study guides future investigations by restricting the range where desirable properties for permanent magnetic applications can be expected to 0.1 ≲ x ≲ 0.3 and 0.1 ≲ y ≲ 0.3.
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