[en] Highlights: • Ni@Ni_xP_y core-shell microstructures are synthesized by a low-temperature phosphidation route. • The initial Ni/NaH₂PO₂ molar ratio can strongly affect the phase of the final product. • Ni@Ni_xP_y microstructures present good catalytic activity for the reduction of 4-nitrophenol. • The catalytic property of Ni@Ni_xP_y microstructures presents markedly phase-dependent features. - Abstract: Magnetic Ni@Ni_xP_y core-shell microstructures were successfully synthesized by a simple low-temperature phosphidation route, employing Ni nanospheres as the initial nickel source, NaH₂PO₂·H₂O as the phosphorus ion source. The phosphidation reaction was carried out in N₂ atmosphere at 300 °C for 2 h. Experiments showed that the initial Ni/NaH₂PO₂ molar ratio could strongly affect the phase of the final product. When the Ni/NaH₂PO₂ molar ratio of 1:2.77 was used, hexagonal Ni₂P phase was obtained; while the Ni/NaH₂PO₂ molar ratio was 1:5.54, hexagonal Ni₅P₄ phase was produced. It was found that the magnetic properties of the products gradually decreased with the conversion from Ni nanospheres, to Ni@Ni₂P and to Ni@Ni₅P₄. Moreover, initial Ni nanospheres, Ni@Ni₂P and Ni@Ni₅P₄ core-shell microstructures presented outstanding catalytic activities for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in excess NaBH₄ solution. Ni@Ni₅P₄ core-shell microstructures owned the best catalytic activity.