Synthesis and Characterization of Discrete FexNi2-xP Nanocrystals (0 < x < 2): Compositional Effects on Magnetic Properties

Ternary FexNi2-xP (0 < x < 2) phases exhibit a range of useful properties that can be augmented or tuned by confinement to the nanoscale including hydrotreating catalytic activity for small x and near-room temperature ferromagnetism for high x. In this Work, a solution-phase arrested-precipitation method was developed for the synthesis of FexNi2-xP over all values of x (0 < x < 2). The synthesis involves preparation of Ni-P amorphous particles, introduction of the Fe precursor to form amorphous Fe-Ni-P particles, and high-temperature conversion of Fe-Ni-P particles into crystalline ternary phosphide nanocrystals. The ternary FexNi2-xP nanocrystals crystallize in the hexagonal Fe2P-type structure, and the morphology of the nanocrystals showed a distinct compositional dependence, transitioning from about 11 nm diameter spheres to rods with aspect ratios approaching 2 as the Fe fraction is increased (x >= 1.2). Lattice parameters do not follow Vegard's law, consistent with Mossbauer data showing preferential site occupation by Fe of the tetrahedral over the square pyramidal sites at low Fe concentrations, and the opposite effect for x > 0.8. Magnetic measurements of FexNi2-xP (x = 1.8, 1.4, and 1.2) nanorods showed a strong compositional dependence of the Curie temperature (T-C) that differs from observations in bulk phases, with the highest T-C (265 K) obtained for x = 1.4.