Selective Metal Exsolution in BaFe(2-y)M(y)(PO4)2 (M = Co(2+), Ni(2+)) Solid Solutions

Area of Science:

• Materials Science
• Solid-state Chemistry
• Magnetism

Background:

• Barium iron phosphates are known for their 2D-Ising ferromagnetic properties.
• Metal exsolution in these materials can lead to the formation of nanosized oxide clusters.
• Understanding metal substitution effects is crucial for tuning material properties.

Purpose of the Study:

• To investigate the feasibility and selectivity of metal exsolution in BaFe(2-y)M(y)(PO4)2 solid solutions, where M is Co(2+) or Ni(2+).
• To analyze the impact of varying M concentrations on the exsolution process and the resulting nanostructure.
• To characterize the magnetic behavior of the modified materials.

Main Methods:

• Synthesis of BaFe(2-y)M(y)(PO4)2 solid solutions (M = Co, Ni; y = 0, 0.5, 1, 1.5).
• Thermal treatment in air at 600 °C to induce metal exsolution.
• (57)Fe Mössbauer spectroscopy, Thermogravimetric Analysis (TGA), Transmission Electron Microscopy (TEM), microprobe analysis, and X-ray Absorption Near Edge Structure (XANES) for characterization.
• Magnetic property measurements.

Main Results:

• Complete oxidation of Fe(2+) to Fe(3+) and stable M(2+) ions were confirmed for all compounds after thermal treatment.
• The size of nanometric α-Fe2O3 clusters depends strongly on the M concentration.
• Iron diffusion is hindered in M-rich phases, leading to superparamagnetic α-Fe2O3 particles.
• Ordered rearrangement of Fe(3+) and Co(2+) ions observed in BaFe(0.67)Co1(PO4)2.
• Magnetic behavior is dominated by antiferromagnetic exchange, with Co(2+)-rich compounds exhibiting metamagnetic transitions.

Conclusions:

• Co(2+) and Ni(2+) ions can be incorporated into the BaFe2(PO4)2 structure, influencing iron exsolution.
• The concentration of substituted metals significantly affects the size and magnetic state of exsolved α-Fe2O3 nanoparticles.
• The crystal structure allows for ordered ion rearrangement, offering pathways to tailor magnetic properties in these phosphate systems.