Selective Metal Exsolution in BaFe2–yMy(PO4)2 (M = Co2+, Ni2+) Solid Solutions

The 2D-Ising ferromagnetic phase BaFe2+2(PO4)2 shows exsolution of up to one-third of its iron content (giving BaFe3+1.33(PO4)2) under mild oxidation conditions, leading to nanosized Fe2O3 exsolved clusters. Here we have prepared BaFe2–yMy(PO4)2 (M = Co2+, Ni2+; y = 0, 0.5, 1, 1.5) solid solutions to investigate the feasibility and selectivity of metal exsolution in these mixed metallic systems. For all the compounds, after 600 °C thermal treatment in air, a complete oxidation of Fe2+ to Fe3+ leaves stable M2+ ions, as verified by 57Fe Mössbauer spectroscopy, TGA, TEM, microprobe, and XANES. The size of the nanometric α-Fe2O3 clusters coating the main phase strongly depends on the yM metal concentration. For M-rich phases the iron diffusion is hampered so that a significant fraction of superparamagnetic α-Fe2O3 particles (100% for BaFe0.5–xCo1.5(PO4)2) was detected even at 78 K. Although Ni2+ and Co2+ ions tend to block Fe diffusion, the crystal structure of BaFe0.67Co1(PO4)2 demonstrates a fully ordered rearrangement of Fe3+ and Co2+ ions after Fe exsolution. The magnetic behaviors of the Fe-depleted materials are mostly dominated by antiferromagnetic exchange, while Co2+-rich compounds show metamagnetic transitions reminiscent of the BaCo2(PO4)2 soft helicoidal magnet.