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

The 2D-Ising ferromagnetic phase BaFe22+(PO4)(2) shows exsolution of up to one-third of its iron content (giving BaFe1.333+(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 degrees C thermal treatment in air, a complete oxidation of Fe2+ to Fe3+ leaves stable M2+ ions, as verified by Fe-57 Mossbauer spectroscopy, TGA, TEM, microprobe, and XANES. The size of the nanometric alpha-Fe2O3 clusters coating the main phase strongly depends on the y(M) metal concentration. For M-rich phases the iron diffusion is hampered so that a significant fraction of superparamagnetic alpha-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.