On the solubility of rare earths in M-type SrFe12O19 hexaferrite compounds

Sr1-xRExFe12O19 and Sr1-xRExFe12-xCoxO19 (x = 0-0.4 and RE = Pr, Nd) M-type hexaferrite powders were produced by a conventional ceramic process. Structural investigations made by x-ray diffraction and Mossbauer spectrometry reveal that the solubility of the rare earth ion in the M-type phase depends on both the nature of the rare earth and the presence of Co. The solubility of Pr in the M-type phase is higher than the solubility of Nd, and the presence of Co increases the solubility of the rare earth ion. These results were interpreted in the frame of the published literature. It appears that only light rare earths can enter the M-type structure, with a solubility that is related to the shape of the 4f electronic charge distribution and to its surroundings in the crystal structure. Rare earth ions are located in the Sr2+ site, whose surroundings favour an oblate electronic distribution. Co2+ ions modify the surroundings of the Sr2+ site, improving the introduction of rare earth ions with oblate electronic distribution.