The synthesis of barium hexaferrite from iron oxide and barium carbonate: 1. General features of the reaction

The reaction between BaCO3 and Fe2O3 in the proportions necessary for the formation of the hexaferrite has been studied in air. The decarbonation reaction was followed by measuring weight loss isothermally, the formation of hexaferrite by measurement of saturation magnetisation and by X-ray diffractometry. The only kinetically important intermediate found was the monoferrite and its rate of formation could be correlated with the rate of decarbonation. An unexpected feature of this process was the rapid apparent disintegration of the barium carbonate lattice: this was far more rapid than the weight loss suggested. An examination of the pseudobinary system BaO-Fe2O3 in air at 1000°C using the electron micro-probe analyser on reaction products at an interface between partly reacted Fe2O3 and BaCO3 revealed the high barium ferrites 2BaO.Fe2O3; 3BaO.Fe2O3; 5BaO.Fe2O3 but no compound intermediate in composition between mono- and hexaferrite, nor any solid solution of Fe2O3 in monoferrite. With powdered reactants the high barium phases were not formed in sufficient concentration and or crystallinity to be revealed by X-ray diffraction. But their presence could be inferred, because of their reaction with water vapour and/or carbon dioxide, from infrared (i.r.) absorption in the range 700–900 cm−1. An absorption peak at 880 cm−1 appeared to be associated with a very small amount of incompletely reacted material as saturation magnetisation approached the maximum achievable. In the presence of only 0.5% lithium fluoride both decarbonation and the conversion of mono- to hexaferrite were greatly accelerated; scanning electron microscopy revealed a big change in crystal size and the form suggested a mechanism involving a liquid phase.