Preparation of Various β″-Alumina Type Ferrites by Ion Exchange Method

Abstract

The K⁺ ions in K⁺-β″-ferrite were exchanged with Na^t, Rb⁺, Cs⁺, Ag⁺, T1⁺ and NH₄⁺ions in the corresponding molten nitrates at 200--450°C or with H30⁺ ions in concentrated H₂SO₄ at 90°C, respectively, to form M⁺-β″-ferrites. The lattice constant c_o, which depends on the thickness of alkali layers, increased with increasing ionic radius of the ion inserted in the layers. In addition, the K⁺ ions in K⁺-β″-ferrite were found to be replaced by Ba²⁺ ions in Ba(NO₃)₂ melt at 650°C to form Ba²⁺-β″-ferrite. However, when the K+-β″-ferrite was immersed in a molten salt of other divalent cations (Mg²⁺, Co²⁺, Ca²⁺, Zn²⁺, Cd²⁺, Pb²⁺ or Sr²⁺ ion), the structure of K⁺-β″-ferrite was rearranged to form a s pinel ferrite, α-Fe₂O₃, γ-Fe₂O₃ or SrFe₂O₄.
The Rb⁺- and Cs⁺-β″-ferrites were partially decomposed to form α-Fe₂0₃ at about 800°C, and then were transformed to β phase at above 1050-1150°C. Ag⁺- and T1⁺-β″-ferrites were rapidly decomposed to form α-Fe₂0₃ at above 600°C. NH₄⁺- and H₃0⁺-β″-ferrites were decomposed at the low temperature of 300°C to form γ-Fe₂O₃T as an intermediate phase, and this phase was transformed to α-Fe₂0₃ at 450°C. The formation of this intermediate phase may be due to the structural similarity of β″-ferrite to γ-Fe₂0₃.