A study of Fe3+ ions in NH4Al(SO4)2 12H20; using electron paramagnetic resonance and the Mössbauer effect

Abstract

Electron Paramagnetic Resonance (EPR) of Fe(H2O)6 3+ in a single crystal of ammonium aluminium alum, 1 Fe to 400 Al, is partially interpreted by the spin Hamiltonian - This assumes that the crystalline field is an admixture of cubic and trigonal components. It was found that the crystalline axes are trigonal (111) and the best fit values of the parameters are, g = 1.978 ± 0.007, D = ±270 gauss, a = ±4, F = ±14g. A broad resonance at g = 2 is interpreted as an exchange narrowed absorption of ferric ions at an imperfection. Both EPR and the Mossbauer effect indicate a slightly distorted ion, for which the quadrupole splitting, ∆EQ = 0.03 mm/sec. The ion is trivalent as its isomer shift is between 0.4 to 0.5 mm/sec relative to metallic iron. A possible explanation of the broad Mossbauer resonance at zero magnetic field is that the electron spins are ordered within the crystal and there are two types of sites. One of these is the trivalent ion position of alum and the other is a range of environments, consequent of some crystal imperfection.