The 9 and 33 GHz EPR Spectra of Mn2+ impurity ions in polycrystalline calcite—Central transition

The 293 K EPR spectra of Mn²⁺ impurity ions in polycrystalline natural and synthetic calcite and in the calcite impurity in various coal samples have been studied at 9.2 and 33.3 GHz. The spectra obtained at these two frequencies were found to differ dramatically. At 9.2 GHz, the lineshapes of the central transition hyperfine resonances are in good agreement with previous measurements. The 33.3 GHz EPR spectrum has some features which have not been reported previously at Mn²⁺ concentrations below 1000 ppm. Computer simulation programs for both spin-Hamiltonian perturbation and diagonalization techniques were used to describe the mono- and polycrystalline spectra. It was found that the simulated spectrum using the spin-Hamiltonian diagonalization method agrees with previous 9.3 GHz monocrystal data within an error of 0.1 mT for the central transitions and of 0.13 mT for the noncentral transitions, whereas the simulation using the perturbation method had errors of 0.2 and 0.6 mT, respectively. An analysis of the polycrystalline hyperfine resonance lineshapes observed at 9.2 and 33.3 GHz for 1, 50, and 520 ppm Mn²⁺ concentrations has been performed using both simulation methods. The hyperfine resonance lineshapes observed at 9.2 GHz are due to the magnetic field angular variation caused by the axial zero field splitting (ZFS) term C₀²(S). The hyperfine resonance lineshapes observed at 33.3 GHz are shown to be due to the combined effect of A-tensor and g-tensor anisotropy, the axial ZFS, and the spin-spin interaction. The Mn²⁺ impurity ion EPR spectrum observed in an Alberta coal and several Argonne coal samples is shown, using lineshape analysis, to originate from their calcite mineral impurity. The lineshape broadening observed in coal samples is characteristic of a strong spin-spin interaction and may be due to interactions with the free radicals and other impurity ions as well as between Mn²⁺ ions.