HF2EPR spectroscopy of Fe(III) impurities in a blue hercynite-based pigment

The nature and the behavior of a paramagnetic Fe(III) impurity, occurring in a new charge-transfer hercynite-based blue pigment and determined through conventional electron paramagnetic resonance (EPR) and Mِssbauer spectroscopy, was investigated and ascertained through high-field high-frequency EPR (HF2EPR) spectroscopy. The blue hercynite-based pigment is biphasic, containing both hercynite and corundum. The HF2EPR spectral features, together with their temperature dependence allowed to assess the intensity and the symmetry of the Fe(III) crystal field interactions. Numerical simulations were used to determine the relevant hamiltonian parameters. The final attribution of Fe(III) to the octahedral sites in corundum was achieved. esence of corundum was found to remove ferric iron, eventually formed during the synthesis, from hercynite. The dilution of Fe(III) in the Al2O3 phase results in a quenching of its coloring effects. As a consequence, corundum acts as a buffer during the synthesis, stabilizing the pigment chromatic yield.