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

The nature and the behaviour 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 Mossbauer spectroscopy, was investigated and ascertained through high-field high-frequency EPR (HF2EPR) spectroscopy. The blue hercynite-based pigment was biphasic, containing both hercynite and corundum. The HF2EPR spectral features, together with their temperature dependence allowed an assessment of 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. The presence of corundum removed ferric iron, which was eventually formed from hercynite during the synthesis. The dilution of Fe(III) in the Al2O3 phase resulted in a quenching of its colouring effects. As a consequence, corundum acted as a buffer during the synthesis, stabilising the pigment chromatic yield. 13 refs.