HF2EPR investigation of a Cr-bearing gahnite pigment

A pink gahnite pigment, containing the chromophore ion Cr3+, has been synthesised under industrial conditions and characterised by X-ray powder diffraction (XRPD), electronic spectroscopy, electron paramagnetic resonance spectroscopy (EPR) and HF2EPR. The paramagnetic resonance techniques, in particular, have been applied to determine the incorporation of the chromophore in the spinel structure and to study its distribution. The pigment was found to be formed by aluminium borate and, mainly, gahnite (ZnAl2O4). The Cr3+ ion was revealed to be incorporated only in the octahedral site of this latter phase. The chromophore is subjected to a strong crystal field (Δ0 = 18800 cm−1), which determines its colouring properties. A large zero field splitting interaction was observed in the X-band EPR spectra and interpreted by comparison with the high-frequency W-band spectra. The axial symmetry of the crystal field surrounding Cr3+ was ascertained by both the techniques. The observed spectroscopic features are due to the single ion properties of Cr3+ in a strong field, thus ruling out any possible clustering of the chromophore within the structure.