HF 2EPR investigation of a Cr-bearing gahnite pigment

Abstract

A pink gahnite pigment, containing the chromophore ion Cr 3+, has been synthesised under industrial conditions and characterised by X-ray powder diffraction (XRPD), electronic spectroscopy, electron paramagnetic resonance spectroscopy (EPR) and HF 2EPR. 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 (ZnAl 2O 4). The Cr 3+ ion was revealed to be incorporated only in the octahedral site of this latter phase. The chromophore is subjected to a strong crystal field ( Δ 0 = 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 Cr 3+ was ascertained by both the techniques. The observed spectroscopic features are due to the single ion properties of Cr 3+ in a strong field, thus ruling out any possible clustering of the chromophore within the structure.