New spectroscopic and diffraction data to solve the vanadium-doped zircon pigment conundrum

The literature on V-doped zircon pigments offers a puzzling situation with unanswered questions about the origin of turquoise colour. In this study, new diffraction (XRPD) and spectroscopic (EPR, EAS) data were obtained on four industrial pigments, whose vanadium content is much higher than samples from previous investigations. Both EPR features and XRPD results indicate the occurrence of tetrahedrally-coordinated V4+ at the zircon interstitial site 16g. EPR spectra at 5 K exhibit a structured multiplet attributed to isolated V4+ ions and an exchange-narrowed line resulting by clustering of V4+ ions. The intensity of optical bands is stronger in samples affected by V4+ clusters. However, the turquoise colour stems from the window of transmitted green-bluish light, which depends on both the tail of the main optical band (B-2 <- (2)A) and the onset of charge transfer phenomena. Small variations in these two bands readily turn into a chromatic shift from light blue to green.