Updating of the interpretation of the optical absorption and emission of Verneuil synthetic and natural metamorphic blue sapphire: the role of V²⁺, V³⁺ and Cr²⁺

In the blue colored sapphires of metamorphic origin and Verneuil synthetic studied here, the absorption-emission properties in the VIS-NIR range are largely determined by Cr³⁺ and Ti³⁺, as we have been able to demonstrate recently. In that work a sharp radio-luminescence band occurring at 870 nm in Verneuil blue sapphires was left unattributed: here we give evidence for the attribution of that band to the ²E emission transition of V²⁺, and for the existence of such an emission also in natural samples of metamorphic origin. After such a result, we accurately evaluated by EDXRF the V concentrations in various samples and found the ion more ubiquitous than foreseen. We then searched for and found, weak but diagnostic spin forbidden transitions and phonon sequencies in the absorption spectra of samples sufficiently rich in V. The experimental results just mentioned allowed us to discuss the effects of the overlap of V³⁺ and Cr³⁺ spin-allowed absorption bands on the spectrum of the varieties of corundum under study. To complete the updating of the interpretation, we spent a further effort to strengthen the attribution of the absorption band at 14500 cm⁻¹ (currently interpreted as an IVCT (Fe²⁺ → Fe³⁺)) to the ⁵E→⁵T₂ transitions of Cr²⁺.