Mechanism of Be-Thermodiffusion in Rutile Inclusions of Fancy Sapphires

The solid-state chemistry of inclusions in Al2O3 is of high relevance in several applications, spanning from photonics to materials engineering and gemology. Be-thermodiffusion is a common treatment of corundum crystals to improve their optical properties. The chemistry of such a high-temperature process is still unclear. Particularly, the interconversion between corundum host and guest inclusion species involves many chemical species and several steps, whose knowledge would guide the tuning of corundum chemical preparation. Herein, 43 thermodiffusion-treated sapphires are investigated, with a focus on 14 samples endowed with blue halos, via optical microscopy, scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS) quantitative chemical analyses, Raman microspectroscopy, and X-ray powder diffraction. This study searches for the snapshots of the chemical path and then recomposes them to propose a novel chemical mechanism in which Be reduces TiO2 inclusion, which is then converted, mediated by iron ions, to several titanates. The chemical mechanism is discussed in terms of applications not limited to only gemology but to any derivative corundum crystal preparation.