Mechanism of Be-Thermodiffusion in Rutile Inclusions of Fancy Sapphires

Abstract

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.

Vibrational properties of OH groups associated with divalent cations in corundum (α-Al2O3)

“…Based on a comparison with other X-H vibrational frequencies and on the theoretical properties of a Be-H model, the band at 2490 cm −1 was ascribed to the formation of Be-H groups (Jollands and Balan, 2022). Although the high-temperature Be treatment of corundum may be done in reduced conditions, which would also lead to the formation of Ti 3+ ions in Fe-Ti sapphire (Rossi et al, 2022), the systematic presence of this band in samples presumably treated under different conditions was unclear. Considering the presence of absorption bands in the same frequency range as that associated with other divalent cations that are unlikely to form strong X-H bonds, the present theoretical results support the alternative assignment of the 2490 cm −1 band to a bending overtone of the (Be) Al -OH in configuration.…”