Luminescence study of transition metal ions in natural magmatic and metamorphic yellow sapphires

Abstract

Optical absorption and luminescence spectra of yellow corundum have been analyzed, both in magmatic and metamorphic materials, looking at the role of localized electronic transitions of transition metal ions at substituted Al sites. By the aid of energy dispersed x-ray fluorescence (EDXRF) elemental analysis and electron paramagnetic resonance (EPR) measurements, the results confirm that Fe 3+ is the dominant impurity ion. However, the results also evidence that Cr 3+ and Ti 3+ contribute in determining the optical absorption and emission properties of this variety of gem-quality corundum, as we have recently found in Type 1 blue sapphires of metamorphic origin. Furthermore, preferentially but not exclusively in samples of metamorphic origin, Mn plays a role never evaluated up to now. Here we show how few ppm of Mn -below the detection limit of EDXRF, not revealed in the optical absorption spectra, but barely detected by EPR spectroscopy as Mn 2+ ion -are active in photoluminescence, showing up with the characteristic emissions of Mn 2+ , Mn 3+ and Mn 4+ .

Defect chemistry of Ti and Fe impurities and aggregates in Al2O3

“…It is still under debate if this is due to intravalence d-d transitions within a single impurity, or to an intervalence charge transfer between the divalent and trivalent valences of iron. 26,31,68 This colour arises from visible light absorption in the energy region of between 2.88 eV and 3.26 eV.…”

Microscopic origin of the optical processes in blue sapphire

“…Additional Fe(II) may also be formed, e.g., through interaction with lattice defects such as oxygen vacancies; however, in naturally occurring sapphires, the predominance of Fe(III) has been noted. 16 Additionally, we have calculated whether the pairs of impurity ions are more stable as isolated species, or when situated in neighbouring lattice sites. Our results show a stabilisation of 0.64 eV when Fe(II) and Ti(IV) ions aggregate, due to their electrostatic attraction.…”

Orange luminescence of α‐Al2O3 related to clusters consisting of F centers and divalent cations

“…It is attributed to the 2 E → 4 A 2 radiative transition of Mn 4+ in synthetic α‐Al 2 O 3 [18, 32–35]. However, in natural α‐Al 2 O 3 , it has been assigned to Mn 4+ only once [36]. However, Mn is detected in ICP‐MS in samples 2 (45 ppm) and 4 (6 ppm).…”