Defect chemistry of Ti and Fe impurities and aggregates in Al₂O₃

Abstract

We report a theoretical evaluation of the properties of iron and titanium impurities in sapphire (corundum structured α-Al2O3). Calculations using analytical force fields have been performed on the defect structure with the metals present in isolated, co-doped and tri-cluster configurations. Crystal field parameters have been calculated with good agreement to available experimental data. When titanium and iron are present in neighbouring face and edge-sharing orientations, the overlap of the d-orbitals facilitates an intervalence charge transfer (FeIII/TiIII → FeII/TiIV) with an associated optical excitation energy of 1.85 eV and 1.76 eV in the respective configurations. Electronic structure calculations based on density functional theory confirm that FeIII/TiIII is the ground-state configuration for the nearest-neighbour pairs, in contrast to the often considered FeII/TiIV pair. Homonuclear intervalence charge transfer energies between both FeIII/FeII and TiIV/TiIII species have also been calculated, with the energy lying in the infra-red region. Investigation of multiple tri-clusters of iron and titanium identified one stable configuration, TiIII–(TiIV/FeII), with the energy of electron transfer remaining unchanged

Cause analysis of chatoyancy of sapphires from Shandong, China

“…Sapphires have long received attention as technologically sensitive materials because of their mechanical and optical properties; thus, sapphires are widely valued in the material industry. [22][23][24][25] Considering the importance of sapphires, it is necessary to study them in all aspects, including their special optical effects. The aim of this study was to help determine the classication, systematic study, and quality evaluation of sapphires and provide research ideas for the cause analysis of cat's eye phenomena of other gemstones.…”

Hybrid‐Functional Study of Native Point Defects and Ti/Fe Impurities in α‐Al2O3

“…This agrees well with previous work reporting that the transition of intra-atomic electrons in metal impurities is the origin of colors in Ti-doped or Fe-doped sapphires. [9,24]…”

“…However, there has been a controversy about whether the Ti IV -Fe II or Ti III -Fe III pair is the lowest energy structure of Ti-Fe pair. [9,13,24,25,56] In this part, we thoroughly investigated the Ti-Fe pair by considering all possible spin configurations, as shown in Figure 1. There are two possible configurations to replace two Figure 7).…”

“…[ 8 ] The cause of the energy absorption in this range was described by the transition of intravalence electrons arising within the metals, and the interaction with the phonon lattice is attributed to the cause of the energy absorption in this range. [ 8,9 ] Once Ti is incorporated into α ‐Al 2 O 3 , it would substitute for Al's site with 3+ oxidation state. However, it has been reported that Ti can also exist in 4+ oxidation state, but the concentration of Ti IV is quite low in natural sapphire.…”

“…However, it has been reported that Ti can also exist in 4+ oxidation state, but the concentration of Ti IV is quite low in natural sapphire. [ 9 ] Regarding iron (Fe)‐doped α ‐Al 2 O 3 , its color can be in the range from yellow to green crystals, and it can be used in stress sensor and optical waveguide applications. [ 10 ] Furthermore, it has been reported that Fe is mostly present in 3+ oxidation state, whereas the concentration of Fe II can be increased by applying heat.…”