Fe-Ti vs. Fe-Fe charge transfers: A comprehensive review and its applications in minerals and glasses

Iron-titanium (Fe-Ti) charge transfer is mentioned in numerous articles as the source of the coloration of many natural minerals and some manufactured materials, but no global review of this phenomenon has been provided so far. Iron and titanium are ubiquitous in nature and are often found in the same material as Fe²⁺ and Fe³⁺, and Ti⁴⁺ (more rarely Ti³⁺). When Fe and Ti ions are in close geometric proximity in an oxide or (alumino)silicate structure, charge transfer can occur between the two ions, even though their concentration might be below 100 ppm. This results in a variety of absorption features that contribute to the color of minerals. A debate remains on the exact nature of Fe/Ti electronic transition, i.e., Fe²⁺ + Ti⁴⁺ → Fe³⁺ + Ti³⁺ or the reverse, but solving this issue is not within the scope of the present work. Ascertaining a metal-metal charge transfer is often not straightforward. This review compiles existing knowledge on Fe-Ti charge transfer in both crystalline and amorphous materials and identifies several key characteristics in more than 40 different materials. A charge transfer is associated with broad, intense, optical absorption bands that decrease in intensity at elevated temperatures. It is also strongly pleochroic in non-isotropic materials. Until now, Fe-Ti charge transfer transitions have been primarily described in the 2.25 to 3.1 eV range, corresponding to yellow to orange to brown colors, with notable exceptions such as blue sapphire or kyanite, and green andalusite. This review suggests that Fe-Ti charge transfer can occur across the entire visible spectrum, and the position of the absorption band correlates with the Fe-Ti interatomic distance. This correlation highlights the presence of multiple crystallographic sites for both Fe and Ti in many oxides, leading to multiple Fe-Ti bands within these materials (e.g., sapphire, ilmenite, pseudobrookite). Finally, the use of metal-metal distances is suggested to differentiate this heteronuclear Fe-Ti charge transfer from the common homonuclear charge transfer Fe²⁺-Fe³⁺.