Fe-Ti
vs. Fe-Fe charge transfers:
A
comprehensive review and its applications in minerals and glasses
Vigier M1.2, Evans H3, Rossman GR3, Jobic S2, Fritsch E2
1Conditions Extrêmes et
Matériaux: Haute Température et Irradiation (CEMHTI), UPR 3079 CNRS,
45100 Orléans, France.
2Nantes Université, CNRS, Institut des Matériaux de Nantes
Jean Rouxel, IMN, F-44000, Nantes, France.
3Division of Geological and Planetary Sciences, California
Institute of Technology, Pasadena, California 91125-2500, U.S.A
Abstract
The iron-titanium charge transfer is mentioned in numerous articles as
the source of the coloration of many natural minerals and some man-made
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 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 contributes 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 with a decreasing intensity with high temperature. It
is also strongly pleochroic in non-isotropic materials. Until now,
Fe-Ti charge transfers have been primarily described in the 2.25 to 3.1
eV range, corresponding to yellow-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⁺.