Magneli

Magneli refers to a family of mixed-valence titanium oxides with the general formula Ti_nO_{2n-1}. These compounds are characterized by the presence of both Ti^3+ and Ti^4+ ions within their crystal structure, which leads to their unique electrical and optical properties. The Magneli phases were first discovered by Arne Magneli and his colleagues in the 1950s.

The general structure of Magneli phases can be visualized as a rutile TiO2 framework with regularly spaced "shear planes" where oxygen atoms are missing. These shear planes effectively create crystallographic shear (CS) planes. The specific value of 'n' in the formula Ti_nO_{2n-1} determines the specific Magneli phase and the spacing and arrangement of these CS planes. Common examples include Ti4O7 (n=4), Ti5O9 (n=5), and Ti7O13 (n=7).

Magneli phases exhibit a range of interesting properties. They are generally conductive, with electrical conductivity increasing as the Ti^3+/Ti^4+ ratio increases. This conductivity arises from the hopping of electrons between Ti^3+ and Ti^4+ sites. They also display thermochromism, meaning their color changes with temperature, often transitioning from black to dark blue or green. These properties make them candidates for applications in areas such as electrocatalysis, solid oxide fuel cells, and sensors. Research continues to explore their potential in various technological fields due to their unique structural and electronic characteristics.