Figure 12.

Visual representation of the pseudo Jahn–Teller effect for a TiO₆ octahedron. O1 and O4 are directly above and below Ti⁴⁺ in (a); blue lobes are (+) and red (−). In the high-symmetry configuration (a), the overlap integral between the HOMO |t _1u z〉 (from O²⁻) and LUMO |3d_yz〉 (from Ti⁴⁺) is null. However, displacement of Ti⁴⁺ along the y axis (b) results in both an increase in positive overlap (+/+ and −/−) and a decrease in negative overlap (+/−), thus resulting in spontaneous distortion.