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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 |t1uz〉 (from O2−) and LUMO |3dyz〉 (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.

IUCrJ
Volume 7| Part 4| July 2020| Pages 581-629
ISSN: 2052-2525
https://doi.org/10.1107/S2052252520005928