Origin of the early degeneracy of the nonequivalent M points

Determine why the nonequivalent M and M′ points in orthorhombic CsPbBr₃ become degenerate at approximately 200 K, despite their Jahn–Teller-induced splitting at lower temperatures.

Background

Angle-resolved photoemission measurements show that the two nonequivalent M points of orthorhombic CsPbBr₃ are split at low temperatures, with the energy difference decreasing as temperature increases. The observations support a temperature-dependent lifting of degeneracy associated with octahedral tilts and rotations and therefore with the proposed second-order Jahn–Teller effect.

However, the two points become degenerate at approximately 200 K, substantially below the temperature range where the orthorhombic structural distortion persists. The paper does not determine the origin of this early degeneracy; surface effects are mentioned only as a speculation, and measurements performed in a different geometry do not establish the temperature at which the degeneracy occurs.

References

However, the fact that M and M$^ become degenerate as early as $\sim$200~K remains unexplained.

— Large Jahn-Teller shifts and splittings observed in halide perovskite CsPbBr3  (2608.23510 - Sajedi et al., 24 Aug 2026) in Section “Energy shift and splitting,” discussion surrounding Fig. 4