What drives the charge/orbital ordering in La0.5Sr1.5MnO4: Jahn-Teller   distortion versus orbital ordering
  Temperature dependences of a magnetic and an orbital Bragg reflection at the Mn L2 and L3 edges of La0.5Sr1.5MnO4 have been collected. The temperature evolution of the orbital reflection depend strongly on the x-ray energy and reflects the different order parameteres for orbitals and Jahn-Teller distortions. The gradual increase of the orbital order parameter indicates that the orbital order and not the Jahn-Teller distortion drives the metal-insulator transition. Correspondingly, the orbital-orbital (super) exchange interaction dominates over the Jahn-Teller strain-field interaction. 
U. Staub, V. Scagnoli, A.M. Mulders, M. Janousch, Z. Honda and J.M.   Tonnerre