Changes in the Jahn-Teller distortion at the metal-insulator transition in CMB manganites (Pr,Nd)0.7(Sr,Ca)0.3MnO3

CMR manganites (Pr,Nd)(0.7)(Sr, Ca)(0.3)MnO3 were studied by x-ray absorption spectroscopy at the O K-edge above and below the transition temperature Tc. In compounds which keep the insulating behaviour in the whole temperature range, the prepeak at the O K-edge remains unique upon cooling even in the case of the presence of a paramagnetic-ferromagnetic transition. In compounds showing a spontaneous transition from a paramagnetic-insulating state to a ferromagnetic-metallic one, the prepeak at the O K-edge is split into two components. A crystal field analysis of this behaviour is proposed based on the assumption of a strong reduction, upon cooling, of the octahedral distortion due to the dynamic Jahn-Teller splitting present above T-C This reduction of the crystal field distortion at T-C is correlated to the transition to the metallic state and to the presence of the magneto-resistant effect.