Abstract :
Al-doped lithium manganese spinels, with starting composition Li1.02AlxMn1.98−xO4 (0.00<x≤0.06), are investigated to determine the influence of the Al3+ doping on the Jahn–Teller (J–T) cooperative transition temperature TJ–T. X-ray powder diffraction (XRPD), nuclear magnetic resonance, electron paramagnetic resonance, conductivity and magnetic susceptibility data are put into relation with the tetrahedral and octahedral occupancy fraction of the spinel sites and with the homogeneous distribution of the Al3+ ions in the spinel phase. It is observed that Al3+ may distribute between the two cationic sublattices. The J–T distortion, associated with a drop of conductivity near room temperature in the undoped sample, is shifted towards lower temperature by very low substitution. However, for x>0.04 TJ–T it increases with increasing x, as clearly evidenced in low temperature XRPD observations. A charge distribution model in the cationic sublattice, for Al substitution, is proposed to explain this peculiar behavior.
